arXiv subjects

Y. Y. Kovalev

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

At least 181 records · Page 10Linked to original sources

arXiv2009

Fermi Discovery of Gamma-Ray Emission from NGC 1275

We report the discovery of high-energy (E>100 MeV) gamma-ray emission from NGC 1275, a giant elliptical galaxy lying at the center of the Perseus cluster of galaxies, based on observations made with the Large Area Telescope (LAT) of the Fermi Gamma ray Space Telescope. The positional center of the gamma-ray source is only ~3' away from the NGC 1275 nucleus, well within the 95% LAT error circle of ~5'.The spatial distribution of gamma-ray photons is consistent with a point source. The average flux and power-law photon index measured with the LAT from 2008 August 4 to 2008 December 5 are F_gamma = (2.10+-0.23)x 10^{-7} ph (>100 MeV) cm^{-2} s^{-1} and Gamma = 2.17+-0.05, respectively. The measurements are statistically consistent with constant flux during the four-month LAT observing period.Previous EGRET observations gave an upper limit of F_gamma < 3.72x 10 ^{-8} ph (>100 MeV) cm^{-2} s^{-1} to the gamma-ray flux from NGC 1275. This indicates that the source is variable on timescales of years to decades, and therefore restricts the fraction of emission that can be produced in extended regions of the galaxy cluster. Contemporaneous and historical radio observations are also reported. The broadband spectrum of NGC 1275 is modeled with a simple one-zone synchrotron/synchrotron self-Compton model and a model with a decelerating jet flow.

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arXiv2009

A Connection Between Apparent VLBA Jet Speeds and Initial Active Galactic Nucleus Detections Made by the Fermi Gamma-ray Observatory

In its first three months of operations, the Fermi Gamma-Ray Observatory has detected approximately one quarter of the radio-flux-limited MOJAVE sample of bright flat-spectrum active galactic nuclei (AGNs) at energies above 100 MeV. We have investigated the apparent parsec-scale jet speeds of 26 MOJAVE AGNs measured by the Very Long Baseline Array (VLBA) that are in the LAT bright AGN sample (LBAS). We find that the gamma-ray bright quasars have faster jets on average than the non-LBAS quasars, with a median of 15 c, and values ranging up to 34 c. The LBAS AGNs in which the LAT has detected significant gamma-ray flux variability generally have faster jets than the nonvariable ones. These findings are in overall agreement with earlier results based on nonuniform EGRET data which suggested that gamma-ray bright AGNs have preferentially higher Doppler boosting factors than other blazar jets. However, the relatively low LAT detection rates for the full MOJAVE sample (24%) and previously known MOJAVE EGRET-detected blazars (43%) imply that Doppler boosting is not the sole factor that determines whether a particular AGN is bright at gamma-ray energies. The slower apparent jet speeds of LBAS BL Lac objects and their higher overall LAT detection rate as compared to quasars suggest that the former are being detected by Fermi because of their higher intrinsic (unbeamed) gamma-ray to radio luminosity ratios.

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arXiv2009

On the relation between AGN gamma-ray emission and parsec-scale radio jets

We have compared the radio emission from a sample of parsec-scale AGN jets as measured by the VLBA at 15 GHz, with their associated gamma-ray properties that are reported in the Fermi LAT 3-month bright source list. We find in our radio-selected sample that the gamma-ray photon flux correlates well with the quasi-simultaneously measured compact radio flux density. The LAT-detected jets in our radio-selected complete sample generally have higher compact radio flux densities, and their parsec-scale cores are brighter (i.e., have higher brightness temperature) than the jets in the LAT non-detected objects. This suggests that the jets of bright gamma-ray AGN have preferentially higher Doppler-boosting factors. In addition, AGN jets tend to be found in a more active radio state within several months from LAT-detection of their strong gamma-ray emission. This result becomes more pronounced for confirmed gamma-ray flaring sources. We identify the parsec-scale radio core as a likely location for both the gamma-ray and radio flares, which appear within typical timescales of up to a few months of each other.

