arXiv subjects

Y. Y. Kovalev

Publications and source records attributed to Y. Y. Kovalev.

At least 91 records · Page 5Linked to original sources

arXiv2018

Constraining the photon coupling of ultra-light dark-matter axion-like particles by polarization variations of parsec-scale jets in active galaxies

Ultra-light dark matter may consist of axion-like particles with masses below 10^(-19) eV. Two-photon interactions of these particles affect the polarization of radiation propagating through the dark matter. Coherent oscillations of the Bose condensate of the particles induce periodic changes in the plane of polarisation of emission passing through the condensate. We estimate this effect and analyze MOJAVE VLBA polarization observations of bright downstream features in the parsec-scale jets of active galaxies. Through the non-observation of periodic polarization changes, we are able to constrain the photon coupling of the ultra-light dark-matter axion-like particles at the level of <~ 10^(-12)/GeV for masses between ~5*10^(-23) eV and ~1.2*10^(-21) eV.

astro-ph.CO↗

arXiv2018

Multi-frequency study of the gamma-ray flaring BL Lac object PKS 2233-148 in 2009-2012

We study the jet physics of the BL Lac object PKS 2233-148 making use of synergy of observational data sets in the radio and gamma-ray energy domains. The four-epoch multi-frequency (4-43 GHz) VLBA observations focused on the parsec-scale jet were triggered by a flare in gamma-rays registered by the Fermi-LAT on April 23, 2010. We also used 15 GHz data from the OVRO 40-m telescope and MOJAVE VLBA monitoring programs. Jet shape of the source is found to be conical on scales probed by the VLBA observations setting a lower limit of about 0.1 on its unknown redshift. Nuclear opacity is dominated by synchrotron self-absorption, with a wavelength-dependent core shift $r_{\text{core[mas]}}\approx0.1λ_{[\text{cm}]}$ co-aligned with the innermost jet direction. The turnover frequency of the synchrotron spectrum of the VLBI core shifts towards lower frequencies as the flare propagates down the jet, and the speed of this propagation is significantly higher, about 1.2 mas/yr, comparing to results from traditional kinematics based on tracking bright jet features. We have found indications that the gamma-ray production zone in the source is located at large distances, 10-20 pc, from a central engine, and could be associated with the stationary jet features. These findings favour synchrotron self-Compton, possibly in a combination with external Compton scattering by infrared seed photons from a slow sheath of the jet, as a dominant high-energy emission mechanism of the source.

astro-ph.HE↗

arXiv2018

A quantitative analysis of systematic differences in positions and proper motions of Gaia DR2 with respect to VLBI

We have analyzed the differences in positions of 9081 matched sources between the Gaia DR2 and VLBI catalogues. The median position uncertainty of matched sources in the VLBI catalogue is a factor of two larger than the median position uncertainty in the Gaia DR2. There are 9% matched sources with statistically significant offsets between both catalogues. We found that reported positional errors should be re-scaled by a factor of 1.3 for VLBI and 1.06 for Gaia, and in addition, Gaia errors should be multiplied by the square root of chi square per degree of freedom in order to best fit the normalized position differences to the Rayleigh distribution. We have established that the major contributor to statistically significant position offsets is the presence of optical jets. Among the sources for which the jet direction was determined, the position offsets are parallel to the jet directions for 62% of the outliers. Among the matched sources with significant proper motion, the fraction of objects with proper motion directions parallel to jets is a factor of 3 greater than on average. Such sources have systematically higher chi square per degree of freedom. We explain these proper motions as a manifestation of the source position jitter caused by flares that we have predicted earlier. Therefore, the assumption that quasars are fixed points and therefore, differential proper motions determined with respect to quasar photocenters can be regarded as absolute proper motions, should be treated with a great caution.

astro-ph.GA↗

arXiv2018

Frequency-Dependent Core Shifts in Ultracompact Quasars

Results of a pilot project with the participation of the "Kvazar-KVO" radio interferometry array in observations carried out with the European VLBI Network are presented. The aim of the project was to conduct and analyze multi-frequency (1.7, 2.3, 5.0, 8.4 GHz) observations of the parsec-scale jets of 24 active galactic nuclei. Three observing sessions were successfully carried out in October 2008. Maps of the radio intensity distributions have been constructed in all four frequencies using phase referencing. A method for measuring the frequency-dependent shift of the position of the VLBI core by applying relative astrometry to observations of close triplets of radio sources has been developed. The fundamental possibility of detecting core shifts in ultra-compact sources for which traditional methods based on the achromatic positions of optically thin regions of the jet are not suitable is demonstrated. The conditions for successful measurement of this shift are discussed; these are determined by the closeness of the calibrator used, the effective resolution of the system, the quality of the filling of the $uv$ plane, the relative orientations of the jets in the triplets, and the brightnesses of the sources.

