arXiv subjects

A. P. Lobanov

Publications and source records attributed to A. P. Lobanov.

At least 91 records · Page 5Linked to original sources

arXiv2005

Resolution limits in astronomical images

A method is introduced to derive resolution criteria for various a priori defined templates of brightness distribution fitted to represent structures and objects in astronomical images. The method is used for deriving criteria for the minimum and maximum resolvable sizes of a template. The minimum resolvable size of a template is determined by the ratio of (SNR-1)/SNR, and the maximum detectable size is determined by the ratio of 1/SNR. Application of these criteria is discussed in connection to data from filled-aperture instruments and interferometers, accounting for different aperture shapes and the effects of Fourier sampling, tapering, apodization and visibility weighting. Specific resolution limits are derived for four different templates of brightness distribution: (1) two-dimensional Gaussian, (2) optically thin spherical shell, (3) disk of uniform brightness, and (4) ring. The limiting resolution for these templates changes with SNR similarly to the quantum limit on resolution. Practical application of the resolution limits is discussed in two examples dealing with measurements of maximum brightness temperature in compact relativistic jets and assessments of morphology of young supernova remnants.

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arXiv2004

Simulations of the relativistic parsec-scale jet in 3C273

We present a hydrodynamical 3D simulation of the relativistic jet in 3C273, in comparison to previous linear perturbation analysis of Kelvin-Helmholtz instability developing in the jet. Our aim is to assess advantages and limitations of both analytical and numerical approaches and to identify spatial and temporal scales on which the linear regime of Kelvin-Helmholtz instability can be applied in studies of morphology and kinematics of parsec-scale jets.

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arXiv2004

Mergers and binary systems of SMBH in the contexts of nuclear activity and galaxy evolution

The dynamic evolution of binary systems of supermassive black holes (SMBH) may be a key factor affecting a large fraction of the observed properties of active galactic nuclei (AGN) and galaxy evolution. Different classes of AGN can be related in general to four evolutionary stages in a binary SMBH: 1) early merger stage; 2) wide pair stage; 3) close pair stage; and 4) pre-coalescence stage. This scheme can explain a variety of properties of AGN: radio and optical luminosity differences between different classes of AGN, long-term and short-term variability, quasi-periodic nuclear flares, recurrent formation of relativistic outflows in AGN and their apparent morphology and kinematics.

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arXiv2004

A supermassive binary black hole in the quasar 3C345

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 3C345, 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 3C345 is described by two equal-mass black holes with masses of approximately 7*10^8 solar masses separated by ~0.33 pc and orbiting with a period of ~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 3C345. The timescale of quasi-periodic flaring activity in 3C345 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.

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arXiv2004

The Twin--Jet System in NGC 1052: VLBI-Scrutiny of the Obscuring Torus

NGC 1052 offers the possibility to study the obscuring torus around a supermassive black hole, predicted by the standard model of active galactic nuclei, over a wide range of wavelengths from the radio to the X-ray regime. We present a detailed VLBI study of the parsec-scale structure of the ``twin-jet'' system in NGC 1052 in both total and polarized intensity and at multiple frequencies. We report the detection of linearly polarized emission from the base of the eastern jet at 5 GHz. While the radio spectrum in this region might be still consistent with synchrotron self absorption, the highly inverted spectrum of the western jet base represents a clear sign of pronounced free-free absorption in a circumnuclear torus. We observe an abrupt change of the brightness temperature gradient at a distance of $\sim 0.2$ pc to 0.3 pc from the central engine. This might provide an observational signature of the edge of the central torus, where the transition from an external pressure-dominated jet regime to a more or less freely expanding jet takes place. We determine the absorbing column density towards the western jet core to be $\sim 2.2 \times 10^{22}$ cm$^{-2}$ in good agreement with the values derived from various X-ray observations. This suggests that the nuclear X-ray emission and the jet emission imaged by VLBI originate on the same scales.

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arXiv2003

Quasi-Periodic Changes in the Parsec-Scale Jet of 3C345

We have observed the QSO 3C345 with VSOP at 1.6GHz and 5GHz and with the VLBA at 22GHz and 43GHz, with 24 epochs in total, from 1997.40 to 1999.69. This study, combined with previous observations, reveals periodic changes in the parsec-scale jet. This quasi-periodicity can be found in the initial position angles, the trajectories and the flux density evolution of the jet components, with a period of about 9 years. This period should reflect the dynamics of the material in the central region of 3C345 and it is possibly related to a putative binary black hole in the center of this object.

