Radio observations of active galactic nuclei with mm-VLBI
TThe Astronomy and Astrophysics Review manuscript No. (will be inserted by the editor)
Radio observations of active galactic nuclei with mm-VLBI
B.Boccardi · T.P. Krichbaum · E. Ros · J.A. Zensus
Received: date / Accepted: date
Abstract
Over the past few decades, our knowledge of jets produced by active galactic nuclei (AGN) hasgreatly progressed thanks to the development of very-long-baseline interferometry (VLBI). Nevertheless,the crucial mechanisms involved in the formation of the plasma flow, as well as those driving its exceptionalradiative output up to TeV energies, remain to be clarified. Most likely, these physical processes take place atshort separations from the supermassive black hole, on scales which are inaccessible to VLBI observationsat centimeter wavelengths. Due to their high synchrotron opacity, the dense and highly magnetized regionsin the vicinity of the central engine can only be penetrated when observing at shorter wavelengths, in themillimeter and sub-millimeter regimes. While this was recognized already in the early days of VLBI, itwas not until the very recent years that sensitive VLBI imaging at high frequencies has become possible.Ongoing technical development and wide band observing now provide adequate imaging fidelity to carryout more detailed analyses.In this article we overview some open questions concerning the physics of AGN jets, and we discussthe impact of mm-VLBI studies. Among the rich set of results produced so far in this frequency regime, weparticularly focus on studies performed at 43 GHz (7 mm) and at 86 GHz (3 mm). Some of the first findingsat 230 GHz (1 mm) obtained with the Event Horizon Telescope are also presented.
Keywords high angular resolution · jets · active galaxies B. Boccardi; T.P. Krichbaum; J.A. ZensusMax-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, D-53121 Bonn, GermanyE-mail: [email protected]. RosMax-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, D-53121 Bonn, GermanyObservatori Astron`omic, Universitat de Val`encia, C. Catedr´atico Jos´e Beltr´an 2, E-46980 Paterna, Val`encia, Spain · Departamentd’Astronomia i Astrof´ısica, Universitat de Val`encia, C. Dr. Moliner 50, E-46100 Burjassot, Val`encia, Spain a r X i v : . [ a s t r o - ph . H E ] N ov B.Boccardi et al.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1
The quest for angular resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 mm-VLBI arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1
General concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2
A brief history and current status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Extragalactic jets as seen by VLBI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1
Where do we stand? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
AGN science with mm-VLBI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.1
High-energy emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.2
Internal structure and morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.3
Polarization and magnetic field topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.4
Launching, acceleration and collimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.5
Sagittarius A* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
The future of mm-VLBI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35adio observations of active galactic nuclei with mm-VLBI 3 Introduction
Astrophysical jets count among the most spectacular and powerful objects in the Universe. Theyare collimated outflows of plasma observed in a variety of astronomical sources – young stellarobjects (YSOs), X-ray binaries (XRBs), active galactic nuclei (AGN) and γ -ray bursts (GRBs) –spanning many order of magnitudes both in the energy domain and in linear scale. Despite thediversity of environments where they can originate, all jets share some common features. Theirpower source is believed to be the gravitational potential of a compact and accreting central object(Salpeter, 1964; Lynden-Bell, 1969), whose mass largely determines the scaling properties (Samset al., 1996; Heinz and Sunyaev, 2003; Merloni et al., 2003; Falcke et al., 2004). Moreover, alljets are magnetized, as they are, especially in the radio band, copious emitters of synchrotronradiation. Analyzing the similarities as well as the differences between the classes of jets is crucialfor ultimately understanding the formation and propagation of the outflows, and the connectionbetween the accretion properties and the jet activity (e.g., Meier, 2003; Belloni, 2010).AGN jets, powered by supermassive black holes at the center of some active galaxies, certainlyform the most studied class. These highly collimated outflows, with opening angles of few degrees,propagate often undisturbed up to kiloparsec and sometimes megaparsec distances, and radiate overa broad interval of the electromagnetic spectrum. Most of their total power, varying in the range10 − erg / s (Ghisellini et al., 2014), is however not radiative, but carried in different forms.Close to the launching site it may be purely electromagnetic, while on larger scales it converts tomostly kinetic as the bulk flow accelerates (e.g., Meier, 2012), reaching terminal Lorentz factorsof the order of ten (Lister et al., 2016). Ultimately, the energy gets dissipated in the form of radia-tion, giving rise to irregular structures of diffuse radio emission known as radio lobes, sometimespunctuated by compact hotspots. A beautiful representation of the large scale morphology in theradio galaxy Hercules A is shown in Figure 1. Although, according to the unification schemes(e.g., Barthel, 1989; Urry and Padovani, 1995), all jets are intrinsically elongated and two-sided asin Hercules A, many of the sources we can observe appear highly compact and asymmetric. Thisis due to strong relativistic and projection effects arising from the close alignment of the jet axiswith our line of sight, which make the jet properties even more dramatic, but also more difficult tostudy. The quest for angular resolutionStarting in 1963, when Maarten Schmidt revealed the extragalactic nature of the radio source3C 273 (Schmidt, 1963), astronomers have extensively investigated the physical processes whichcould lead to those tremendous energy outputs. The puzzle became even more complicated whenthe emission was observed to vary on extremely short timescales t var (see Rees, 1970, and refer-ences therein). Based on the light travel time argument, the sizes of the emitting regions l ≤ c · t var (with c indicating the speed of light) were determined to be as small as few light months. Theextreme compactness implied was not only a challenge for theorists, but also for observers. Ex-pressed in angular dimensions, the relevant scales for an AGN jet are in fact of the order of themilli-arcsecond or smaller (for typical distances up to few Giga-parsecs), reaching far beyond thediffraction limit of a single telescope.Already in the 1940s, radio astronomers had been looking for smart solutions aimed at increas-ing the resolving power of their instruments. This goal was brilliantly achieved with the develop-ment of aperture synthesis (Ryle and Vonberg, 1946; Ryle and Hewish, 1960), acknowledged by B.Boccardi et al.
Fig. 1
Composite image of the radio galaxy Hercules A. In blue is the radio emission associated with the jets and the lobes; in pink theX-ray emission from the heated surrounding gas; in white, orange, and blue the host galaxy and the background optical field. (Credit:X-ray: NASA/CXC/SAO, Optical: NASA/STScI, Radio: NSF/NRAO/VLA). the first Nobel prize for astronomical research, in 1974. Aperture synthesis is an interferometrictechnique aimed at synthesizing a very large effective aperture from an array of telescopes. Thiselegant method is applied in its utmost complexity in very-long-baseline interferometry (VLBI),in which the astronomical signal collected by physically unconnected telescopes can be combined.The angular resolution of an interferometer is proportional to the ratio λ / b max , where λ is thewavelength of the radiation and b max is the maximum baseline length, i.e., the maximum (pro-jected) separation between two elements of the array. The major VLBI arrays currently in use, theVery Long Baseline Array (VLBA) and the European VLBI Network (EVN), are characterizedby maximum baselines of the order of 10000 kilometers, and perform most of their observationsat centimeter wavelengths. Their operation is crucial in the investigation of compact radio sources,and has advanced significantly our knowledge of the physical conditions of the plasma flow onmilli-arcsecond scales, i.e., on projected spatial extents of the order of few parsecs.However, the quest for a deeper understanding of these objects is not over. Based on observa-tional and theoretical grounds, the fundamental mechanisms involved in the energy production andin the very formation of the jet outflow are expected to take place on even smaller scales, com-parable to the Schwarzschild radius R S = GM BH / c ( G is the gravitational constant and M BH isthe black hole mass) of the supermassive black hole. Radio interferometric observations capableof probing such scales are therefore decisive for obtaining a complete picture of the AGN phe- nomenon, particularly if used in synergy with high-frequency studies, in the optical, X-ray, and γ -ray bands (e.g. Marscher, 2005).When aiming at improving the angular resolution of a radio interferometer, two main ap-proaches can be followed. The first consists in increasing the maximum baseline by having oneor more telescopes orbiting in space. After some pioneer experiments performed in the 80’s, space-VLBI was successfully realized with the VLBI Space Observatory Program (VSOP) (Hirabayashiet al., 2000) and, more recently, with the RadioAstron mission (Kardashev et al., 2013). In the sec-ond approach, ground telescopes can be equipped with receivers operating at shorter wavelengths,in the millimeter or sub-millimeter bands. Both methods enable resolutions of a few tens of micro-arcseconds, which in the closest objects translate into linear sizes as small as few Schwarzschildradii. However, since the nuclear environment is dense and highly magnetized, a truly sharp view ofradio cores in AGN can only be obtained by penetrating the opacity barrier shrouding them. Bothsynchrotron and free-free opacity can affect significantly the cm-wave emission, but are expectedto be much reduced in the millimeter band. Millimeter VLBI combines in a unique manner a highspatial resolution with a spectral domain where source-intrinsic absorption effects vanish, and istherefore ideally suited for the imaging of the still unexplored regions in the vicinity of the blackhole.In this article, we discuss some open questions concerning the physics of compact radio sources,and the impact of mm-VLBI observations towards a more detailed physical understanding. In Sec-tion 2 we briefly summarize the historical development of this sophisticated technique and wereport on the capabilities of current mm-VLBI arrays. Section 3 is intended to provide the readerwith an overview of the basic properties of jets as inferred from VLBI studies, and of the theoret-ical models that best describe them. In Section 4 we expand the discussion on the scientific topicsthat mm-VLBI can address and we present the main results produced so far. We conclude with anoutlook of the future developments and goals of the mm-VLBI science (Sect. 5). mm-VLBI arrays General conceptsVery-long-baseline interferometry (VLBI) is an elegant observing technique which, as of this writ-ing, provides the highest possible angular resolution in astronomy (see G´omez et al., 2016, for thehighest resolution image produced to date). In the following part of this section, we will detail thecapabilities of VLBI arrays operating in the millimeter regime. First, however, we wish to introducesome necessary terminology and fundamental parameters which define the performance of radiotelescopes and interferometers. The following description is by no means complete, and the readeris referred to the specialized textbooks on interferometry and synthesis imaging (e.g., Thompsonet al., 2017).A radio interferometer is an instrument which enables to combine the radio waves coming froman astronomical object to form interference fringes. By correlating the signals collected simulta-neously by each telescope forming the array, radio interferometers measure the complex visibilityfunction V ( u , v ) , which is the (noise-corrupted) Fourier transform of the brightness distribution ofthe sky. The ( u , v ) coordinates define, in units of wavelength, the East-West and the North-Southcomponent of each baseline projected in the sky, as seen from the source. Thus, the ( u , v ) -planecontains information about the existence or absence of a visibility measurement in a certain point. B.Boccardi et al.