astro-ph.GA↗

arXiv2018

MOJAVE XVI: Multi-Epoch Linear Polarization Properties of Parsec-Scale AGN Jet Cores

We present an analysis of the core linear polarization properties of 387 parsec-scale active galactic nuclei (AGN) jets. Using 15 GHz VLBA data, we revisit the conclusions of the first paper in this series with multi-epoch measurements and more detailed analysis of a larger AGN sample which spans a broader range of synchrotron peak frequencies. Each AGN has been observed for at least five epochs between 1996 and 2017. We find that BL Lac objects have core electric vector position angles (EVPAs) which tend towards alignment with the local jet direction; compared to flat spectrum radio quasars (FSRQs), their EVPAs are also less variable over time. The AGN cores which are most fractionally polarized and least variable in polarization have EVPAs that are closely aligned with the local jet direction; they also have low variability in EVPA. These results support the popular model of a standing transverse shock at the base of the jet which collimates the jet magnetic field perpendicular to the jet direction, increasing the fractional polarization and leading to greater polarization stability over time. High-synchrotron-peaked (HSP) BL Lac objects form a low luminosity, low fractional polarization population. The five narrow-line Seyfert 1 galaxies in our sample have low fractional polarization and large EVPA-jet misalignments. Although AGN detected at gamma-rays are thought to be more Doppler boosted than non-detected AGN, we find no significant differences in fractional polarization based on detection by Fermi-LAT; the gamma-loud AGN are, however, more variable in core EVPAs.

astro-ph.GA↗

arXiv2018

Reversals in the Direction of Polarization Rotation in OJ 287

We have obtained a smooth time series for the Electric Vector Position Angle (EVPA) of the blazar OJ 287 at centimeter wavelengths, by making $\pm nπ$ adjustments to archival values from 1974 to 2016. The data display rotation reversals in which the EVPA rotates counter-clockwise (CCW) for 180 deg and then rotates clockwise (CW) by a similar amount. The time scale of the rotations is a few weeks to a year, and the scale for a double rotation, including the reversal, is one to three years. We have seen four of these events in 40 years. A model consisting of two successive outbursts in polarized flux density, with EVPAs counter-rotating, superposed on a steady polarized jet, can explain many of the details of the observations. Polarization images support this interpretation. The model can also help to explain similar events seen at optical wavelengths. The outbursts needed for the model can be generated by the super-magnetosonic jet model of Nakamura et al. (2010) and Nakamura and Meier (2014), which requires a strong helical magnetic field. This model produces forward and reverse pairs of fast and slow MHD waves, and the plasma inside the two fast/slow pairs rotates around the jet axis, but in opposite directions.

astro-ph.GA↗

arXiv2018

A wide and collimated radio jet in 3C 84 on the scale of a few hundred gravitational radii

Understanding the launching, acceleration, and collimation of jets powered by active galactic nuclei remains an outstanding problem in relativistic astrophysics. This is partly because observational tests of jet formation models suffer from the limited angular resolution of ground-based very long baseline interferometry that has thus far been able to probe the transverse jet structure in the acceleration and collimation zone of only two sources. Here we report radio interferometric observations of 3C 84 (NGC 1275), the central galaxy of the Perseus cluster, made with an array including the orbiting radio telescope of the RadioAstron mission. The obtained image transversely resolves the edge-brightened jet in 3C 84 only 30 microarcseconds from the core, which is ten times closer to the central engine than what has been possible in previous ground-based observations, and it allows us to measure the jet collimation profile from ~ 100 to ~10000 gravitational radii from the black hole. The previously found, almost cylindrical jet profile on scales larger than a few thousand r_g is now seen to continue at least down to a few hundred r_g from the black hole and we find a broad jet with a transverse radius larger than about 250 r_g at only 350 r_g from the core. If the bright outer jet layer is launched by the black hole ergosphere, it has to rapidly expand laterally on scales smaller than 100 r_g. If this is not the case, then this jet sheath is likely launched from the accretion disk.

