C. Casadio

The awakening of BL Lacertae: observations by Fermi, Swift and the GASP-WEBT

Since the launch of the Fermi satellite, BL Lacertae has been moderately active at ?-rays and optical frequencies until 2011 May, when the source started a series of strong flares. The exceptional optical sampling achieved by the GLAST–AGILE Support Program of the Whole Earth Blazar Telescope in collaboration with the Steward Observatory allows us to perform a detailed comparison with the daily ?-ray observations by Fermi. Discrete correlation analysis between the optical and ?-ray emission reveals correlation with a time lag of 0 ± 1 d, which suggests cospatiality of the corresponding jet emitting regions. A better definition of the time lag is hindered by the daily gaps in the sampling of the extremely fast flux variations. In general, optical flares present more structure and develop on longer time-scales than corresponding ?-ray flares. Observations at X-rays and at millimetre wavelengths reveal a common trend, which suggests that the region producing the mm and X-ray radiation is located downstream from the optical and ?-ray-emitting zone in the jet. The mean optical degree of polarization slightly decreases over the considered period and in general it is higher when the flux is lower. The optical electric vector polarization angle (EVPA) shows a preferred orientation of about 15°, nearly aligned with the radio core EVPA and mean jet direction. Oscillations around it increase during the 2011–2012 outburst. We investigate the effects of a geometrical interpretation of the long-term flux variability on the polarization. A helical magnetic field model predicts an evolution of the mean polarization that is in reasonable agreement with the observations. These can be fully explained by introducing slight variations in the compression factor in a transverse shock waves model.

Variability of the blazar 4C 38.41 (B3 1633+382) from GHz frequencies to GeV energies

Context. After years of modest optical activity, the quasar-type blazar 4C 38.41 (B3 1633+382) experienced a large outburst in 2011, which was detected throughout the entire electromagnetic spectrum, renewing interest in this source. Aims. We present the results of low-energy multifrequency monitoring by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT) consortium and collaborators, as well as those of spectropolarimetric/spectrophotometric monitoring at the Steward Observatory. We also analyse high-energy observations of the Swift and Fermi satellites. This combined study aims to provide insights into the source broad-band emission and variability properties. Methods. We assemble optical, near-infrared, millimetre, and radio light curves and investigate their features and correlations. In the optical, we also analyse the spectroscopic and polarimetric properties of the source. We then compare the low-energy emission behaviour with that at high energies. Results. In the optical-UV band, several results indicate that there is a contribution from a quasi-stellar-object (QSO) like emission component, in addition to both variable and polarised jet emission. In the optical, the source is redder-when-brighter, at least for R ≳ 16. The optical spectra display broad emission lines, whose flux is constant in time. The observed degree of polarisation increases with flux and is higher in the red than the blue. The spectral energy distribution reveals a bump peaking around the U band. The unpolarised emission component is likely thermal radiation from the accretion disc that dilutes the jet polarisation. We estimate its brightness to be R QSO ∼ 17.85-18 and derive the intrinsic jet polarisation degree. We find no clear correlation between the optical and radio light curves, while the correlation between the optical and γ-ray flux apparently fades in time, likely because of an increasing optical to γ-ray flux ratio. Conclusions. As suggested for other blazars, the long-term variability of 4C 38.41 can be interpreted in terms of an inhomogeneous bent jet, where different emitting regions can change their alignment with respect to the line of sight, leading to variations in the Doppler factor δ. Under the hypothesis that in the period 2008-2011 all the γ-ray and optical variability on a one-week timescale were due to changes in δ, this would range between ∼7 and ∼21. If the variability were caused by changes in the viewing angle θ only, then θ would go from ∼2.6° to ∼5°. Variations in the viewing angle would also account for the dependence of the polarisation degree on the source brightness in the framework of a shock-in-jet model.

The kinematic of HST-1 in the jet of M 87

Aims. We aim to constrain the structural variations within the HST-1 region downstream of the radio jet of M87, in general as well as in connection to the episodes of activity at very high energy (VHE). Methods. We analyzed and compared 26 VLBI observations of the M87 jet, obtained between 2006 and 2011 with the Very Long Baseline Array (VLBA) at 1.7 GHz and the European VLBI Network (EVN) at 5 GHz. Results. HST-1 is detected at all epochs; we model-fitted its complex structure with two or more components, the two outermost of which display a significant proper motion with a superluminal velocity around ∼4c. The motion of a third feature that is detected upstream is more difficult to characterize. The overall position angle of HST-1 has changed during the time of our observations from −65 ◦ to −90 ◦ , while the structure has moved by over 80 mas downstream. Our results on the component evolution suggest that structural changes at the upstream edge of HST-1 can be related to the VHE events.