The filling of the ( u , v ) -plane, i.e., the ( u , v ) - coverage , is by definition incomplete for any inter-ferometer, and can be improved by adding more telescopes to the array and by increasing theon-source time up to 12 hours, so that the Earth rotation enables a single baseline to sample a fulltrack (an ellipse) in the ( u , v ) -plane. Each baseline of projected length b is characterized by its ownfringe pattern, and is only sensitive to source structures on scales comparable to the fringe spacing λ / b . Therefore, the better the sampling of the ( u , v ) -plane, the more reliable will be the reconstruc-tion of the sky brightness distribution. The smallest angular scale an interferometer can probe, itsresolution, coincides, in general, with the diffraction limit ∼ λ / b max , where b max is the maximumbaseline length.While the angular resolution achieved at a given wavelength depends, in principle, only on thegeometrical properties of the array, the sensitivity of interferometric observations is largely depen-dent on the characteristics of the single telescopes forming the array and of the recording systemsin use. Given an array formed by N telescopes, the sensitivity of a single telescope with geometricalarea A is quantified by the system equivalent flux density SEFD, a parameter which accounts for thecombined sensitivity of the antenna and of the receiving system. The SEFD [ Jy ] is given by the ratio T sys / g , where the system temperature T sys [ K ] is the sum of all the noise contributions, both fromthe electronics and from the astronomical source, while the gain g [ K / Jy ] quantifies the radio noisepower received from a source of unit flux density. The gain is a measure of the antenna efficiency η , an elevation-dependent parameter which determines the effective collective area A eff = η A ofthe dish. At short radio wavelengths, the efficiency is especially limited by the antenna surfaceaccuracy. For instance, a surface accuracy s = λ /
16 implies a reduction of the effective collectingarea by a factor η s = exp ( − π s / λ ) (cid:39) . n s . However, the number of samples cannot be arbitrarily high, being limited by thebandwidth of the signal ∆ ν . According to the Nyquist sampling theorem, samples taken over timeintervals ∆ t shorter than 1 / ∆ ν are not independent. Therefore the number of samples n s = ∆ ν t has to be increased by increasing the observing time t and/or by extending the observing bandwidth ∆ ν . According the central limit theorem, averaging over a large number of samples will reduce therms noise σ by a factor of √ n s , which allows us to write the radiometer equation as: σ = √ T sys √ ∆ ν t = T sys √ ∆ ν t . (1)Among the possible ways to achieve a higher sensitivity, increasing the bandwidth is the mostcost-effective. For this reason, the technical improvements in VLBI, and in radio interferometry ingeneral, have been especially oriented towards the development of wide bandwidth receivers andrecording systems.In VLBI mode, radio signals are digitized and recorded at each telescope on magnetic tapes,together with extremely precise time stamps usually provided by an hydrogen maser clock. Thedigitization process has also an impact on sensitivity, and reduces the signal to noise ratio by afactor η c ≤
1, called VLBI system efficiency, with respect to an ideal analog recording. The VLBIsystem efficiency depends on the number of bits used to represent each sample, with η c ∼ .
63 fora 1-bit representation and η c ∼ .
87 for a 2-bit representation.The sensitivity provided by a certaindigital recording system is therefore determined by the data rate, expressed in bit / s.Ultimately, the theoretical thermal noise σ im of a VLBI image depends both on the SEFD ofeach telescope and on the data rate, and can be expressed in units of Jy / beam as adio observations of active galactic nuclei with mm-VLBI 7
160 2010.50.30.20.150.120.10 301001401802202603000102030405060708090100 1009080706050403020100 O p ac it y ( on e - w a y , % ) T r a n s m i ss i on ( on e - w a y , % ) Frequency (GHz) 90 GHzwindow135 GHzwindow 35 GHzwindowWater-vapor absorption bandsWater-vapor absorption bandsOxygen absorption bandsWavelength (cm)
Fig. 2
Percentage transmission through the Earth’s atmosphere, in the vertical direction, under clear sky conditions (Ulaby et al., 2014).Strong molecular absorption is caused by the water vapor and the oxygen in the troposphere. σ im = η c SEFD ∗ (cid:112) N ( N − ) ∆ ν t int , (2)where SEFD ∗ = (cid:113) ∑ N ; i < ji , j = ( SEFD i × SEFD j ) and t int is the total integration time on source in sec-onds. A brief history and current statusThe possibility of conducting VLBI experiments at millimeter wavelengths was first explored inthe late 70’s. Only a few years later, in the early 80’s, pioneering experiments were successfullyperformed through the observation of the strong radio source 3C 84 at 86 GHz (3 mm) (Readheadet al., 1983) and 43 GHz (7 mm) (Marcaide et al., 1985). The choice of these observing bandswas naturally determined by the behavior of atmospheric transmission in the radio window. Asshown in Fig. 2, strong opacity spikes due to molecular absorption in the troposphere (mainly fromwater vapor and oxygen) overlay the general trend of steeply increasing opacity as a function offrequency, and bracket three main observing windows at around 35 GHz, 90 GHz, and 135 GHz.Since those first detections, the performance of mm-VLBI arrays has been steadily improving,until a sensitive imaging (with dynamic range > B.Boccardi et al.
Table 1
Properties of the antennas operating at 86 GHz. Col. 1: Name of the telescope. Col. 2: Its location. Col. 3: Effective diameter inmeters. Col. 4: Typical value of the system equivalent flux density (SEFD), which is a measurement of the telescope sensitivity. Lowervalues indicate a better performance.
Station Location Effective diameter [ m ] SEFD [ Jy ] Effelsberg Germany 80 1000Onsala Sweden 20 5100Plateau de Bure France 34 820Pico Veleta Spain 30 650Yebes Spain 40 1700Mets¨ahovi Finland 14 17000Green Bank United States 100 140VLBA ( ×
8) United States 25 2500KVN ( ×
3) South Korea 21 3200ALMA Chile 85 60 the local oscillator systems of the receiver, antenna-pointing errors, uncertainties in the antennaspositions (see e.g., Rogers and Moran, 1981). At high radio frequencies, however, phase drifts aremostly of atmospheric origin (see Rogers et al., 1984, for a discussion), being primarily caused bywater vapor in the troposphere. High altitude and/or dry sites are the optimal choice for minimizingthe degradation of the signal in such observations. While excellent instruments, like the Plateau deBure interferometer or the Atacama Large Millimeter Array (ALMA), were built ad hoc in optimalsites in the following years, initial millimeter arrays could only count on the existing telescopes,most of which were conceived for operating at centimeter-waves. This, combined with the poor ( u , v ) -coverage, limited bandwidths and inaccurate calibration procedures, has severely hinderedthe imaging capabilities of past mm-VLBI arrays.An important step forward was made with the foundation of global arrays operating at 86 GHz,the Coordinated Millimeter VLBI Array (CMVA) (Rogers et al., 1995), established in 1995, and itssuccessor, the Global Millimeter VLBI Array (GMVA) (Krichbaum et al., 2006), whose activityis ongoing since 2003. The participation of a larger number of telescopes ( > >
10) gradually led toan improvement of the performance. This fact can be immediately recognized when consideringthe detection rate of 86 GHz VLBI surveys conducted between 1997 and 2008. This increasedfrom a percentage of less than 25% obtained in the first campaigns (Beasley et al., 1997; Lonsdaleet al., 1998; Rantakyr¨o et al., 1998) to more than 90% achieved in the latest ( ? Lee et al., 2008;Gopalakrishnan Nair et al., 2016).Currently, the GMVA comprises up to 18 telescopes (Effelsberg, Onsala, Plateau de Bure, PicoVeleta, Yebes, Mets¨ahovi, Green Bank, eight VLBA stations and the three telescopes forming theKorean VLBI Network - KVN) spread over three continents (Europe, America, and Asia – Table1, Fig. 3). The phased ALMA, located in Chile, has also participated for the first time in mm-VLBIexperiments in April 2017. To date, most of the GMVA stations are observing at a bandwidth of512 MHz (which corresponds to a data rate of 2048 Mbit/s), and further upgrades are expected inthe near future thanks to the fast development of digital VLBI recording systems. Global VLBI VLBA – Mauna KeaVLBA – Mauna KeaVLBA – Kitt PeakVLBA – Kitt PeakVLBA – Owens ValleyVLBA – Owens ValleyVLBA – BrewsterVLBA – Brewster VLBA – Pie TownVLBA – Pie Town VLBA – Fort DavisVLBA – Fort Davis IRAM – Pico Veleta 30mIRAM – Pico Veleta 30m OAN – Yebes 40mOAN – Yebes 40m IRAM - NOEMAIRAM - NOEMA KVN – TamnaKVN – TamnaKVN – UlsanKVN – UlsanVLBA – North LibertyVLBA – North LibertyVLBA – Los AlamosVLBA – Los Alamos Phased-ALMAPhased-ALMA Onsala 20mOnsala 20m Metsӓhovi 14mMetsӓhovi 14mNRAO – GBT 100mNRAO – GBT 100m KVN – YonseiKVN – YonseiMPIfR – Effelsberg 100mMPIfR – Effelsberg 100m INAF- NotoINAF- Noto
Fig. 3
The telescopes forming the Global Millimeter VLBI Array (GMVA), operating at the frequency of 86 GHz. Credit: HelgeRottmann. observations at 86 GHz can achieve a typical angular resolution of 50-70 micro-arcseconds, and anarray sensitivity of ∼ project. When completed, the EHT will be able resolve the black hole surroundings onscales comparable with the event horizon. The expected resolution is of 20–30 µ as at 230 GHz andof 13–20 µ as at 345 GHz. Due to the large apparent size of their event horizon, Sagittarius A* andM 87 are the best candidates for achieving such a goal, and therefore are considered as primarytargets for EHT observations. Extragalactic jets as seen by VLBI
In VLBI images, the radio emission from an AGN can be typically ascribed to a compact, bright,and unresolved feature called the “core”, and to a one-sided jet emanating from it (Fig. 4). Thisclassical morphology is the result of selection effects, involving both the relativistic nature of theflow and the sensitivity of VLBI arrays.Similarly to Hercules A (Fig. 1), all jets are thought to be characterized by a roughly symmetrictwo-sided structure. However, due to the relativistic nature of the flow, the emission from the sideapproaching the observer can be highly enhanced as a consequence of relativistic Doppler boost-ing. By applying the Lorentz transformations, it can be shown that the observed flux density S o differs from the intrinsic one S e by a factor δ n − α , where δ is the Doppler factor, α is the spec-tral index, and n is a parameter varying between 2 and 3 (see Scheuer and Readhead, 1979). TheDoppler factor depends on the speed of the flow β = v / c , which determines the bulk Lorentz fac-tor Γ = / (cid:112) ( − β ) , and on the jet orientation, defined by the angle θ between the direction ofpropagation of the outflow and the line of sight of the observer: δ = Γ ( − β cos θ ) . (3)Given a population of jets oriented randomly and spanning a certain range of Lorentz factors, itis clear from Eq. 3 that Doppler boosting increases the chances to detect the fastest objects and/orthose oriented at small angles (naturally, there is also a dependence on the intrinsic luminosityfunction of the jet population, see e.g. Lister and Marscher, 1997). The frequent observation ofsuperluminal motion, i.e., of apparently faster-than-light speeds β app of the plasma features in thejet, confirms the selection bias affecting the jets samples. This purely geometrical phenomenon,predicted by Martin Rees in 1966 (Rees, 1966) and observed for the first time in 3C 273 (Gubbayet al., 1969), occurs more prominently in fast flows seen at small angles, as described by the relation β app = β sin θ − β cos θ . (4)Another selection effect in VLBI is related to sensitivity. It is not sufficient for an astronomicalsource to be a strong radio emitter to be seen by VLBI. Only “high intensity” objects, i.e., withhigh brightness temperature T b , are suitable targets. The minimum brightness temperature T minb detectable by an interferometer depends on the flux density S ν , the baseline length b , and theBoltzmann constant k as T minb = π k b S ν , (5)and is typically in the range 10 − K. The latter implies that both thermal emission ( T B < K)and non-thermal emission of insufficient compactness (e.g., from the radio lobes) are completelyresolved out in a VLBI observation. In conclusion, the samples targeted by VLBI are dominated bythe highly boosted compact objects, also known as blazars . While studies of blazars may providean incomplete view of the general jet phenomenon in AGN, these sources represent a unique cosmiclaboratory for investigating the physics of relativistic plasmas in extreme conditions. According to the unification schemes (Urry and Padovani, 1995), blazars come in two flavors: FSRQs (Flat Spectrum RadioQuasars), which show broad emission lines and powerful, highly collimated jets, and BL Lacs, which lack emission or absorptionfeatures and have less powerful jets. The parent population of jets oriented at larger viewing angles is formed by the radio galaxies,Fanaroff-Riley II and Fanaroff-Riley I respectively (Fanaroff and Riley, 1974).adio observations of active galactic nuclei with mm-VLBI 11