astro-ph.GA↗

arXiv2018

Parsec-scale Faraday rotation and polarization of 20 active galactic nuclei jets

We perform polarimetry analysis of 20 active galactic nuclei (AGN) jets using the Very Long Baseline Array (VLBA) at 1.4, 1.6, 2.2, 2.4, 4.6, 5.0, 8.1, 8.4, and 15.4 GHz. The study allowed us to investigate linearly polarized properties of the jets at parsec-scales: distribution of the Faraday rotation measure (RM) and fractional polarization along the jets, Faraday effects and structure of Faraday-corrected polarization images. Wavelength-dependence of the fractional polarization and polarization angle is consistent with external Faraday rotation, while some sources show internal rotation. The RM changes along the jets, systematically increasing its value towards synchrotron self-absorbed cores at shorter wavelengths. The highest core RM reaches 16,900 rad/m^2 in the source rest frame for the quasar 0952+179, suggesting the presence of highly magnetized, dense media in these regions. The typical RM of transparent jet regions has values of an order of a hundred rad/m^2. Significant transverse rotation measure gradients are observed in seven sources. The magnetic field in the Faraday screen has no preferred orientation, and is observed to be random or regular from source to source. Half of the sources show evidence for the helical magnetic fields in their rotating magnetoionic media. At the same time jets themselves contain large-scale, ordered magnetic fields and tend to align its direction with the jet flow. The observed variety of polarized signatures can be explained by a model of spine-sheath jet structure.

astro-ph.HE↗

arXiv2018

Multiwavelength observations of the blazar BL Lacertae: a new fast TeV gamma-ray flare

Combined with very-long-baseline interferometry measurements, the observations of fast TeV gamma-ray flares probe the structure and emission mechanism of blazar jets. However, only a handful of such flares have been detected to date, and only within the last few years have these flares been observed from lower-frequency-peaked BL~Lac objects and flat-spectrum radio quasars. We report on a fast TeV gamma-ray flare from the blazar BL~Lacertae observed by VERITAS, with a rise time of $\sim$2.3~hr and a decay time of $\sim$36~min. The peak flux above 200 GeV is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photon} \;\text{m}^{-2}\; \text{s}^{-1}$ measured with a 4-minute-binned light curve, corresponding to $\sim$180\% of the flux which is observed from the Crab Nebula above the same energy threshold. Variability contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in VLBA observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models which invoke relativistic plasma passing stationary shocks.

astro-ph.HE↗

arXiv2018

The extreme blazar AO 0235+164 as seen by extensive ground and space radio observations

Clues to the physical conditions in radio cores of blazars come from measurements of brightness temperatures as well as effects produced by intrinsic opacity. We study the properties of the ultra compact blazar AO 0235+164 with RadioAstron ground-space radio interferometer, multi-frequency VLBA, EVN and single-dish radio observations. We employ visibility modeling and image stacking for deriving structure and kinematics of the source, and use Gaussian process regression to find the relative multi-band time delays of the flares. The multi-frequency core size and time lags support prevailing synchrotron self absorption. The intrinsic brightness temperature of the core derived from ground-based VLBI is close to the equipartition regime value. In the same time, there is evidence for ultra-compact features of the size of less than 10 $μ$as in the source, which might be responsible for the extreme apparent brightness temperatures of up to $10^{14}$ K as measured by RadioAstron. In 2007--2016 the VLBI components in the source at 43 GHz are found predominantly in two directions, suggesting a bend of the outflow from southern to northern direction. The apparent opening angle of the jet seen in the stacked image at 43 GHz is two times wider than that at 15 GHz, indicating a collimation of the flow within the central 1.5 mas. We estimate the Lorentz factor $Γ= 14$, the Doppler factor $δ=21$, and the viewing angle $θ= 1.7^\circ$ of the apparent jet base, derive the gradients of magnetic field strength and electron density in the outflow, and the distance between jet apex and the core at each frequency.

astro-ph.GA↗

arXiv2017

Linear Polarization Properties of Parsec-Scale AGN~Jets

We used 15 GHz multi-epoch Very Long Baseline Array (VLBA) polarization sensitive observations of 484 sources within a time interval 1996--2016 from the MOJAVE program, and also from the NRAO data archive. We have analyzed the linear polarization characteristics of the compact core features and regions downstream, and their changes along and across the parsec-scale active galactic nuclei (AGN) jets. We detected a significant increase of fractional polarization with distance from the radio core along the jet as well as towards the jet edges. Compared to quasars, BL Lacs have a higher degree of polarization and exhibit more stable electric vector position angles (EVPAs) in their core features and a better alignment of the EVPAs with the local jet direction. The latter is accompanied by a higher degree of linear polarization, suggesting that compact bright jet features might be strong transverse shocks, which enhance magnetic field regularity by compression.

