Andrei P. Lobanov

A GLOBAL 86 GHZ VLBI SURVEY OF COMPACT RADIO SOURCES

We present results from a large 86 GHz global very long baseline interferometry (VLBI) survey of compact radio sources. The main goal of the survey is to increase by factors of 3-5 the total number of objects accessible for future 3 mm VLBI imaging. The survey observations reach a baseline sensitivity of 0.1 Jy and an image sensitivity of better than 10 mJy beam–1. A total of 127 compact radio sources have been observed. The observations have yielded images for 109 sources, extending the database of the sources imaged at 86 GHz with VLBI observation by a factor of 5, and only six sources have not been detected. The remaining 12 objects have been detected but could not be imaged due to insufficient closure phase information. Radio galaxies are less compact than quasars and BL Lac objects on the sub-milliarcsecond scale. The flux densities and sizes of the core and jet components of all imaged sources have been estimated using Gaussian model fitting. From these measurements, brightness temperatures have been calculated, taking into account the resolution limits of the data. The cores of 70% of the imaged sources are resolved. The core brightness temperatures of the sources peak at ~1011 K and only 1% have brightness temperatures higher than 1012 K. The cores of intraday variable (IDV) sources are smaller in angular size than those of non-IDV sources, and so yield higher brightness temperatures.

A supermassive binary black hole in the quasar 3C 345

Radio loud active galactic nuclei present a remarkable variety of signs indicating the presence of periodical processes possibly originating in binary systems of supermassive black holes, in which orbital motion and precession are ultimately responsible for the observed broad-band emission variations, as well as for the morphological and kinematic properties of the radio emission on parsec scales. This scenario, applied to the quasar 3C 345, explains the observed variations of radio and optical emission from the quasar, and reproduces the structural variations observed in the parsec-scale jet of this object. The binary system in 3C 345 is described by two equal-mass black holes with masses of ≈7.1 × 10 8 Mseparated by ≈0.33 pc and orbiting with a period ∼480 yr. The orbital motion induces a precession of the accretion disk around the primary black hole, with a period of ≈2570 yr. The jet plasma is described by a magnetized, relativistic electron-positron beam propagating inside a wider and slower electron-proton jet. The combination of Alfven wave perturbations of the beam, the orbital motion of the binary system and the precession of the accretion disk reproduces the variability of the optical flux and evolution of the radio structure in 3C 345. The timescale of quasi-periodic flaring activity in 3C 345 is consistent with typical disk instability timescales. The present model cannot rule out a small-mass orbiter crossing the accretion disk and causing quasi-periodic flares.

Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron. I. Imaging BL Lacertae at 21 μas resolution

This research has been supported by the Spanish Ministry of Economy and Competitiveness grant AYA2013-40825-P, by the Russian Foundation for Basic Research (projects 13-02-12103, 14-02-31789, and 15-02-00949), and St. Petersburg University research grant 6.38.335.2015. The research at Boston University (BU) was funded in part by NASA Fermi Guest Investigator grant NNX14AQ58G. Y.M. acknowledges support from the ERC Synergy Grant >BlackHoleCam-Imaging the Event Horizon of Black Holes> (Grant 610058). Part of this work was supported by the COST Action MP1104 >Polarization as a tool to study the Solar System and beyond.> The RadioAstron project is led by the Astro Space Center of the Lebedev Physical Institute of the Russian Academy of Sciences and the Lavochkin Scientific and Production Association under a contract with the Russian Federal Space Agency, in collaboration with partner organizations in Russia and other countries.

Internal structure and dynamics of the kiloparsec-scale jet in M87

Abstract Detailed analysis of HST and VLA observations enables quantitative modeling of the internal structure of the kiloparsec-scale jet in M87. The jet transverse profiles can be modeled by double-Gaussian profiles. The jet structure is dominated by two threads resembling closely a double helix visible until about 20″ distance. It is produced by a combination of the helical mode and of the elliptical mode of Kelvin–Helmholtz instability. The elliptical mode dominates the pre-knot A jet, while the helical mode becomes prominent at larger distances. Modeling the jet structure constrains well the instability pattern speed, β w =0.5 c and jet viewing angle, θ j =40°. The estimated bulk Lorentz factor of the jet is 3.4. The model reproduces the jet structure on scales of up to 50″ (accounting for the jet curvature).