Fig. 4
Where do we stand?The physical conditions of the plasma forming a jet evolve significantly as the jet propagates fromthe central engine through the medium. According to the current paradigm, it is possible to identifyfour distinct regions (Fig. 5): launching, acceleration and collimation, kinetic-flux dominated, anddissipation.Jets are thought to be launched in the immediate vicinity of the supermassive black hole, atdistances of ≤ R S (Meier et al., 2001). In the launching region, the plasma is channeled throughthe action of strong magnetic fields, which extract part of the energy stored in the accretion disk(Blandford and Payne, 1982) and/or in the rotating black hole (Blandford and Znajek, 1977). Thecomposition of the loaded matter is not well determined: the jet may consist of a normal electron-proton plasma (e.g., Celotti and Fabian, 1993) or it may be formed by light particles only, electronsand positrons (e.g., Reynolds et al., 1996).This purely electromagnetic flow is then accelerated and collimated. In the second region, reach-ing up to scales of 10 − R S ( ∼ sub-parsec to parsec) (Vlahakis and K¨onigl, 2003, 2004), the initially broad stream is rapidly focused thanks to the confinement provided by the magnetic fieldand/or by the external medium, and is accelerated to relativistic speeds by magnetic pressure gra-dients (e.g., Spruit et al., 1997; Komissarov et al., 2007; Lyubarsky, 2009). At the end of thisprocess, a large part of the magnetic energy has been converted to kinetic energy. The kinetic-fluxdominated jet extends between 10 − R S (parsecs to kiloparsecs). In this region, the magneticfield is expected to become dynamically unimportant, and the jet can be appropriately describedby the physical laws of gas dynamics (Daly and Marscher, 1988). Being subject to hydrodynamicshocks (Blandford and K¨onigl, 1979; Marscher and Gear, 1985) and plasma instability (Blake,1972; Hardee, 1979), the flow ultimately looses its collimation and dissipates its energy in the formof radiation. Within the lobes formed at distances ≥ R S ( > kiloparsecs), compact hotspots areoften observed, suggesting that part of the plasma can still be highly relativistic on large scales(Georganopoulos and Kazanas, 2004).Over the past 40 years, radio interferometric observations have provided us with a detailed de-scription of the two outermost regions. In particular, VLBI studies at centimeter wavelengths havesignificantly improved our understanding of parsec-scale flows (also known as compact jets). Bothstatistical studies of large samples – first among all, those performed at 2cm within the MOJAVE program (Lister et al., 2009) – and detailed analyses of single targets were able to address severalkey topics. How fast does the plasma flow? What is the energy balance between the magnetic fieldand the particles? Is the magnetic field ordered? What are jets made of? And what is that we reallyobserve with VLBI? Providing a complete summary of these results is a challenging task, and be-yond the scope of this review. The interested reader is referred to the review articles from Zensus(1997) and Marscher (2006). In the following we discuss some highlights, with the aim of bettercontextualizing the scientific questions that are more relevant for mm-VLBI studies.Centimeter VLBI imaging has shown that a large fraction of the parsec-scale emission is pro-duced by the core component, the upstream feature from which the jet appears to emanate. Theradio core, characterized by a flat spectrum and weak polarization and usually located at parsec-distances from the black hole, is classically interpreted as marking the transition between syn-chrotron self-absorbed regions and optically thin regions (Blandford and K¨onigl, 1979; K¨onigl,1981). The flatness of the spectrum is explained as the result of the superposition of differentsynchrotron self-absorbed components in a conical geometry (see also Marscher, 1996). In thisscenario, the synchrotron opacity associated with the core component implies that its position isnot fixed, but has a frequency-dependence. Specifically, the emission should be detected at progres-sively smaller distances from the central engine with increasing observing frequency. This effect,known as core-shift , is observed in many jets (Marcaide and Shapiro, 1984; Lobanov, 1998), al-though not in all (Pushkarev et al., 2012).The emission from the jet is largely ascribed to the development of relativistic hydrodynamicshocks and plasma instability. The Kelvin-Helmoltz instability (Blandford and Pringle, 1976; Tur-land and Scheuer, 1976; Ferrari et al., 1978), formed in the presence of velocities shears betweentwo fluids, is likely to dominate the emission and shape the flow on hectoparsec and larger scales(Hardee, 2000; Lobanov and Zensus, 2001). Shocks, instead, are a distinctive feature of parsec-scale flows (Blandford and K¨onigl, 1979; Marscher and Gear, 1985). Indeed, most of the jets im-aged with VLBI do not appear as continuous flows, but can be well modeled as a sum of discretefeatures, known as blobs or knots . Shocks originate either from pressure mismatches at the bound-aries with the external medium or from major changes at the base of the flow (e.g., new plasmaejections or directional changes). They are sites of efficient particle acceleration through the Fermi Fig. 5
Schematic view of the main regions of a relativistic jet, according to the current paradigm for magnetically-driven flows. Theradial separation from the black hole is represented in a logarithmic scale, in units of Schwarzschild radii R S . We also report thecorresponding distance in units of parsecs (pc) for a black hole mass of 10 M (cid:12) . The extension of each region is approximate, and mayvary in different jets. VLBI observations at millimeter wavelengths are suited for probing the magnetically dominated jet base and thetransition to the kinetic-flux dominated region.4 B.Boccardi et al. mechanism (e.g., Bell, 1978) and of local amplification of the magnetic field. Accordingly, strongvariability of the emission and enhanced polarization can often be associated with shocked regionsin VLBI images of jets. Shocks are observed to move superluminally, with apparent speeds as highas ∼ c , but more commonly below 10 c (Lister et al., 2016).In addition to moving features, jets are also punctuated by bright stationary spots (e.g. Keller-mann et al., 2004; Lister et al., 2009; Agudo et al., 2012a). These have generally been interpreted asstanding shocks, formed, e.g., as a consequence of prominent recollimation events (e.g., Daly andMarscher, 1988; Gomez et al., 1995). Stationary features in the vicinity of the jet base are thoughtto play an important role for the production of non-thermal high-energy emission in AGN (e.g.,Le´on-Tavares et al., 2010; Arshakian et al., 2010), and may mark fundamental transition regionsof the jet. For instance, the presence of a recollimation shock may be expected at the end of theacceleration and collimation region, when the jet becomes causally disconnected from the centralengine (e.g., Polko et al., 2010; Cohen et al., 2014).In some objects, particularly in those not showing an appreciable core-shift, the VLBI core it-self may coincide with such a recollimation feature or, more generally, with the first and brighteststanding shock developing in the flow (Marscher, 2008, 2009). The nature of the radio core, stilldebated, is clearly a crucial element to understand, and much of the recent theoretical work hasbeen aimed at identifying the dissipative events which could account for the broadband emission,variability and polarization properties observed in its vicinity (e.g., Sikora et al., 2009; Sironi et al.,2015, and references therein). Radio observations indicate that, whichever mechanism is takingplace, it must lead to very efficient energy dissipation. This is inferred, for instance, from mea-surements of the brightness temperature of individual components in VLBI images, obtained afterestimating their flux density S ν and angular dimension d as: T b = . · S ν ( + z ) d ν . (6)At centimeter wavelengths, the intrinsic brightness temperature T = T b / δ of the core reaches(Kellermann et al., 1998; Homan et al., 2006) and sometimes exceeds (Kovalev et al., 2016) themaximum theoretical value expected for a synchrotron self-absorbed component. This limit isposed by the onset of the inverse Compton catastrophe , i.e., by the strong enhancement of theinverse Compton cooling in a synchrotron-emitting region whose temperature has approached athreshold of ∼ × K (Kellermann and Pauliny-Toth, 1969). High brightness temperatures areespecially measured during flares, and indicate that VLBI cores are strongly particle-dominated. Inthe jet, instead, a rapid temperature drop is observed, with values which are usually in very goodagreement with the equipartition limit of ∼ × K (Readhead, 1994), i.e., with the temperatureexpected when the energy of the radiating particles equals the energy stored in the magnetic field.The details of the processes taking place between the jet launching region, where the jet is apurely electromagnetic stream, and the matter-dominated cm-VLBI core are not known yet. De-cisive constraints can only come from direct observational probes of the relevant spatial scales,which is where mm-VLBI comes into play. AGN science with mm-VLBI
In the following part of this review we focus on the discussion of the main open questions concern-ing the physics of AGN jets that can be ideally addressed by the use of the mm-VLBI technique. adio observations of active galactic nuclei with mm-VLBI 15
The results presented are mainly from observations at 43 GHz and 86 GHz, but include some of thefirst findings at 230 GHz. At 43 GHz, an important contribution comes from the blazar monitoringprogram run by the Boston University (Jorstad and Marscher, 2016) with the VLBA.We start by describing the possible mechanisms giving rise to the high-energy emission andhow mm-VLBI observations can help constraining its location (Sect. 4.1). From this discussion,the importance of probing the internal structure of the flow and its morphological aspects willemerge, a topic which is expanded in Sect. 4.2. We will then address the broad subject of jetformation, investigated through polarization studies (Sect. 4.3) or through the direct imaging of thejet launching region (Sect. 4.4). We will conclude with a short overview of the first scientific resultsand future goals in the study of the compact radio source in our Galaxy, Sagittarius A*, with theEvent Horizon Telescope. High-energy emissionSince the launch of