astro-ph.HE↗

arXiv2017

Multi-Frequency VLBA Polarimetry and the Twin-Jet Quasar 0850+581

We present the first multi-frequency VLBA study of the quasar 0850+581 which appears to have a two-sided relativistic jet.Apparent velocity in the approaching jet changes from 3.4c to 7c with the separation from the core. The jet-to-counter-jet ratio of about 5 and apparent superluminal velocities suggest that the observing angle of the inner jet is $\leq33^\circ$. It is likely that this orientation significantly changes downstream due to an interaction of the jet with the surrounding medium, signs of this are seen in polarization. A dense inhomogeneous Faraday screen is detected in the innermost regions of this quasar. We suggest that there is a presence of ionized gas in its nucleus, which might be responsible for the free-free absorption of the synchrotron emission in the jet and counter-jet at frequencies below 8.4~GHz. The experiment makes use of slowly varying instrumental polarisation factors (polarization leakage or D-terms) in time. We report application of the "D-term connection" technique for the calibration of an absolute orientation of electric vector position angle (EVPA) observed by VLBA at 4.6, 5.0, 8.1, 8.4, 15.4, 22.3, and 43.3 GHz bands during the 2007--2011.

astro-ph.HE↗

arXiv2017

MOJAVE: XV. VLBA 15 GHz Total Intensity and Polarization Maps of 437 Parsec-Scale AGN Jets From 1996-2017

We present 5321 milliarcsecond-resolution total intensity and linear polarization maps of 437 active galactic nuclei (AGNs) obtained with the VLBA at 15 GHz as part of the MOJAVE survey, and also from the NRAO data archive. The former is a long-term program to study the structure and evolution of powerful parsec-scale outflows associated with AGNs. The targeted AGNs are drawn from several flux-limited radio and gamma-ray samples, and all have correlated VLBA flux densities greater than about 50 mJy at 15 GHz. Approximately 80% of these AGNs are associated with gamma-ray sources detected by the Fermi LAT instrument. The vast majority were observed with the VLBA on 5 to 15 occasions between 1996 January 19 and 2016 December 26, at intervals ranging from a month to several years, with the most typical sampling interval being six months. A detailed analysis of the linear and circular polarization evolution of these AGN jets are presented in other papers in this series.

astro-ph.GA↗

arXiv2017

The high brightness temperature of B0529+483 revealed by RadioAstron and implications for interstellar scattering

The high brightness temperatures, $T_\mathrm{b}\gtrsim 10^{13}$ K, detected in several active galactic nuclei by RadioAstron space VLBI observations challenge theoretical limits. Refractive scattering by the interstellar medium may affect such measurements. We quantify the scattering properties and the sub-mas scale source parameters for the quasar B0529+483. Using RadioAstron correlated flux density measurements at 1.7, 4.8, and 22 GHz on projected baselines up to 240,000 km we find two characteristic angular scales in the quasar core, about 100 $μ$as and 10 $μ$as. Some indications of scattering substructure are found. Very high brightness temperatures, $T_\mathrm{b}\geq 10^{13}$ K, are estimated at 4.8 GHz and 22 GHz even taking into account the refractive scattering. Our findings suggest a clear dominance of the particle energy density over the magnetic field energy density in the core of this quasar.

astro-ph.HE↗

arXiv2017

VLBA polarimetric monitoring of 3C 111

We aim to better understand the dynamics within relativistic magneto-hydrodynamical flows in the extreme environment and close vicinity of supermassive black holes. To do so, we analyze the peculiar radio galaxy 3C 111, for which long-term polarimetric observations are available. We make use of the high spatial resolution of the VLBA network and the MOJAVE monitoring program, which provides high data quality also for single sources and allows us to study jet dynamics on parsec scales in full polarization with an evenly sampled time-domain. We additionally consider data from the IRAM 30-m Telescope as well as the SMA. Jet properties such as the electric vectors, the (polarized) flux density, feature size, and brightness temperature, describe a complex evolution of the polarized jet. The electric vector position angles (EVPAs) of features traveling down the jet perform a large and smooth rotation of $\gtrsim 180^{\circ}$ across a distance of about 20 pc. In contrast, the EVPAs are strongly variable within the first parsecs of the jet. We find a tendency towards transverse EVPAs across the jet with a local anomaly of aligned vectors in between. The transverse extent of the flow decreases coincident with a jump in brightness temperature around where we observe the EVPAs to turn into alignment with the jet flow. Also the gradients of the feature size and particle density with distance steepen in that region. We interpret the propagating polarized features with shocks and the observed local anomalies with the interaction of these shocks with a recollimation shock of the underlying flow. Together with a sheared magnetic field, this shock-shock interaction can explain the large rotation of the EVPA. The superimposed variability of the EVPAs close to the core is likely related to a clumpy Faraday screen, which also contributes significantly to the observed EVPA rotation in that region.