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arXiv2009

Spectral Analysis of the Accretion Flow in NGC 1052 with Suzaku

We present an analysis of the 101 ks, 2007 Suzaku spectrum of the LINER galaxy NGC 1052. The 0.5-10 keV continuum is well-modeled by a power-law modified by Galactic and intrinsic absorption, and it exhibits a soft, thermal emission component below 1 keV. Both a narrow core and a broader component of Fe K emission centered at 6.4 keV are robustly detected. While the narrow line is consistent with an origin in material distant from the black hole, the broad line is best fit empirically by a model that describes fluorescent emission from the inner accretion disk around a rapidly rotating black hole. We find no evidence in this observation for Comptonized reflection of the hard X-ray source by the disk above 10 keV, however, which casts doubt on the hypothesis that the broad iron line originates in the inner regions of a standard accretion disk. We explore other possible scenarios for producing this spectral feature and conclude that the high equivalent width (EW ~ 185 keV) and full-width-half-maximum velocity of the broad iron line (v ~ 0.37c) necessitate an origin within d ~ 8 gravitational radii of the hard X-ray source. Based on the confirmed presence of a strong radio jet in this galaxy nucleus, the broad iron line may be produced in dense plasma near the base of the jet, implying that emission mechanisms in the centralmost portions of active galactic nuclei are more complex than previously thought.

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arXiv2009

Parsec-scale properties of GHz-Peaked Spectrum sources from 2.3 and 8.6 GHz VLBI surveys

We investigate the sample of 213 GPS sources selected from simultaneous multi-frequency 1-22 GHz observations obtained with RATAN-600 radio telescope. We use publicly available data to characterize parsec-scale structure of the selected sources. Among them we found 121 core dominated sources, 76 Compact Symmetric Object (CSO) candidates (24 of them are highly probable), 16 sources have complex parsec-scale morphology. Most of GPS galaxies are characterized by CSO-type morphology and lower observed peak frequency (~1.8 GHz). Most of GPS quasars are characterized by "core-jet"-type morphology and higher observed peak frequency (~3.6 GHz). This is in good agreement with previous results. However, we found a number of sources for which the general relation CSO - galaxy, core-jet - quasar does not hold. These sources deserve detailed investigation. Assuming simple synchrotron model of a homogeneous cloud we estimate characteristic magnetic field in parsec-scale components of GPS sources to be B ~ 10 mG.

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arXiv2009

Parsec-scale Jet in the Distant Gigahertz-Peaked Spectrum Quasar PKS 0858-279

The high redshift GPS quasar PKS 0858-279 exhibits the following properties which make the source unusual. Our RATAN-600 monitoring of 1-22 GHz spectrum has detected broad-band radio variability with high amplitude and relatively short time scale. In the same time, the milliarcsecond scale structure observed in a snapshot VLBA survey turned out to be very resolved which is not expected from the fast flux density variations. We performed 1.4-22 GHz VLBA observations of this quasar in 2005-2007. It has revealed a core-jet morphology. A high Doppler factor delta is suggested for the jet, its nature is discussed in this report on the basis of the multi-frequency VLBA and RATAN data collected. Synchrotron self-absorption was confirmed to be dominating at low frequencies, the magnetic field strength of the dominating jet feature is estimated of an order of 0.1*delta mG.

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arXiv2009

Multiwavelength Observations of Markarian 421 in 2005 - 2006

Since September 2005, the Whipple 10m Gamma-ray Telescope has been operated primarily as a blazar monitor. The five Northern Hemisphere blazars that have already been detected at the Whipple Observatory, Markarian 421, H1426+428, Markarian 501, 1ES 1959+650 and 1ES 2344+514, are monitored routinely each night that they are visible. We report on the Markarian 421 observations taken from November 2005 to June 2006 in the gamma-ray, X-ray, optical and radio bands. During this time, Markarian 421 was found to be variable at all wavelengths probed. Both the variability and the correlations among different energy regimes are studied in detail here. A tentative correlation, with large spread, was measured between the X-ray and gamma-ray bands, while no clear correlation was evident among the other energy bands. In addition to this, the well-sampled spectral energy distribution of Markarian 421 (1101+384) is presented for three different activity levels. The observations of the other blazar targets will be reported separately.