Polarization angle swings in blazars: The case of 3C 279

International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Max Planck Institute for Radio Astronomy; Universities of Bonn and Cologne; Academy of Finland project [274477]; NASA Fermi GI grant [NNX11AQ03G]; Russian Foundation for Basic Research [13-02-12103, 14-02-31789]; RFBR [12-02-01237a]; UNAM DGAPA-PAPIIT [IN116211-3]; Ramon y Cajal grant of the Spanish Ministry of Economy and Competitiveness (MINECO); Spanish Ministry of Economy and Competitiveness (Spain); Regional Government of Andalucia (Spain) [AYA2010-14844, AYA2013-40825-P, P09-FQM-4784]; Fermi Guest Investigator [NNX08AW56G, NNX09AU10G, NNX12AO93G, NNX14AQ58G]; Russian RFBR [15-02-00949]; St. Petersburg University research [6.38.335.2015]; Shota Rustaveli National Science Foundation [FR/638/6-320/12, 31/77]

THE CONNECTION BETWEEN THE RADIO JET AND THE GAMMA-RAY EMISSION IN THE RADIO GALAXY 3C 120

We present the analysis of the radio jet evolution of the radio galaxy 3C 120 during a period of prolonged -ray activity detected by the Fermi satellite between December 2012 and October 2014. We nd a clear connection between the -ray and radio emission, such that every period of -ray activity is accompanied by the aring of the mm-VLBI core and subsequent ejection of a new superluminal component. However, not all ejections of components are associated with -ray events detectable by Fermi. Clear ray detections are obtained only when components are moving in a direction closer to our line of sight. This suggests that the observed -ray emission depends not only on the interaction of moving components with the mm-VLBI core, but also on their orientation with respect to the observer. Timing of the -ray detections and ejection of superluminal components locate the -ray production to within 0.13 pc from the mm-VLBI core, which was previously estimated to lie about 0.24 pc from the central black hole. This corresponds to about twice the estimated extension of the broad line region, limiting the external photon eld and therefore suggesting synchrotron self Compton as the most probable mechanism for the production of the -ray emission. Alternatively, the interaction of components with the jet sheath can provide the necessary photon eld to produced the observed -rays by Compton scattering. Subject headings: galaxies: active | galaxies: jet | galaxies: individual (3C120) | radio continuum: galaxies

A RECOLLIMATION SHOCK 80 mas FROM THE CORE IN THE JET OF THE RADIO GALAXY 3C 120: OBSERVATIONAL EVIDENCE AND MODELING

We present Very Long Baseline Array observations of the radio galaxy 3C 120 at 5, 8, 12, and 15 GHz designed to study a peculiar stationary jet feature (hereafter C80) located ~80 mas from the core, which was previously shown to display a brightness temperature ~600 times larger than expected at such distances. The high sensitivity of the images—obtained between 2009 December and 2010 June—has revealed that C80 corresponds to the eastern flux density peak of an arc of emission (hereafter A80), downstream of which extends a large (~20 mas in size) bubble-like structure that resembles an inverted bow shock. The linearly polarized emission closely follows that of the total intensity in A80, with the electric vector position angle distributed nearly perpendicular to the arc-shaped structure. Despite the stationary nature of C80/A80, superluminal components with speeds up to 3 ± 1 c have been detected downstream from its position, resembling the behavior observed in the HST-1 emission complex in M87. The total and polarized emission of the C80/A80 structure, its lack of motion, and brightness temperature excess are best reproduced by a model based on synchrotron emission from a conical shock with cone opening angle η = 10°, jet viewing angle θ = 16°, a completely tangled upstream magnetic field, and upstream Lorentz factor γ u = 8.4. The good agreement between our observations and numerical modeling leads us to conclude that the peculiar feature associated with C80/A80 corresponds to a conical recollimation shock in the jet of 3C 120 located at a de-projected distance of ~190 pc downstream from the nucleus.

Comprehensive Monitoring of Gamma-Ray Bright Blazars. I. Statistical Study of Optical, X-Ray, and Gamma-Ray Spectral Slopes

We present -ray, X-ray, ultraviolet, optical, and near-infrared light curves of 33 -ray bright blazars over four years that we have been monitoring since 2008 August with multiple optical, ground-based telescopes and the Swift satellite, and augmented by data from the Fermi Gamma- ray Space Telescope and other publicly available data from Swift. The sample consists of 21 flat-spectrum radio quasars (FSRQs) and 12 BL Lac objects (BL Lacs). We identify quiescent and active states of the sources based on their -ray behavior. We derive -ray, X-ray, and optical spectral indices, �γ, �X, ando, respectively (Fν ∝ � α ), and construct spectral energy distributions (SEDs) during quiescent and active states. We analyze the relationships between different spectral indices, blazar classes, and activity states. We find (i) significantly steeper -ray spectra of FSRQs than for BL Lacs during quiescent states, but a flattening of the spectra for FSRQs during active states while the BL Lacs show no significant change; (ii) a small difference ofX within each class between states, with BL Lac X-ray spectra significantly steeper than in FSRQs; (iii) a highly peaked distribution of X-ray spectral slopes of FSRQs at ∼ −0.60, but a very broad distribution ofX of BL Lacs during active states; (iv) flattening of the optical spectra of FSRQs during quiescent states, but no statistically significant change ofo of BL Lacs between states; and (v) a positive correlation between optical and -ray spectral slopes of BL Lacs, with similar values of the slopes. We discuss the findings with respect to the relative prominence of different components of high-energy and optical emission as the flux state changes.