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arXiv2002

A Multi-Frequency Study of 3C309.1

Here we summarize our results from a detailed multi-frequency study of the QSO 3C309.1 based on the Very Long Baseline Array (VLBA) observations made in mid 1998. From our images, we find a curved jet extending up to 100 milliarcseconds (mas) to the east at low frequencies with two main components, A and B. A preliminary astrometric analysis (Ros and Lobanov 2001) provides a determination of the core position at different frequencies by phase-referencing to a nearby radio source, QSO S5 1448+76. The changes of the core position with frequency suggest high opacity close to the core caused by synchrotron self-absorption. Due to the large astrometric uncertainties we cannot draw any conclusions about the values of the opacity gradients at high frequencies. We believe that a detailed analysis of the frequency depedence of the core position will reveal the profile of the matter distribution in the broad line region, as was initially suggested by Lobanov (1998).

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arXiv2002

NGC1052 - A study of the pc-scale twin jet

We present results of a VLBA multi-frequency study of the pc-scale twin jet in NGC1052. We observed this object at epoch 1998.99 with the VLBA at 5, 8.4, 22 and 43 GHz both in total and linearly polarized intensity. The spectral analysis confirms the necessity of a free-free absorbing medium, obscuring the innermost part of both jets. At 5 GHz we found a compact linearly polarized emission region at the base of the eastern jet with a degree of polarization of 1.5%. At higher frequencies there is no evidence for polarization in our data. A core shift analysis constrains the position of the central engine to ~0.03 pc. The shift rates of the apparent core position with frequency confirm the strong influence of free-free absorption in conjunction with steep pressure gradients at the bases of both jets.

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arXiv2001

Opacity in the jet of 3C309.1

The "core" of a radio source is believed to mark the frequency-dependent location where the optical depth to synchrotron self--absorption tau_s~1. The frequency dependence can be used for derive physical conditions of the radio emitting region and the ambient environment near the central engine of the radio source. In order to test and improve the models to derive this information, we made multi-frequency dual-polarization observations of 3C309.1 in 1998.6, phase-referenced to the QSOs S5 1448+76 and 4C72.20 (S5 1520+72) (4d26' and 1d49' away, respectively). We present here preliminary results from these observations: total and polarized intensity maps, spectral information deduced from these images, and the relative position of 3C309.1 with respect to S5 1448+76 at different frequencies. Finally, we discuss briefly the observed shift of the core position.

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arXiv2001

The quasar 3C345 at the highest resolution with mm- and space-VLBI

The quasar 3C345 is one of the best examples of extragalactic jet with superluminal components moving on helical paths. To get better constraints on the jet physics, we have imaged the source at the highest possible resolution, using mm-VLBI and space-VLBI observations. We observed 3C345 at 5 different epochs so far (1 epoch with 3-mm ground VLBI, and 4 epochs with space VLBI at 6cm and 18cm). These measurements expand significantly our ongoing monitoring of 3C345. In this contribution, we discuss the properties of the jet and its moving components in the immediate vicinity of the core (<3mas).

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arXiv2000

VSOP observation of the quasar PKS 2215+020: a new laboratory for core-jet physics at z=3.572

We report results of a VSOP (VLBI Space Observatory Programme) observation of a high redshift quasar PKS 2215+020 (z=3.572). The ~1 milliarcsecond resolution image of the quasar reveals a prominent `core-jet' structure on linear scales from 5/h to 300/h pc ($H_0=100*h km/(s*Mpc). The brightness temperatures and sizes of bright features identified in the jet are consistent with emission from relativistic shocks dominated by adiabatic energy losses. The jet is powered by the central black hole with estimated mass of ~4*10^9 solar masses. Comparisons with VLA and ROSAT observations indicate a possible presence of an extended radio/X-ray halo surrounding 2215+020.

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arXiv2000

III Zw 2, the first superluminal jet in a Seyfert galaxy

So far all relativistically boosted jets with superluminal motion have only been detected in typical radio galaxies with early type host galaxies. We have now discovered superluminal motion in the Seyfert I galaxy III Zw 2, classified as a spiral. Superluminal motion was first inferred from the spectral evolution of the source and then confirmed by VLBI observations. The lower limit for the apparent expansion speed is 1.25 +/- 0.09 c. The fact that the spectral and spatial evolution are closely linked demonstrates that we are dealing with real physical expansion. Prior to this rapid expansion we have seen a period of virtually no expansion with an expansion speed less than 0.04 c. Since III Zw 2 is also part of a sample of so called radio-intermediate quasars (RIQ), it confirms earlier predictions of superluminal motion for this source, based on the argument that RIQs could be relativistically boosted jets in radio-weak quasars and Seyfert galaxies.