astro-ph.GA↗

arXiv2017

Multiwavelength observations of the blazar BL Lacertae: a new fast TeV $γ$-ray flare

Observations of fast TeV $γ$-ray flares from blazars reveal the extreme compactness of emitting regions in blazar jets. Combined with very-long-baseline radio interferometry measurements, they probe the structure and emission mechanism of the jet. We report on a fast TeV $γ$-ray flare from BL Lacertae observed by VERITAS, with a rise time of about 2.3 hours and a decay time of about 36 minutes. The peak flux at $>$200 GeV measured with the 4-minute binned light curve is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photons} \;\text{m}^{-2}\, \text{s}^{-1}$, or $\sim$180% the Crab Nebula flux. Variability in GeV $γ$-ray, X-ray, and optical flux, as well as in optical and radio polarization was observed around the time of the TeV $γ$-ray flare. A possible superluminal knot was identified in the VLBA observations at 43 GHz. The flare constrains the size of the emitting region, and is consistent with several theoretical models with stationary shocks.

astro-ph.HE↗

arXiv2017

Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron II. Observations of 3C 273 at minimum activity

RadioAstron is a 10 m orbiting radio telescope mounted on the Spektr-R satellite, launched in 2011, performing Space Very Long Baseline Interferometry (SVLBI) observations supported by a global ground array of radio telescopes. With an apogee of about 350 000 km, it is offering for the first time the possibility to perform μas-resolution imaging in the cm-band. We present observations at 22 GHz of 3C 273, performed in 2014, designed to reach a maximum baseline of approximately nine Earth diameters. Reaching an angular resolution of 0.3 mas, we study a particularly low-activity state of the source, and estimate the nuclear region brightness temperature, comparing with the extreme one detected one year before during the RadioAstron early science period. We also make use of the VLBA-BU-BLAZAR survey data, at 43 GHz, to study the kinematics of the jet in a 1.5-year time window. We find that the nuclear brightness temperature is two orders of magnitude lower than the exceptionally high value detected in 2013 with RadioAstron at the same frequency (1.4x10^13 K, source-frame), and even one order of magnitude lower than the equipartition value. The kinematics analysis at 43 GHz shows that a new component was ejected 2 months after the 2013 epoch, visible also in our 22 GHz map presented here. Consequently this was located upstream of the core during the brightness temperature peak. These observations confirm that the previously detected extreme brightness temperature in 3C 273, exceeding the inverse Compton limit, is a short-lived phenomenon caused by a temporary departure from equipartition. Thus, the availability of interferometric baselines capable of providing μas angular resolution does not systematically imply measured brightness temperatures over the known physical limits for astrophysical sources.

astro-ph.GA↗

arXiv2017

Milky Way scattering properties and intrinsic sizes of active galactic nuclei cores probed by very long baseline interferometry surveys of compact extragalactic radio sources

We have measured the angular sizes of radio cores of active galactic nuclei (AGN) and analyzed their sky distributions and frequency dependencies to study synchrotron opacity in AGN jets and the strength of angular broadening in the interstellar medium. We have used archival very long baseline interferometry (VLBI) data of more than 3000 compact extragalactic radio sources observed at frequencies, $ν$, from 2 to 43 GHz to measure the observed angular size of VLBI cores. We have found a significant increase in the angular sizes of the extragalactic sources seen through the Galactic plane ($|b|\lesssim10^\circ$) at 2, 5 and 8 GHz, about 1/3 of which show significant scattering. These sources are mainly detected in directions to the Galactic bar, the Cygnus region, and a region with galactic longitudes $220^\circ\lesssim l\lesssim260^\circ$ (the Fitzgerald window). The strength of interstellar scattering of the AGNs is found to correlate with the Galactic H$α$ intensity, free-electron density, and Galactic rotation measure. The dependence of scattering strengths on source redshift is insignificant, suggesting that the dominant scattering screens are located in our Galaxy. The observed angular size of Sgr A$^\ast$ is found to be the largest among thousands of AGN observed over the sky; we discuss possible reasons of this strange result. Excluding extragalactic radio sources with significant scattering, we find that angular size of opaque cores in AGN scales typically as $ν^{-1}$ confirming predictions of a conical synchrotron jet model with equipartition.

astro-ph.HE↗