CGRO/EGRET (Compton Gamma-Ray Observatory/Energetic Gamma-RayExperiment Telescope, Thompson et al., 1993),
AGILE (Astro-rivelatore Gamma a Immagini Leg-gero, Tavani et al., 2008), and especially of the
Fermi /LAT (Large Area Telescope, Atwood et al.,2009), blazars have been established as the most numerous class of γ -ray sources in the sky. To date, Fermi has identified over 1000 blazars (Acero et al., 2015), a fraction of which is also detected atTeV energies .The most frequently invoked process to account for the high-energy emission observed in AGNis the inverse Compton (IC) scattering of soft photons by the population of relativistic electronsforming the jet. In this scenario, the electrons are giving rise to both the low-frequency synchrotroncomponent and the high-frequency bump characterizing the spectral energy distribution (SED).In principle, the reservoir of soft photons can originate in various regions of the AGN. In thesynchrotron self-Compton mechanism, the same synchrotron photons emitted by the jet are up-scattered to higher energies (Maraschi et al., 1992). Concerning the “external” reservoirs, instead,several possibilities have been proposed. In order of increasing distance from the central engine,these include optical/UV photons from the accretion disk (Dermer and Schlickeiser, 1993) andfrom the broad line region (Sikora et al., 1994), infra-red emission from the torus (Bła˙zejowskiet al., 2000), and, on larger scales, CMB photons (e.g., Celotti et al., 2001). Alternatively, ifultra-relativistic protons are also present in the jet, the γ -ray emission may result from proton-synchrotron or from p γ photopion production (Mannheim and Biermann, 1992; Aharonian, 2000).Especially in hadronic models, but also in the leptonic ones, extremely efficient acceleration mech-anisms are required and must be at play in AGN jets.Statistical studies of large samples (Ackermann et al., 2011; Fuhrmann et al., 2014; Ramakrish-nan et al., 2015) have inferred the existence of significant correlations between the broad-band (0 . < E <
300 GeV)
Fermi light-curves and the radio ones obtained from single-dish monitoringprograms at centimeter and millimeter wavelengths (see e.g., the
F-GAMMA legacy Program ),suggesting a common origin of the emission in the two bands. In the aforementioned studies, theradio variability is usually found to be delayed with respect to the high frequencies. Similar con-clusions were reached through a VLBI analysis of the MOJAVE sample (Pushkarev et al., 2010) showing, in addition, that the radio- γ correlations are highly significant when considering the radioproperties of the core, rather than those of the jet.From a theoretical standpoint, the observed behavior may be conveniently explained if the ac-tivity – triggered, for instance, by a prominent change in particle density of the plasma – arisesin a synchrotron self-absorbed region located in the innermost surroundings of the central engine.Here, highly relativistic electrons in the jet can up-scatter the optical/UV photons from the broad-line region to γ -ray energies. At energies above 1 GeV, this dense photon-field is also expectedto be highly opaque to the emerging γ -rays through the reaction γγ → e ± (see, e.g., Ghiselliniand Tavecchio, 2009), which could account for the GeV spectral break observed in several blazars(Poutanen and Stern, 2010). Moreover, one of the main arguments in favor of the “close dissi-pation” scenario is given by variability timescales as short as few hours, or even minutes (e.g.,Aharonian et al., 2007; Ackermann et al., 2016), which constrain the size of the emitting regionsinvolved to be highly compact and have led to the conclusion that γ -rays must originate within 10 cm, i.e., << EGRET -detectedblazars, Valtaoja and Terasranta (1995) were among the first to propose that, especially in quasars,the γ -ray flares are likely to originate in the millimeter-wave emitting regions (see also Valtaojaand Teraesranta, 1996; L¨ahteenm¨aki and Valtaoja, 2003; Le´on-Tavares et al., 2011) rather than inthe vicinity of the broad line region. Thanks to its ability to detect detailed structural changes inthe flow, mm-VLBI has now provided compelling evidence that, in fact, the high-energy eventsand the millimeter-wave emission are often co-spatial, corroborating physical scenarios where the γ -ray emission is produced at parsec distances from the central engine. Some of the mm-VLBIstudies supporting this idea, as well as findings possibly requiring alternative interpretations, aresummarized in the following.Already before the launch of Fermi , a systematic VLBI study of a large sample of γ -ray blazarsdetected by EGRET was conducted at frequencies up to 43 GHz by Jorstad et al. (2001). In alarge number of sources, a clear connection is observed between the occurrence of a γ -ray flareand the ejection of a new superluminal component. The γ -ray flare is found, on average, to followthe ejection, and to nearly coincide with a local maximum in the polarized radio flux density. Byinterpreting the newly ejected components as traveling shocks, this delay could coincide with thetime required for the shock to fully develop and for the electrons to be efficiently accelerated at itsforward layer.More recent VLBI monitoring campaigns of single objects of 43 GHz have confirmed that, ingeneral, a tight relation exists between the γ -ray emission and the properties of the mm-VLBIcore region. In sources like 1156+295 (Ramakrishnan et al., 2014), PKS 1510 −
089 (Aleksi´c et al.,2014) and 0954+658 (Morozova et al., 2014), the γ -ray outbursts are found to be triggered bythe passage of new superluminal components through the mm-VLBI core. In the case of 3C 120(Casadio et al., 2015a) and CTA 102 (Casadio et al., 2015b), this trend is confirmed, but only whenthe newly ejected features are traveling in a direction closer to the observer’s line of sight. Indeed,a close connection between the jet orientation and the occurrence of γ -ray flares is evident at astatistical level in the MOJAVE survey (Pushkarev et al., 2009), since the class of γ -ray loud jets is adio observations of active galactic nuclei with mm-VLBI 17 found to be oriented at smaller viewing angles with respect to the general radio-loud population.Flaring events triggered by the ejection of new components are often accompanied by increasedactivity in the optical and X-rays, and by systematic rotations of the optical electric vector positionangle (e.g., Marscher et al., 2008; Jorstad et al., 2010; Marscher et al., 2010). F l u x ( x - c m - s - ) γ -rays ≥
100 MeV
Fermi 0 1 2 2007 2008 2009 2010 2011 2012 2013 2014 2015
86 GHz
F-GAMMA 86 GHz86 GHz VLBI coreC1C2C3 0 1 2 3 4 F l u x den s i t y ( Jy )
43 GHz
F-GAMMA 43 GHz43 GHz VLBI coreC1C2C3 0 1 2 3 4 5
Time (MJD)
15 GHz
F-GAMMA 15 GHz15 GHz VLBI coreC1C2 0 1 2 354000 54500 55000 55500 56000 56500 57000
Fig. 6
Light curves of the blazar PKS 1502+106, which showed a very prominent γ -ray flare in 2009 (Karamanavis et al., 2016a).From top to bottom: 1) monthly binned Fermi /LAT γ -ray light curve at energies E >
100 MeV; 2) F-GAMMA single-dish radio lightcurve at 86 GHz, core and component light curves from the VLBI flux density decomposition; 3) same as above, at 43 GHz; 4) sameas above, at 15 GHz. A newly ejected VLBI knot, labeled C3, is visible at 43 GHz and 86 GHz. Its ejection time, designated by the redsolid line and estimated based on the VLBI kinematic analysis, follows the onset of the γ -ray flare. The latter originates ∼ In several cases, the mm-core where the high-energy emission is produced has been identifiedas a standing shock (e.g., D’Arcangelo et al., 2007; Marscher et al., 2008, 2010), possibly markingthe end of the acceleration and collimation zone. Standing shocks playing a major role in the high-energy flaring events have been pinpointed with mm-VLBI also downstream in the jet. In OJ 287,a secondary standing shock at a distance of 14 parsecs from the innermost stationary feature wasproposed to give rise to a prominent γ -ray flare as it was crossed by a turbulent moving blob(Agudo et al., 2011). In 3C 345, Schinzel et al. (2012) showed that γ -rays may be produced atmultiple locations over an extended region of 23 parsecs, including the proximity of a stationaryfeature observed ∼
10 parsecs away from the core.While the physical processes triggering the activity may vary from flare to flare, all of the aboveresults point toward a “far dissipation” scenario. Although, as we discussed in Sect. 3, observing atmillimeter wavelengths enables us to unveil emission regions closer to the jet base, the millimetercore is still located at parsec distances from the black hole, at least in the brightest blazars (e.g.,Sikora et al., 2008; Fromm et al., 2015). The inferred co-spatiality of the low and high-energyemission implies that efficient particle acceleration takes place far beyond the broad-line region.At the distances determined by mm-VLBI studies, the most likely mechanisms giving rise to thehigh-energy emission are then the synchrotron self-Compton and the external inverse Compton ofinfra-red photons from the torus.This conclusion may still be valid when the γ -ray activity is found to precede the variations inthe radio band (see, e.g., Rani et al., 2014; Karamanavis et al., 2016a; Lisakov et al., 2017). Forinstance, through a dedicated VLBI campaign at 43 GHz and 86 GHz of the high-redshift blazarPKS 1502+106, Karamanavis et al. (2016a) determined that the high-energy emission originatesfew parsecs upstream of the mm-core (Fig. 6). With the combined effort of single-dish and VLBIstudies, an absolute distance of ∼ . γ -ray activity withrespect to the black hole (Karamanavis et al., 2016b). Since the broad line region is expected toextend on much smaller scales in this object ( ∼ . γ -ray flares, with no structural change on parsecscales, have been frequently observed, e.g., in Mrk 421 (Lico et al., 2014) and CTA 102 (Casadioet al., 2015b). In the misaligned object 3C 84, the variability in the radio and at the very high γ -rayenergies ( E >
10 GeV) appears to be uncorrelated (Suzuki et al., 2012; Nagai et al., 2012). Par-ticularly for the TeV BL Lacs and for radio galaxies, whose Doppler factors inferred from VLBIkinematic analyses are found to be too low to account for the observed energetic processes, a sim-ple “one-zone model” for the high-energy production may be not appropriate. For these objects itwas suggested (Ghisellini et al., 2005) that the inverse Compton emission can be enhanced if the jetis structured transversely, e.g., it is formed by a fast central spine and a slower outer sheath. Underthese conditions, the electrons of one component can up-scatter the beamed photons of the other, sothat the low and the high-energy emissions do not necessarily originate in the same region, and canshow uncorrelated variability. A similar mechanism was theorized by Georganopoulos and Kazanas(2003) for decelerating jets, i.e., those characterized by a radial velocity gradient. 43 GHz VLBIobservations of 3C 84 have revealed that this jet is in fact characterized by a transverse structure ofthe spine-sheath kind (Fig. 7, Nagai et al., 2014). The uncorrelated radio- γ variability, the observedtransverse stratification, and modeling of the SED favor a “two-zone model” for the high-energyproduction in this source (Tavecchio and Ghisellini, 2014). adio observations of active galactic nuclei with mm-VLBI 19 Transversely stratified jets in BL Lacs have been proposed (Tavecchio et al., 2014) as possiblesources of the neutrinos detected by IceCube in the (0.1-1) PeV range (Aartsen et al., 2013). Thesedetections have opened new frontiers for the study of the high-energy emission in AGN, sinceseveral recent studies have identified a number of blazars as possible counterparts of the IceCubesignals (e.g., Kadler et al., 2016; Padovani et al., 2016). Future mm-VLBI analysis of the structuralcharacteristics of these candidates may provide further clues for the correctness of the proposedassociations.