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arXiv1999

Total intensity and polarized emission of the parsec-scale jet in 3C345

We have studied the parsec-scale structure of the quasar 3C345 at λλ1.3, 2, 3.6, and 6 cm, using total intensity and linear polarization images from 3 epochs of VLBA observations made in 1995.84, 1996.41, and 1996.81. The images show an unresolved "core" and a radio jet extending over distances up to 25 milliarcseconds. At 1.3 and 2 cm, images of the linearly polarized emission reveal an alignment of the electric vector with the extremely curved inner section of the jet. At 6 cm, the jet shows strong fractional polarization (m~15%), with the electric vector being oriented perpendicular to the jet. This is consistent with the existence of a transition from the shock-dominated to the plasma interaction-dominated emission regimes in the jet, occurring at a distance of ~1.5 milliarcseconds from the "core".

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arXiv1999

Spectral Evolution of The Parsec-Scale Jet in The Quasar 3C345

The long-term evolution of the synchrotron emission from the parsec-scale jet in the quasar 3C345 is analysed, on the basis of multi-frequency monitoring with very long baseline interferometry (VLBI) and covering the period 1979-1994. We combine the model fits from 44 VLBI observations of 3C345 made at 8 different frequencies between 2.3 and 100GHz. We calculate the turnover frequency, turnover flux density, integrated 4-25GHz flux and 4-25GHz luminosity of the core and the moving features. The core has an estimated mean luminosity of 7.1+/-3.5 * 10^42 erg/s; the estimated total luminosity of 3C345 on parsec scales is ~3*10^43 erg/s (about 1% of the observed luminosity of the source between the radio to infrared regimes). The spectral changes in the core can be reconciled with a shock or dense plasma condensation traveling through the region where the jet becomes optically thin. We are able to describe the evolution of the core spectrum by a sequence of 5 flare-like events characterized by an exponential rise and decay of the particle number density of the material injected into the jet. The same model is also capable of predicting the changes in the flux density observed in the core. The flares occur approximately every 3.5-4 years, roughly correlating with appearances of new moving features in the jet.

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arXiv1999

Unusual radio variability in the BL Lac object 0235+164

We present radio observations at three frequencies and contemporaneous optical monitoring of the peculiar BL Lac object AO 0235+164. During a three-week campaign with the VLA we observed intraday variability in this source and found a distinct peak which can be identified throughout the radio frequencies and tentatively connected to the R-band variations. This event is characterized by unusual properties: its strength increases, and its duration decreases with wavelength, and it peaks earlier at 20cm than at 3.6 and 6cm. We discuss several generic models (a `standard' shock-in-jet model, a precessing beam, free-free-absorption in a foreground screen, interstellar scattering, and gravitational microlensing), and explore whether they can account for our observations. Most attempts at explaining the data on 0235+164 require an extremely small source size, which can be reconciled with the $10^{12}$ K inverse Compton limit only when the Doppler factor of the bulk flow is of order 100. However, none of the models is completely satisfactory, and we suggest that the observed variability is due to a superposition of intrinsic and propagation effects.

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arXiv1999

Radiospectra and kinematics in blazars

Broad band spectra of total and compact--scale radio emission from blazars, used in combination with kinematic information inferred from VLBI monitoring programs, can be applied successfully to better constrain the models of compact jets. We discuss here a ``hands-on'' approach for tying together the kinematic and spectral properties of radio emission from blazars.

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arXiv1999

Multiband polarimetric and total intensity imaging of 3C345

We monitored the superluminal QSO 3C 345 at three epochs during a one-year period in 1995--1996, observing with the VLBA at 22, 15, 8.4, and 5 GHz. We imaged the radio source both in total and in polarized intensity. In the images at 5 and 8.4 GHz, the jet emission is traced up to 20 milliarcseconds (mas) from the jet core. In the 15 and 22 GHz images, we identify several enhanced emission regions moving at apparent speeds of 5c. Images of the linear polarized emission show predominantly an alignment of the electric vector with the extremely curved jet along the inner part of the high frequency jet. At 5 GHz, the jet shows remarkably strong fractional polarization (m~15%) with the electric vector perpendicular to the jet orientation.

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arXiv1999

VLBA Monitoring of the QSO 3C345

We report results on the QSO 3C345 obtained from VLBA observations carried out on epochs 1995.84, 1996.41, and 1996.81, at 22, 15, 8.4 and 5 GHz. Total and polarized images VLBI images were obtained. We observed superluminal motions of components C8 and C7 with respect to the core component D. We imaged also the remarkably complex polarization structure near the core.

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