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arXiv2009

A sample of GHz-peaked spectrum sources selected at RATAN-600: spectral and variability properties

We describe a new sample of 226 GPS (GHz-Peaked Spectrum) source candidates selected using simultaneous 1-22 GHz multi-frequency observations with the RATAN-600 radio telescope. Sixty objects in our sample are identified as GPS source candidates for the first time. The candidates were selected on the basis of their broad-band radio spectra only. We discuss the spectral and variability properties of selected objects of different optical classes.

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arXiv2008

Probing parsec scale jets in AGN with geodetic VLBI

We report on an ongoing effort to image active galactic nuclei simultaneously observed at 2.3 and 8.6 GHz in the framework of a long-term VLBI project RDV (Research and Development - VLBA) started in 1994 aiming to observe compact extragalactic radio sources in the astrometric/geodetic mode. Observations of bright extragalactic sources are carried out bi-monthly making up to six sessions per year with participation of all ten VLBA antennas and up to nine additional (geodetic and EVN) radio telescopes. Analysis of single-epoch results for 370 quasars, BL Lacs and radio galaxies is presented. We discuss VLBI core properties (flux densities, sizes, brightness temperatures), spectral characteristics of the cores and jets, evolution of brightness temperatures in the jets.

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arXiv2008

Adiabatic expansion and magnetic fields in AGN jets

Results of high-resolution simultaneous multi-frequency 8.1-15.4 GHz VLBA polarimetric observations of relativistic jets in active galactic nuclei (the MOJAVE-2 project) are analyzed. We compare characteristics of VLBI features with jet model predictions and test if adiabatic expansion is a dominating mechanism for the evolution of relativistic shocks in parsec-scale AGN jets. We also discuss magnetic field configuration, both predicted by the model and deduced from electric vector position angle measurements.

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arXiv2008

MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. V. Multi-epoch VLBA Images

We present images from a long term program (MOJAVE: Monitoring of Jets in AGN with VLBA Experiments) to survey the structure and evolution of parsec-scale jet phenomena associated with bright radio-loud active galaxies in the northern sky. The observations consist of 2424 15 GHz VLBA images of a complete flux-density limited sample of 135 AGN above declination -20 degrees, spanning the period 1994 August to 2007 September. These data were acquired as part of the MOJAVE and 2 cm Survey programs, and from the VLBA archive. The sample selection criteria are based on multi-epoch parsec-scale (VLBA) flux density, and heavily favor highly variable and compact blazars. The sample includes nearly all the most prominent blazars in the northern sky, and is well-suited for statistical analysis and comparison with studies at other wavelengths. Our multi-epoch and stacked-epoch images show 94% of the sample to have apparent one-sided jet morphologies, most likely due to the effects of relativistic beaming. Of the remaining sources, five have two-sided parsec-scale jets, and three are effectively unresolved by the VLBA at 15 GHz, with essentially all of the flux density contained within a few tenths of a milliarcsecond.

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arXiv2008

15 GHz VLBI detection of the HST-1 feature in the M87 jet

A bright feature 100 pc away from the core in the powerful jet of M 87 shows mysterious properties. Earlier radio, optical and X-ray observations have shown that this feature, labelled HST-1, is superluminal, and is possibly connected with the TeV flare detected by HESS in 2005. To examine the possible blazar-like nature of HST-1, we analyzed 2 cm VLBA data from dedicated full-track observations and the 2 cm survey/MOJAVE VLBI monitoring programs observed from 2000 to 2008. Applying wide-field imaging techniques, the HST-1 region was imaged at milliarcsecond resolutions. Here we present the first 15 GHz VLBI detection of this feature and discuss the connection between our radio findings and the TeV detection.

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arXiv2008

High-resolution studies of the twin jet in NGC 1052

The radio loud galaxy NGC 1052 is being studied in an intensive multi-band campaign including X-ray brigthness monitoring and spectroscopic observations, single-dish radio brightness monitoring at centimetre wavelengths, and a high-frequency very-long-baseline interferometry monitoring program. Here we present a progress report on our studies from this program. The final goal of our observations is to relate the findings from the high-resolution radio images with the observed variations in the X-ray regime, to address the accretion processes and their relationship with the radio jet activity.