Internal structure and morphologyAs evident from the previous section, probing the morphology and the internal structure of theinnermost regions of AGN jets is essential for truly understanding the physical processes occurringin the plasma flow. Analyzing the jet transverse structure is not only important for the correctmodeling of the high-energy emission, as explained, but also for probing the development of plasmainstabilities, which can affect crucially the jet propagation on larger scales.In order to obtain meaningful insights into the internal jet structure, it is necessary to resolve theflow in the transverse direction. This condition is usually not met in centimeter VLBI observations,especially in the most compact regions at the jet base. Interferometric observations with enhancedresolution, like space or mm-VLBI, are instead adequate for achieving such a goal in several nearbyobjects.A powerful tool for testing the development of instabilities is the analysis of the jet ridge line,i.e., of the evolution of the location of the peak of emission in the transverse direction as a functionof distance from the core. In the case of the strong jet in S5 0836+710 (Perucho et al., 2012), VLBIobservations up to 43 GHz have shown that the ridge line is likely tracing a pattern of helicallytwisted pressure maxima due to the formation of Kelvin–Helmoltz instabilities. A similar conclu-sion was reached in the analysis of 3C 273 through space VLBI observations at 5 GHz (Lobanovand Zensus, 2001). In this case, a double helical ridge line is observed, the jet being bright at itslimbs and dimmer close to the central axis.The class formed by the so-called “structured jets”, i.e., those characterized by a transversegradient in the intensity, in the velocity, or in the polarization properties (see Sect. 4.3), is gettingmore and more numerous as the resolution of the images improves. Several limb-brightened jetscould recently be imaged through mm-VLBI, e.g., in 3C 84 (Nagai et al., 2014, Fig. 7), in Mrk 501(Giroletti et al., 2008), in Mrk 421 (Piner et al., 2010), in M 87 (Junor et al., 1999; Walker et al.,2016; Mertens et al., 2016), and in Cygnus A (Boccardi et al., 2016b,a). In the latter source, thestratification is visible not only in the approaching jet, but also in the counter-jet, and a similarfeature is hinted in the weak counter-jet of M 87. The limb-brightening characterizing these jetscould reflect the presence of instability patterns, as for 3C 273, or it may be a direct result of thejet formation mechanism. This second scenario appears especially plausible for M 87, where thedouble ridge line is visible already at few tens of Schwarzschild radii from the central engine. Theintensity gradient can, in this case, be explained in two ways: 1) the jet emissivity is intrinsicallylower closer to the jet axis or 2) the jet emissivity is apparently lower close to the jet axis due tothe existence of a spine-sheath velocity gradient. In fact, limb-brightening will naturally arise insufficiently misaligned jets due to the differential boosting of each filament or to the de-boosting ofthe fast central spine. A spine-sheath velocity structure has been long proposed for relativistic jets(Sol et al., 1989), and is compatible with findings from recent kinematic studies in Cygnus A (Boc-cardi et al., 2016b) and M 87 (Mertens et al., 2016). In particular, Mertens et al. (2016) were able to
200 400 600 M illi A r c sec ond s MilliArc seconds1.5 1.0 0.5 0.0 -0.5 -1.0 -1.50.50.0-0.5-1.0-1.5-2.0-2.5-3.0
Fig. 7
The limb-brightened jet structure of the peculiar radio galaxy 3C 84 as revealed by VLBI observations at 43 GHz. (Imagereproduced with permission from Nagai et al. (2014), copyright by AAS) z o b s ( m a s ) − . − . . . . . . . R e l a t i v e D EC ( m a s ) − − − − − − Relative RA (mas) mas ≈ . pc ≈ R s − − − − Jy / beam
Fig. 8
Velocity structure of the jet in M 87 derived from the analysis of a multiepoch VLBI data set at 43 GHz. Motion is detected alongthree main filaments, two outer limbs and a central body. Two overlapping velocity components can be identified: a mildly relativisticone and a faster streamline with a Lorentz factor of 2.5. (Image reproduced with permission from Mertens et al. (2016) copyright byESO)adio observations of active galactic nuclei with mm-VLBI 21 obtain a detailed 2-D velocity field in the inner regions of M 87, clearly showing the displacementof features along three main filaments of the flow, two outer limbs and a central body (Fig. 8).Two speed components, which appear to overlap across the jet, are detected: a mildly relativisticspeed likely associated with an outer disk wind or with an instability pattern and a faster streamlinefeaturing a Lorentz factor of ∼ . Polarization and magnetic field topologyPolarimetric studies can be a powerful tool for deriving fundamental constraints on the jet physics,as well as on the properties of the jet environment. The interpretation of polarization data requires,however, particular caution. The likely co-existence of relativistic, projection and other effectsmakes it challenging to reconstruct the intrinsic orientation and strength of the magnetic fields (seee.g., Lyutikov et al., 2005). Moreover, the very nature of these fields is still highly debated. In par-ticular, it is unclear whether the observed polarization has to be ascribed to the local ordering, e.g.,due to compression in a shock, of an otherwise random ambient magnetic field, or to the presenceof a large-scale, ordered field permeating the plasma flow. While a certain degree of ambiguity islikely inevitable when investigating this subject, we discuss here how VLBI observations at mil-limeter wavelengths enable many of the uncertainties to be reduced.From synchrotron theory, the predicted fractional linear polarization for an ensemble of rela-tivistic electrons moving in a uniform magnetic field is 70%–75%, in the case of optically thinemission, and 10%–12% for optically thick emission (Pacholczyk, 1970). However, both single-dish radio surveys (e.g., Aller et al., 1985, 1992) and VLBI studies of large samples (e.g., Pollack et al., 2003; Lister and Homan, 2005; Helmboldt et al., 2007) have inferred much lower degreesthan expected on this theoretical basis. Typical values measured in the proximity of the VLBI coreare below 5%, while there is a trend of increasing fractional polarization, up to tens of percent, atlarger distances from the jet base (e.g., Lister and Homan, 2005).These results have often been interpreted as evidence for the random nature of the magneticfield (see Hughes, 2005, and references therein). Under the assumption that large part of the jetemission originates in shocked regions, polarization degrees of 20–30%, or higher, can be explainedin the framework of the shock-in-jet model (Marscher and Gear, 1985; Hughes et al., 1985), as aconsequence of the compression and amplification of the component of the magnetic field orientedparallel to the shock front. In the case of a transverse shock, the electric vector position angle(EVPA) is then expected to be oriented in the direction parallel to the jet axis, a configuration whichis frequently found to characterize the compact jet knots in polarimetric VLBI images, especiallyin BL Lac objects (Gabuzda et al., 2000). Enhanced polarization can also result from the formationof oblique and conical shocks (Cawthorne and Cobb, 1990, and references therein) or from theshear of the magnetic field lines at the boundaries between the jet and the ambient medium (Laing,1980).Although a largely random field can be adequate for reproducing the observed properties ofAGN jets, its assumption poses some difficulties for theoretical models describing the launching,acceleration, and collimation of the flow. In fact, most of these models require the existence ofan ordered, large scale field at the base of the jet (e.g., Spruit, 2010, and references therein). Ifthis prediction is correct, the aforementioned observational results can then be explained in twoways. A first possibility is that the field gets disrupted and tangled, e.g., owing to magnetic in-stabilities (Giannios and Spruit, 2006), already before the jet becomes visible at the VLBI core.This scenario is plausible especially in the case of the most highly boosted blazars, where, as wesaid, the mm-VLBI core appears to be located quite far from the central engine, at a distance of10 − Schwarzschild radii (e.g., Marscher et al., 2008; Fromm et al., 2015). Alternatively, alarge scale field may be preserved on VLBI scales, and the radio emission may be intrinsicallyhighly polarized, but a strong depolarization occurs as the radiation propagates to the observer.In general, depolarization results as a consequence of the integration of polarized emission frommultiple emission regions and/or along the line of sight. In relativistic jets, a decrease of the netpolarization could be observed in the following cases: – Line of sight integration through opaque material
Assuming that the VLBI core actually marks, at a given frequency, the transition region to theoptically thin regime, then at least part of its emission is expected to be opaque. The line ofsight integration through a partially optically thick material could be responsible for the low netpolarization observed at the core location. This effect may apply especially to sources whichare seen at a very small viewing angles. – Beam depolarization
VLBI observations are characterized by a finite resolution. If the observing beam is not negli-gible when compared to the size of the coherent polarization structure of the source, depolar-ization may result from the averaging of polarized emission inside the interferometer beam. – Faraday depolarization