Very Long Baseline polarimetry and the γ-ray connection in Markarian 421 during the broadband campaign in 2011

Context. This is the third paper in a series devoted to the analysis of the multiwavelength data from a campaign on the nearby (z = 0.03) TeV blazar Mrk 421 during 2011. Aims. We investigate the structure of the high angular resolution polarization, the magnetic topology, the total intensity light curve, the γ-ray flux, and the photon index. We describe how they evolve and how they are connected. Methods. We analyzed data in polarized intensity obtained with the Very Long Baseline Array (VLBA) at twelve epochs (one observation per month from January to December 2011) at 15, 24, and 43 GHz. For the absolute orientation of the electric vector position angles (EVPA) we used the D-terms method; we also confirm its accuracy. We also used γ-ray data from the Fermi Large Area Telescope on weekly time bins throughout 2011. Results. The source shows polarized emission, and its properties vary with time, frequency, and location along the jet. The core mean polarization fraction is generally between 1% and 2%, with a 4% peak at 43 GHz in March; the polarization angle is variable, mainly at 15 GHz, where it changes frequently, and less so at 43 GHz, where it oscillates in the range 114 ◦ −173 ◦ . The jet polarization properties are more stable, with a fractional polarization of around 16% and a polarization angle nearly perpendicular to the jet axis. The average flux and photon index at γ-ray energies are (17.4 ± 0.5) ×10 −8 ph cm −2 s −1 and Γ= 1.77 ± 0.02. The γ-ray light curve shows variability, with a main peak of (38 ± 11) × 10 −8 ph cm −2 s −1 at the beginning of March and two later peaks centered on September 8 and November 13. The first γ-ray peak appears to be associated with the peak in the core polarized emission at 43 GHz, as well as with the total intensity light curve. A discrete correlation function analysis yields a correlation coefficient of 0.54 at zero delay, with a significance level >99.7%. Conclusions. With this multifrequency study, we accurately determine the polarization properties of Mrk 421, both in the core and in the jet region. The radio and γ-ray light curves are correlated. The observed EVPA variability at 15 GHz is partly due to opacity and partly to a variable Faraday rotation effect. To explain the residual variability of the intrinsic polarization angle and the low degree of polarization in the core region, we invoke a blend of variable cross-polarized subcomponents with different polarization properties within the beam.

3C 286: a bright, compact, stable, and highly polarized calibrator for millimeter-wavelength observations

Context. Several millimeter and submillimeter facilities with linear polarization observing capabilities have started operating during the last years. These facilities, as well as other previous millimeter telescopes and interferometers, require bright and stable linear polarization calibrators to calibrate new instruments and to monitor their instrumental polarization. The current limited number of adequate calibrators implies difficulties in the acquisition of these calibration observations. Aims. Looking for additional linear polarization calibrators in the millimeter spectral range, we started monitoring 3C 286 in mid2006. This source is a standard and highly stable polarization calibrator for radio observations. Methods. Here we present the 3 mm and 1 mm monitoring observations obtained between September 2006 and January 2012 with the XPOL polarimeter on the IRAM 30 m Millimeter Telescope. Results. Our observations show that 3C 286 is a bright source of constant total flux with 3 mm flux density S 3m m = (0.91 ± 0.02) Jy. The 3 mm linear polarization degree (p 3m m = [13.5 ± 0.3]%) and polarization angle (χ 3m m = [37.3 ± 0.8] ◦ , expressed in the equatorial coordinate system) were also constant during the time span of our observations. Although with poorer time sampling and signal-tonoise ratio, our 1 mm observations of 3C 286 are also reproduced by a constant source of 1 mm flux density (S 1m m = [0.30 ± 0.03] Jy), polarization fraction (p 1m m = [14.4 ± 1.8]%), and polarization angle (χ 1m m = [33.1 ± 5.7] ◦ ). Conclusions. This, together with the previously known compact structure of 3C 286 – extended by ∼3.5 �� in the sky – allow us to propose 3C 286 as a new calibrator for both single-dish and interferometric polarization observations at 3 mm, and possibly at shorter wavelengths.

Kinematics of Parsec-scale Jets of Gamma-Ray Blazars at 43 GHz within the VLBA-BU-BLAZAR Program

We analyze the parsec-scale jet kinematics from 2007 June to 2013 January of a sample of