ANATOMY OF HELICAL EXTRAGALACTIC JETS: THE CASE OF S5 0836+710

Helical structures are common in extragalactic jets. They are usually attributed in the literature to periodical phenomena in the source (e.g., precession). In this work, we use very long baseline interferometry data of the radio jet in the quasar S5 0836+710 and hypothesize that the ridgeline of helical jets like this corresponds to a pressure maximum in the jet and assume that the helically twisted pressure maximum is the result of a helical wave pattern. For our study, we use observations of the jet in S5 0836+710 at different frequencies and epochs. The results show that the structures observed are physical and not generated artificially by the observing arrays. Our hypothesis that the observed intensity ridgeline can correspond to a helically twisted pressure maximum is confirmed by our observational tests. This interpretation allows us to explain jet misalignment between parsec and kiloparsec scales when the viewing angle is small, and also brings us to the conclusion that high-frequency observations may show only a small region of the jet flow concentrated around the maximum pressure ridgeline observed at low frequencies. Our work provides a potential explanation for the apparent transversal superluminal speeds observed in several extragalactic jets by means of transversal shift of an apparent core position with time.

Characterization of the Inner Knot of the Crab: the Site of the Gamma-ray Flares?

One of the most intriguing recent discoveries has been the detection of powerful gamma-ray flares from the Crab Nebula. Such events, with a recurrence time of about once per year, can be so dramatic to make the system the brightest source in the gammaray sky as occurred, e.g. in April 2011. These flares challenge our understanding of how pulsar wind nebulae work and defy current astrophysical models for particle acceleration. We present here our study of the inner knot located within a fraction of an arcsecond from the pulsar with the aim of characterizing the feature and asking if this might be the site of the origin of the gamma-ray flares. We took data using Keck, HST, and Chandra obtained as part of our multiwavelength campaign to identify the source of the enigmatic flares. We set an upper limit as to the gamma-ray flux from the knot. We also find that the dimensions, surface brightness, flux, etc. of the optical and infrared knot are all correlated with distance from the pulsar. This distance, in turn, varies with time. In addition to this most thorough characterization of the inner knots properties, we examine the hypothesis that the knot may be the site of the flares by examining the knot separation versus the Fermi/LAT gamma-ray flux. Finally, as part of this research, we make use of a new approach employing singular value decomposition (SVD) for analyzing time series of images and compare the approach to more traditional methods. Our conclusions are only refined but not impacted by using the new approach.

A search for active galactic nuclei in the most extreme UV-selected starbursts using the European VLBI Network

We have used the European VLBI Network (EVN) to observe a sample of Lyman Break Analogs (LBAs), nearby (z < 0.3) galaxies with properties similar to the more distant Lyman Break Galaxies (LBGs). The study of LBGs may help define the feedback relationship between black holes (BHs) and their host galaxies. Previous VLA observations have shown that the kpc-scale radio emission from LBAs is dominated by starbursts. The main targets of this VLBI experiment were selected because they possessed emission-line properties between starbursts and Type 2 (obscured) AGN. Eight targets (three star-forming LBAs, four composite LBAs, and one Type 1 AGN) were observed at 5 GHz, four of which (one star-forming LBA and three composite LBAs) were also observed at 1.7 GHz. One star-forming LBA and one composite LBA were detected above 5σ at 1.7 GHz (only), while the AGN was detected at 5 GHz. In both LBAs, the radio luminosity (LR) exceeds that expected from supernovae (remnants) based on a comparison with Arp220, Arp229A and Mrk273, by factors of 2 8. The composite LBA exhibits a compact core emitting around 10% of the VLA flux density. The high Tb of 3.5×10 7 K and excess core LR with respect to the LR/LX relation of radio-quiet AGN indicate that this LBA possesses an obscured AGN (MBH � 10 5−7 M⊙). In three other composite LBAs detected previously in the X-ray, no radio sources were detected, indicating either variability or the presence of an obscured AGN below our radio sensitivity. While weak AGN may co-exist with the starbursts as shown in at least one of the LBAs, their contribution to the total radio flux is fairly minimal. Our results show that the detection of such weak AGN presents a challenge at radio, X-ray and optical emission-line wavelengths at z � 0.2, indicating the great difficulties that need to be overcome in order to study similar processes at high redshift when these types of galaxies were common.