In addition to the previous effects, further depolarization could arise as a consequence of Fara-day rotation (Burn, 1966), i.e., the rotation of the polarization plane of an electromagnetic wavewhile it propagates through a magnetized plasma. The rotation measure RM quantifies, at agiven wavelength λ , the difference between the intrinsic polarization angle χ and the observed adio observations of active galactic nuclei with mm-VLBI 23 one χ . In the simplest case of a homogeneous medium, RM is proportional to the integral overthe line of sight from the source to the observer of the density of charges in the medium n e timesthe component of the magnetic field along the line of sight B (cid:107) : χ = RM λ + χ , RM ∝ (cid:90) n e B (cid:107) dl . (7)The presence of Faraday screens can cause a reduction of the total polarization whenever theemission experiences differential rotations while propagating to the observer. This can happeneither when the Faraday rotating material is mixed with the emitting plasma (internal depolar-ization) or when the radiation crosses external inhomogeneous media (external depolarization).It is important to note that the strongest rotations are expected to be induced by thermal elec-trons, whose presence is likely abundant in the jet environment. Dense thermal media whichcan potentially act as Faraday screens include both orbiting clouds in the broad- or narrow-lineregion and non-relativistic material embedding the relativistic jet beam (disk winds). Multi-frequency cm-VLBI studies have indeed inferred the presence of such media from the highrotation measures (up to thousands of rad / m ) detected in several blazars (e.g., Taylor, 1998;Zavala and Taylor, 2004; Hovatta et al., 2012). The effect appears to be especially strong atshorter distances from the central engine, indicating a decreasing strenght of the magnetic fieldwith distance and/or a decreasing density of the thermal electrons. The latter is in agreementwith the expectations from the unified scheme for AGN that most of the dense gas surroundingthe accretion disk is concentrated in the central parsecs.Ultimately, while a large scale field may still permeate the plasma flow on small VLBI scales,these very scales are also the most heavily obscured, synchrotron self-absorbed and unresolved.VLBI at millimeter wavelengths is then the optimal technique when aiming at revealing the intrinsicstrength and orientation of the magnetic fields in the jet. Due to its dependence on the square ofthe wavelength (eq. 7), Faraday rotation is expected to be diminished, and so is the depolarizationarising from it. Moreover, being able to penetrate the jet base closer to the central engine, mm-VLBI increases the possibility to image – with enhanced resolution – those regions potentiallycharacterized by dynamically important magnetic fields.A clear example of the potential of mm-VLBI observations is provided, in this context, by thestudy of BL Lac presented by Marscher et al. (2008). At 43 GHz, polarized emission could bedetected from a region upstream of the VLBI core (Fig. 9). The disturbance injected at the jet basepropagates downstream, causing a double flare at optical, X-rays and TeV energies. The secondflare is also observed in radio when the disturbance crosses the VLBI core, which is interpretedas a standing shock. The optical and radio polarization properties suggest the upstream region tobe part of the acceleration and collimation zone of this jet. In particular, the smooth rotation ofthe optical polarization vector by ∼ ◦ observed over few days and its final alignment with theradio polarization vector, parallel to the jet axis, support the presence of a large scale magneticfield with helical geometry which then becomes mostly turbulent close to the standing shock. Inthis scenario, consistent with the findings from RadioAstron at 22 GHz (G´omez et al., 2016) andsimilarly suggested for PKS 1510 −
089 (Marscher et al., 2010), the VLBI core is the location wherethe plasma has attained its terminal speed, and possibly equipartition of the magnetic and particleenergy densities.In the picture proposed by Marscher et al. (2008), the helical structure of the magnetic field isalready disrupted at the mm-VLBI core, located at a distance of ∼ R S from the jet apex. Severalother authors, however, have suggested that the helical geometry may be preserved on much larger Fig. 9
VLBI polarimetric images of BL Lac at 43 GHz. A feature emerging in the region upstream of the core is visible in the first threemaps. The optical and radio polarization properties suggest this region to be part of the jet acceleration and collimation zone. (Imagereproduced with permission from Marscher et al. (2008), copyright by NPG) scales. The existence of a parsec-scale helical field has been proposed for NRAO 150 (Molinaet al., 2014) for explaining the high speed rotation of the emission regions. In the polarimetricanalysis of this object performed up to 86 GHz frequencies, a toroidal magnetic field is observed,although the polarization degree is quite low due to the almost perfectly face-on orientation ofthis jet. In Zamaninasab et al. (2013), the nature of a broad arc-like feature in the inner regionsof 3C 454.3 was investigated through multi-frequency, polarimetric VLBI imaging between 5 GHzand 86 GHz. Transverse gradients across the arc, which is highly polarized, are observed both in thepolarization degree and in the apparent magnetic vector position angle, which varies by more than90 ◦ . Moreover, the rotation measure shows a sign reversal across the feature. These properties canbe best reproduced by assuming the presence of a large-scale helical field. If confirmed, this resulthas the remarkable implication that an ordered field exists at distance of ∼ R S , i.e., much beyondthe expected extension of the acceleration and collimation region. Transverse rotation measuregradients have been observed in several other objects on parsec scales (e.g., Asada et al., 2002;Gabuzda et al., 2004; O’Sullivan and Gabuzda, 2009), and often interpreted as a characteristicsignature of helical fields. The analysis of RM transverse gradients can provide important insightsfor the investigation of this subject. However, as pointed out by Taylor and Zavala (2010), spuriousresults can arise as a consequence of limited transverse resolution in the VLBI images. Therefore,it will be important in the future to verify the aforementioned results through VLBI studies at thehighest possible resolution.Polarimetric imaging at frequencies higher than 43 GHz has been limited in the past due to thenon-standard calibration procedures required. Over the past few years, a pipeline for the calibra-tion of full-polarization, global VLBI data at 86 GHz has been developed, and the feasibility of thetechnique has been demonstrated for the case of the blazar 3C 345 (Mart´ı-Vidal et al., 2012). Re-cently, a polarimetric image of M 87 has also been produced at this frequency (Hada et al., 2016),revealing the presence of a highly polarized feature in the innermost regions of the jet. Concern-ing the earlier experiments, 86 GHz polarization maps had been obtained for mainly three objects:3C 120 (G´omez et al., 1999), 3C 273 (Attridge, 2001; Attridge et al., 2005), and 3C 279 (Attridge, adio observations of active galactic nuclei with mm-VLBI 25 − in the case of 3C 273, are inferred. While, on the one hand, several alternative explanations existsfor the low polarization of VLBI cores, such high rotation measures are not incompatible with thetremendous opacities that can be expected at the jet base. Through recent ALMA observations ofthe high-redshift blazar PKS 1330 −
211 at frequencies up to 300 GHz (1 THz in the source frame),rotation measures of the order or ∼ rad m − have been derived (Mart´ı-Vidal et al., 2015). Thesevalues are consistent with the presence of dynamically important magnetic fields at the jet base,with strengths of tens of Gauss and potentially much higher. The importance of this and othersimilar results is further commented in the following section. Launching, acceleration and collimationVLBI studies at centimeter wavelengths, as well as radio interferometric analyses probing largerscales, have collected substantial observational evidence of one basic fact: jets are self-similar overmany orders of magnitude in length. This self-similarity is expected to break close to the blackhole, where the jet should be dominated by the magnetic energy driving it. Among the proposedmechanisms for jet formation, many of which involve a purely hydrodynamical launching (Bland-ford and Rees, 1974), the magnetic launching model (see e.g., Meier et al., 2001) appears as themost viable option according to numerical simulations (e.g., Tchekhovskoy, 2015, and referencestherein), and is currently largely favored. In this model the jet properties scale trivially with theblack hole mass, conveniently explaining the similarities between galactic and extragalactic jets.Although there is a general agreement on the basic principles of the magnetic launching mech-anism, reproducing the extreme properties observed in relativistic jets, such as opening anglessmaller than 1 degree and bulk Lorentz factors as high as 50, is still among the most challengingtasks for theorists. This is firstly due to the inherent complexity of plasma physics in the immedi-ate surrounding of super-massive black holes, whose description requires a full GRMHD (GeneralRelativistic Magneto-HydroDynamic) formalism. In the past 20 years, important steps forwardhave been made by implementing three-dimensional GRMHD numerical simulations (Koide et al.,1999; McKinney, 2006). These have shown that AGN jets can be efficiently powered by the rota-tional energy of the compact central object, extracted through strong, large scale magnetic fields.The power source can either be the accretion disk, as described in the work of (Blandford andPayne, 1982), or a spinning super-massive black hole, as originally proposed by Blandford andZnajek (1977).At their very base, such magnetically-driven jets are likely to propagate in the form of purePoynting flux, which then gets gradually converted into kinetic flux and, for a small percentage,into radiation. Based on theoretical models (Vlahakis and K¨onigl, 2004; Lyubarsky, 2009) andnumerical simulations (McKinney, 2006; Komissarov et al., 2007), the predicted scales for themechanisms of acceleration and collimation to take place span an interval between few and 10 − Schwarzschild radii, corresponding to sub-parsec or parsec scales (see Fig. 5). In this region,the magnetic field is expected to be dynamically important and well ordered, with a geometrygradually evolving from purely poloidal to helical, or possibly to purely toroidal.