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arXiv2008

Physics of the central region in the quasar 0850+581

The apparent position of the origin (core) of extragalactic radio jets shifts with the observing frequency, owing to synchrotron self-absorption and external absorption. One of the largest core shifts was detected by us in the quasar 0850+581 between 2 and 8 GHz. We have followed this up recently by a dedicated VLBA experiment at 5, 8, 15, 24, and 43 GHz. First results from this study enabled estimating the absolute geometry and physical conditions in the parsec-scale jet origin.

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arXiv2008

Large-amplitude intraday variability in QSO 1156+295 observed during a VLBI experiment

We present here the discovery of rapid, large amplitude intraday variability in the compact flat-spectrum radio quasar 1156+295. The detection of 40% flux density variations at 15 GHz on a timescale of only 2.7 hours was serendipitously made when the source was observed with the Very Long Baseline Array as a part of the MOJAVE survey programme on February 5, 2007. Intraday variability on timescales of a few hours or less is rare, and there exist very few sources that show large-amplitude variations on a timescale as short as what is now observed for 1156+295. The shape of the visibility function of the source changes very little during the observation, although the correlated flux density changes by 40%. This suggests that the variability occurs in a single dominant compact component. The observed variability characteristics are consistent with interstellar scintillation in nearby, highly turbulent medium. The rms amplitude of modulation at 15 GHz is unusually large and it implies a rather high scattering measure along the line-of-sight towards 1156+295.

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arXiv2008

Hydrodynamics of a perturbation in the jet of 3C111

We present results on the modelling of the ejection of a superluminal component in the jet of 3C111. We propose that the component is generated by an injection of dense material followed by a decrease in the injection rate of bulk particles in the jet. Our model is supported by 1D relativistic hydrodynamics and emission simulations, and is capable of reproducing the brightness evolution of two features, as revealed by 15 GHz VLBA observations. We show that other scenarios, such as an increase of the Lorentz factor in the material of the perturbation, fails to reproduce the observed evolution of this flare.

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arXiv2008

Serendipitous VLBI detection of rapid, large-amplitude, intraday variability in QSO 1156+295

We report a serendipitous detection of rapid, large amplitude flux density variations in the highly core-dominated, flat-spectrum radio quasar 1156+295 during an observing session at the Very Long Baseline Array (VLBA). The source was observed as a part of the MOJAVE survey programme with the VLBA at 15 GHz on February 5, 2007. Large amplitude variability in the correlated flux density, unexplainable in terms of the source structure, was first discovered while processing the data, and later confirmed by calibrating the antenna gains using 24 other sources observed in the experiment. The source shows variations in the correlated flux density as high as 40% on a timescale of only 2.7 hours. This places 1156+295 between the classical IDV sources and the so-called intra-hour variables. The observed variability timescale and the modulation index of 13% are consistent with interstellar scintillation by a nearby, highly turbulent scattering screen. The large modulation index at 15 GHz implies a scattering measure that is atypically high for a high galactic latitude source such as 1156+295.

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arXiv2008

On the nature of an ejection event in the jet of 3C111

We present a possible scenario for the ejection of a superluminal component in the jet of the Broad Line Radio Galaxy 3C111 in early 1996. VLBI observations at 15 GHz discovered the presence of two jet features on scales smaller than one parsec. The first component evolves downstream, whereas the second one fades out after 1 parsec. We propose the injection of a perturbation of dense material followed by a decrease in the injection rate of material in the jet as a plausible explanation. This scenario is supported by 1D relativistic hydrodynamics and emission simulations. The perturbation is modeled as an increase in the jet density, without modifying the original Lorentz factor in the initial conditions. We show that an increase of the Lorentz factor in the material of the perturbation fails to reproduce the observed evolution of this flare. We are able to estimate the lifetime of the ejection event in 3C111 to be 36\pm7 days.

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