Dual-Frequency VSOP Imaging of the Jet in S5 0836+710

The luminous high-redshift (z =2 .17) quasar S5 0836+710 was observed in 1997 October with the VSOP at 1.6GHz and 5GHz. We report here a previously unpublished image made from the data at 1.6GHz, and compare the structure of a relativistic jet in this quasar at two frequencies. We present a spectral index image tracing the spectral properties of the jet up to ∼ 40 milliarcsec distance from the nucleus. The curved jet ridge line observed in the images and the spectral index distribution can be described by a Kelvin–Helmholtz instability developing in a relativistic outflow with a Mach number of ∼ 6. In this description, the overall ridge line of the jet is formed by the helical surface mode of the Kelvin–Helmholtz instability, while areas of flatter spectral index embedded into the flow correspond to pressure enhancements produced by the elliptical surface mode of the instability. An alternative explanation involving a sequence of slowly dissipating shocks cannot be ruled out at this point.

Wavelet-based decomposition and analysis of structural patterns in astronomical images

Context. Images of spatially resolved astrophysical objects contain a wealth of morphological and dynamical information, and e ective extraction of this information is of paramount importance for understanding the physics and evolution of these objects. Algorithms and methods employed presently for this purpose (such as, for instance, Gaussian model fitting) often use simplified approaches for describing the structure of resolved objects. Aims. Automated (unsupervised) methods for structure decomposition and tracking of structural patterns are needed for this purpose, in order to be able to deal with the complexity of structure and large amounts of data involved. Methods. A new Wavelet-based Image Segmentation and Evaluation (WISE) method is developed for multiscale decomposition, segmentation, and tracking of structural patterns in astronomical images. Results. The method is tested against simulated images of relativistic jets and applied to data from long-term monitoring of parsecscale radio jets in 3C 273 and 3C 120. Working at its coarsest resolution, WISE reproduces exceptionally well the previous results of model fitting evaluation of the structure and kinematics in these jets. Extending the WISE structure analysis to fine scales provides the first robust measurements of two-dimensional velocity fields in these jets and indicates that the velocity fields are likely to reflect the evolution of Kelvin-Helmholtz instabilities developing in the flow.

Acceleration of Compact Radio Jets on Sub-parsec Scales

Jets of compact radio sources are highly relativistic and Doppler boosted, making studies of their intrinsic properties difficult. Observed brightness temperatures can be used to study the intrinsic physical properties of the relativistic jets, and constrain models of jet formation in the inner jet region. We aim to observationally test such inner jet models. The very long baseline interferometry (VLBI) cores of compact radio sources are optically thick at a given frequency. The distance of the core from the central engine is inversely proportional to the frequency. Under the equipartition condition between the magnetic field energy and particle energy densities, the absolute distance of the VLBI core can be predicted. We compiled the brightness temperatures of VLBI cores at various radio frequencies of 2, 8, 15, and 86~GHz. We derive the brightness temperature on sub-parsec scales in the rest frame of the compact radio sources. We find that the brightness temperature increases with increasing distance from the central engine, indicating that the intrinsic jet speed (the Lorentz-factor) increases along the jet. This implies that the jets are accelerated in the (sub-)parsec regions from the central engine.