In the recent years, mm-VLBI observations have been able to probe these previously unexploredscales following different approaches. In addition to the analysis of the polarization properties,discussed in the previous section, important observational tests could be provided either throughstatistical studies in survey experiments or through a direct and detailed imaging of single, optimaltargets.The most recent surveys conducted at 86 GHz (Lobanov et al., 2000; Lee et al., 2008; Gopalakr-ishnan Nair et al., 2016) enabled to model the observed distribution of brightness temperature, andto investigate its dependence as a function of distance from the central engine. A significant trendhas emerged, indicating that the brightness temperature measured at the location of the 86 GHzcore, peaking at T b ∼ K (Lee et al., 2008), is lower than the one measured at lower frequen-cies. This behavior can be best explained by assuming that plasma acceleration is still taking placein the region between the VLBI cores (Lee et al., 2016), a result which confirms the theoreticalprediction that the acceleration zone extends on parsec scales. These findings, together with manyof the previously discussed polarization and variability studies (e.g. Marscher et al., 2008, 2010),collocate the mm-core in a crucial position: in blazars the mm-core marks the transition regionbetween the magnetic-dominated and the kinetic-dominated regimes.One implication of this scenario is that, in most sources, the bulk of the MHD processes drivingthe outflow may occur in the “invisible” jet, comprised between the supermassive black hole andthe mm-core. There exists, however, a limited sample of objects where a high resolution imagingof the true jet base can be obtained and MHD models for jet formation can be tested in detail.The list of suitable targets mainly includes nearby misaligned jets (Table 2) where, for reasonswhich are not fully clarified (Marscher, 2011) but are most likely related to the reduced impact ofprojection and relativistic effects, the VLBI core appears to be located much closer to the centralblack hole than in blazars. For instance, phase-referencing VLBI observations of M 87 (Hada et al.,2011) have determined a shift of only 14 − R S of the 43 GHz core with respect to the centralengine, while an upper limit of 100 R S could be inferred for NGC 1052 at 86 GHz (Baczko et al.,2016). Table 2 indicates that the resolution provided by VLBI at 86 GHz is sufficient for probingthe acceleration and collimation region in all of the listed objects, while the mechanisms involved inthe jet launching, expected to take place on scales < R S (Sect. 3), can be currently best studiedin M 87. The latter is the optimal target in terms of spatial resolution, although the intermediateorientation of its jet ( θ ∼ ◦ , Mertens et al., 2016) introduces larger uncertainties on the intrinsicparameters than in true radio galaxies like Cygnus A ( θ ∼ ◦ , Boccardi et al., 2016b) or NGC 1052(64 ◦ < θ < ◦ , Baczko et al., 2016).In M 87, the jet formation region has been investigated in numerous studies. The collimationzone is well resolved already at 43 GHz; on sub-parsec scales, the limb-brightened flow appearsmuch broader than on parsec scales, expanding with an opening angle of ∼ ◦ (Junor et al., 1999;Ly et al., 2004). Deeper imaging at 86 GHz (Hada et al., 2016) reveals an even broader jet base withapparent opening angle of ∼ ◦ , indicating that the jet starts poorly collimated and gets rapidlyfocused farther downstream. These findings are confirmed by the analysis of a more recent 86 GHzimage, obtained after stacking five different maps (Fig. 10, Kim et al., 2016). The stacking methodis especially effective when aiming at recovering the full jet cross-section and at investigatingwith better fidelity the jet expansion. Both studies performed at 86 GHz show that the jet has aparabolic shape on scales of ∼
100 R S . Using a rich, multi- frequency VLBI data set, Asada andNakamura (2012) inferred that this shape is preserved also on larger scales, up to approximately ∼ R S . Beyond that, however, the flow appears to be freely expanding, i.e., it has a conicalshape. According to theory, acceleration through magnetohydrodynamic processes is inefficient in adio observations of active galactic nuclei with mm-VLBI 27 Table 2
List of nearby, misaligned objects which are well suited for mm-VLBI observations aimed at imaging the jet formation region.Col. 1: Source name. Col. 2: Luminosity distance ∗ D L . Col. 3: Logarithm of the black hole mass M BH , given in units of solar massesM (cid:12) . Ref: a) Gebhardt et al. (2011) - b) Tadhunter et al. (2003) - c) Scharw¨achter et al. (2013) - d) Woo and Urry (2002) - e) Neumayer(2010). Col. 4: Linear size in units of Schwarzschild radii probed by VLBI at 86 GHz. The objects are listed in order of decreasingspatial resolution. Source D L [ Mpc ] Log ( M BH [ M (cid:12) ]) Linear size of 50 µ as [ R S ]M 87 19 9.8 a b c d e ∗ Assuming a Λ CDM cosmology with H = 69.6 h − km s − Mpc − , Ω M = 0.286, Ω Λ = 0.714. conical flows, and can extend on large scales only if the plasma undergoes differential collimation(see e.g., Komissarov, 2012). The observed transition from a parabolic to a conical shape may thensignal a crucial change in the physical conditions of the plasma, marking the termination of theacceleration zone. Interestingly, the transition occurs in the vicinity of the Bondi radius (wherea change in the ambient pressure gradient is expected) and of a stationary feature in the HST-1complex, a bright region where superluminal speeds of 4 c − c have been measured (Biretta et al.,1999). Then, a natural question to ask is: does the flow show acceleration in the parabolic regioncomprised between its base and HST-1? Previous kinematic studies (Biretta et al., 1995; Kovalevet al., 2007; Ly et al., 2007; Asada et al., 2014) have yielded contrasting results, likely due to thepoor sampling and to the intrinsic difficulty in identifying moving features in a jet that lacks welldefined spots, and is transversely stratified. The method recently employed by Mertens et al. (2016)enabled a detailed two-dimensional kinematic structure to be inferred, and therefore appears as themost appropriate in the case of M 87 (Fig. 8). In this work, accelerating features are detected onscales from 10 to 10 R S ; the speeds, which are mildly relativistic, increase faster up to distancesof 10 R S , while a milder gradient is observed further downstream. These properties could be wellinterpreted in the framework of MHD acceleration and collimation: the bulk of the accelerationoccurs within thousands of Schwarzschild radii, and is followed by a regime of saturation of thePoynting flux conversion which may extend up to HST-1.The acceleration and collimation region could also be imaged in Cygnus A at 43 GHz and86 GHz (Boccardi et al., 2016b,a). A multi-scale representation of this prototypical source is shownin Fig. 11. The observed kinematic properties in this powerful radio galaxy are quite similar to thoseinferred for M 87: the flow is transversely stratified and mildly relativistic (1 < Γ < . ∼ − R S , and shows acceleration with two distinct regimes,a fast regime (with Lorentz factor increasing as fast as the jet radius) up to ∼ R S and a slowregime up to ∼ R S . At 86 GHz, the two-sided jet could be resolved transversely also close tothe apex, revealing a very wide jet base with a minimum width of ( ± ) R S . This value ismuch larger than the radius of the innermost stable circular orbit (ISCO) in the accretion disk(1 < R ISCO < R S ), implying that at least part of the flow must be anchored at large disk radii inthe accretion disk.In this respect, Cygnus A differs from M 87. Pilot observations of M 87 performed at 1 mmwith the Event Horizon Telescope (EHT) (Doeleman et al., 2012) could determine an upper limit Fig. 10
VLBI observations of M 87 at 86 GHz. The image was created after stacking 5 VLBI images taken during 2004-2015. (Imagereproduced with permission from Kim et al. (2016), copyright by the authors) for the transverse size of the jet apex of only 5.5 R S , compatible with later findings by Krichbaumet al. (2014). Unlike in Cygnus A, the initial jet width in M 87 is comparable with the radius of theinnermost stable circular orbit (ISCO), suggesting that the jet is launched from the inner regionsof the disk or from the ergosphere. Very similar conclusions were reached by Mertens et al. (2016)through the analysis of the jet rotation at 43 GHz.All of the aforementioned results agree well with the theoretical predictions for magneticallydominated jets, supporting the currently most favored paradigm. Additional observational groundsfavoring the magnetic launching scenario come from estimates of the magnetic field strength at thejet apex. In a recent analysis of a large number of sources from the MOJAVE survey (Zamaninasabet al., 2014), the nuclear magnetic field strength has been derived from the jet radio luminosity.The inferred forces are extremely high, and comparable with the black hole’s gravitational pull.Magnetic field strengths of the order of ∼ G or more were also derived in GMVA studies ofOJ 287 (Hodgson et al., 2017) and NGC 1052 (Baczko et al., 2016), and may be compatible withthe already discussed ALMA results presented by Mart´ı-Vidal et al. (2015) for PKS 1330 − Magnetically Arrested Disks (MADs), i.e., disks featuring dynami-cally important magnetic fields, was theorized by Narayan et al. (2003). The adjective “arrested”refers to the accretion, which can be suppressed if the magnetic pressure gets strong enough, in theinnermost regions of the disk.Numerical simulations (Tchekhovskoy et al., 2011) confirm that MAD systems can producestrong jets, with powers up to 140% of the accreted rest-mass energy in the case of a maximally adio observations of active galactic nuclei with mm-VLBI 29
Fig. 11
The radio galaxy Cygnus A on scales from hundreds of kilo-parsecs imaged with the Very Large Array (Perley et al., 1984) tothe sub-parsec probed with mm-VLBI (Boccardi et al., 2016b,a). The VLBI images are created after stacking several epochs. Data at 2cm are from the
MOJAVE survey (Lister et al., 2009).0 B.Boccardi et al. spinning black hole. Simply speaking, this means that “more than what comes in goes out”, andthis surplus must be at the expenses of the black hole’s rotational energy. This theoretical predictionagrees very well with another recent observational study of a large sample of blazars (Ghiselliniet al., 2014), showing that the jet luminosity is correlated with the disk luminosity but at the sametime is much larger than it. Again, the contribution of a spinning black hole is necessary to accountfor the jet power. These results indicate that the Blandford-Znajeck process is appropriate for re-producing the properties of relativistic jets in blazars. If, on the other hand, the Blandford-Paynemechanism is also at play, but produces less powerful jets, it is natural to expect that the black-holedriven component will be dominant due to the stronger boosting. The observation of disk winds inhighly misaligned sources like Cygnus A is well placed in such a scenario.A deep investigation of other objects in Table 2 is ongoing. In the future, the possibility ofenlarging the sample of sources where the jet formation region can be imaged will depend on thecompactness of the jet bases at frequencies higher than 86 GHz. Results from the surveys (Lobanovet al., 2000; Lee et al., 2008; Gopalakrishnan Nair et al., 2016) indicate a decrease of brightnesstemperature with increasing frequency, which may hinder the detection of the innermost jet emis-sion at 230 GHz and higher. On the other hand, experiments planned for the near future will alsoprofit from the deployment of the phased ALMA and from the improved sensitivity of global arrays,which may enable to probe the jet formation region in fainter objects.
Sagittarius A*Sagittarius A* (Sgr A*) is one of the three main components forming the Sagittarius A complex,located at the center of the Milky way. This compact object, emitting from radio to γ -rays witha peak at sub-millimeter wavelengths, is believed to mark the position of the supermassive blackhole at the center of our galaxy. With a mass of about 4 millions solar masses (Gillessen et al.,2009) and a distance of ∼ µ as at 215 GHz (Bower et al., 2006). In a three-station VLBI experiment at the latter frequency, ascattering-corrected source size of 37 µ as could be determined (Doeleman et al., 2008). Assum-ing the aforementioned distance and mass parameters, the radius of the black hole event horizonsubtends an angular scales of ∼ µ as, but the apparent horizon is expected to be enlarged due togravitational lensing, to a diameter of ∼ µ as. Therefore, quite surprisingly, the size measured at adio observations of active galactic nuclei with mm-VLBI 31
215 GHz was found to be smaller than the apparent dimension of the event horizon. This remark-able result suggests that the emission from Sgr A* may not be centered on the exact location of theblack hole but it may originate in its surroundings.Several models have been elaborated with the aim of identifying the nature of the emissionregion. Many invoke the emission from an inefficient accretion flow, such as the ADAF (advection-dominated accretion flow, Narayan and Yi, 1994) and the ADIOS (advection-dominated inflow-outflow solutions, Blandford and Begelman, 1999). While these models may explain the observedemission up to the high energies, they have been found to significantly under-predict the low-frequency radio continuum (e.g., Yuan et al., 2002). The latter component can be better reproducedby including the contribution of hot electrons ( T ∼ K), often proposed to originate in a weakrelativistic jet coupled to the accretion flow (e.g., Falcke and Markoff, 2000). With a bolometricluminosity several orders of magnitude below the Eddington limit ( ∼ − L Edd ), Sgr A* is highlyunder-luminous when compared to a classical AGN. The study of this putative jet is thereforefundamental for exploring the jet formation phenomenon under different physical conditions.In the near future, the Event Horizon Telescope will be capable of producing detailed imagingof the radio structure in Sgr A*, possibly shedding light on the nature of the emission. In themeantime, the visibility and closure phase data collected so far at 230 GHz are already providingimportant constraints. The radio emission has been observed to vary on time scales of days (Fishet al., 2011), and a persistent asymmetry in the source structure has been deduced from the non-zero closure phases (Fish et al., 2016b). Moreover, a recent study has shown that the radio emissionis highly polarized, up to ∼ S (Johnson et al., 2015, Fig. 12), and that thepolarization direction varies spatially. The polarization degree decreases by one order of magnitudewhen looking at the short baselines, to values that match very well those measured at the samefrequency on arcsecond scales (e.g., Bower et al., 2003). This comparison shows, on the one hand,that the polarization arises in the very central regions of the source, and is not associated to dustemission in the surroundings. Secondly, it points out the absolute importance of resolving thesecompact regions for unveiling the coherent polarization structures in the source. There is no singleexplanation for the existence of strong ordered magnetic fields in Sgr A*. They could signal thepresence of a magnetically dominated region formed after accumulation of magnetic flux near theevent horizon, similarly to that inferred for the cores of extragalactic jets (Sect. 4.4), or they couldarise as a consequence of instabilities (e.g., the magnetorotational instability - MRI, Balbus andHawley, 1991) in the magnetized and rotating accretion disk. Finally, they could also be associatedwith the emergence of a relativistic jet.The second and most crucial objective of the EHT project will be the direct imaging of theso-called “black hole shadow”. The shadow is the dark region around a black hole (Bardeen, 1973;Luminet, 1979; Falcke et al., 2000), expected to be observable in the presence of a backgroundsource, e.g., an optically thin plasma. The shadow should be surrounded by a thin photon ring,whose shape and size are independent on the physical detail of the accretion process and shouldonly be determined by the basic properties of the black hole, i.e., its mass and spin, and by itsinclination. The photon ring is predicted to be perfectly circular in the case of a non-spinning blackhole with spherical symmetry, while crescent-like morphologies should arise in the case of a Kerrblack hole. Concerning the brightness distribution around the photon ring, this is expected to beasymmetric in all cases due to differential Doppler beaming, but its detailed characteristics will alsodepend on the physical conditions of the accretion flow (see e.g., Mo´scibrodzka et al., 2009; Dexteret al., 2010). Ultimately, the analysis of the geometrical and physical properties of the shadow will Fig. 12
Interferometric fractional polarization measurements for Sgr A* during EHT observations in 2013. Polarization degrees upto 70% −
80% are observed. The telescopes participating to the experiment were the phased CARMA (Combined Array for Researchin Millimeter-wave Astronomy) in California, the SMT (Submillimeter Telescope) in Arizona, the SMA (Submillimeter Array) andthe JCMT (James Clerk Maxwell Telescope) in Hawaii. (Image reproduced with permission from Johnson et al. (2015), copyright byAAAS) provide further constraints on the properties of the plasma flow in SgrA* and, most importantly, adirect test for the validity of the no-hair theorem and of the Einstein’s theory of gravity in general.Through a recent four-site experiment at 230 GHz, providing a resolution of ∼ . ( u , v ) -coverage, influence ofscattering in the images, and time variability during the observations. Several methods are beingdeveloped, concerning non-imaging analysis and addressing the mitigation of those effects (Fishet al., 2014; Johnson and Gwinn, 2015; Johnson, 2016; Lu et al., 2016).The investigation of the Galactic center is considered the main scientific goal of the Event Hori-zon Telescope (EHT) project, whose efforts are shared with –and complemented by– the Black-HoleCam
European collaboration (e.g., Goddi et al., 2017). In this context, vibrant research activ-ity is being carried out at present on multiple fronts, from the refinement of theoretical models andsimulations to the enhancement of VLBI arrays and data analysis techniques. For a recent overviewof the EHT efforts concerning these subjects, the reader is referred to Fish et al. (2016a). The future of mm-VLBI
Fifty years after its birth, VLBI is experiencing a second youth due to its development in themillimeter and sub-millimeter regimes. Unexplored frontiers have now become accessible thanksto it, new science has been produced and more fundamental results are foreseen in the comingyears. adio observations of active galactic nuclei with mm-VLBI 33
The GMVA is conducting successful biannual observations at 86 GHz, and has provided im-portant constraints for theories describing blazar physics and jet formation. In the future, a morereliable description of the jet kinematic properties and variability could be obtained through ahigh-cadence monitoring which, given the short timescales characterizing blazars, is currently toosparse. Significant improvements could also come by implementing multi-frequency receiving sys-tems, following the example given by the Korean VLBI Network (Asada et al., 2017). Besides theobvious advantages it would provide for the accurate characterization of the jet spectral properties,this method also enables to enhance the data quality. By applying a technique known as source-frequency phase referencing , the measurements obtained at the lowest frequencies, which are lessaffected by atmospheric opacity, can in fact be used to correct the phase at higher frequencies.In addition to the Korean VLBI Network, which is at present regularly participating in the ob-serving sessions, other telescopes may enhance the future imaging capabilities at 86 GHz. Theseinclude the NOEMA (NOrthern Extended Millimeter Array), successor of the Plateau du Bure ob-servatory, the Large Millimeter Telescope in Mexico, the phased ALMA (at present commissionedfor observing at 86 GHz and 230 GHz, potentially capable of going down to 43 GHz and up to 345GHz and even beyond), and the Greenland Telescope, starting its commissioning in early 2017.These latter sites are also part of the Event Horizon Telescope, together with APEX in Chile, theJCMT (James Clerk Maxwell Telescope) and the SMA (Sub-Millimeter Array), both located inHawaii, the Arizona Radio Observatory (SMT), the SPT (South Pole Telescope), and the 30-mtelescope atop Pico Veleta in Spain.The ALMA Phasing Project (e.g., Matthews et al., 2014), led by the Haystack Observatoryof the Massachusetts Institute of Technology in collaboration with several institutions worldwide,started in 2011 and completed commissioning in 2015. Its goal was to phase-up the 64 ALMAdishes to form a single VLBI station with equivalent diameter of 84 meters. Additionally to thephasing, the project includes polarization conversion to match the typical circular polarizationrecorded by the VLBI stations with the X/Y (linear) polarization used by ALMA (Mart´ı-Vidalet al., 2016).ALMA has performed its first regular VLBI observations in April 2017 during Cycle 4. TheGMVA was assigned the role of the network provider at 86 GHz, and the Event Horizon Tele-scope consortium (supervised by the National Radio Astronomy Observatory) this role at 230 GHz.Thanks to its large collecting surface, ALMA is expected to provide a substantial boost in sensi-tivity, since the root-mean-square noise of the image will be typically reduced by a factor of three.The first observations including ALMA are still limited by technical reasons, such as a flux den-sity threshold of 500 mJy and the need of long integration time for polarization calibration. Thoselimitations will be loosen in the future, for instance, when a sky model can be introduced to thephasing.The possibility of achieving an excellent sensitivity will be crucial in future EHT experiments,aiming at producing a direct image of the black-hole shadow. High sensitivity and good imagefidelity will be obtained not only through the expansion of the array, which after 2020 should alsoinclude a telescope in Namibia, Africa (AMT - Africa Millimeter Telescope) , but also thanks tothe improvement of the recording capabilities at each site, with subsequent increase in bandwidth.Recording capabilities are now boosted by the use of digital techniques and, from the 512 Mbit / sused several years ago, present developments in digital base band conversion (e.g., Tuccari et al.,2012) bring them to 64 Gbit / s, yielding instantaneous bandwidths of 16 GHz. Millimeter VLBI is an example of cutting edge technology at the service of science. The in-ternational effort recently spent in its further upgrade will enable the exciting scientific questionspresented in this article to be fully addressed, by zooming into some of the most violent phenom-ena in the Universe. Landmark tests for our current knowledge of fundamental physics will beprovided, together with final and direct evidence for the existence of black holes at the center ofgalaxies. adio observations of active galactic nuclei with mm-VLBI 35
Acknowledgements
MOJAVE database that is maintained by the
MOJAVE team(Lister et al., 2009). The European VLBI Network is a joint facility of European, Chinese, South African, and other radio astronomyinstitutes funded by their national research councils.
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