arXiv subjects

Marek Sikora

Publications and source records attributed to Marek Sikora.

72 records · Page 4Linked to original sources

arXiv2007

The Case of the 300 kpc Long X-ray Jet in PKS 1127-145 at z=1.18

The complex X-ray morphology of the 300 kpc long X-ray jet in PKS1127-145 (z=1.18 quasar) is clearly discerned in a ~100 ksec Chandra observation. The jet X-ray surface brightness gradually decreases by an order of magnitude going out from the core. The X-ray spectrum of the inner jet is relatively flat with alpha_X=0.66+/-0.15 and steep in the outer jet with alpha_X=1.0+/-0.2. The X-ray and radio jet intensity profiles are strikingly different, with the radio emission peaking strongly at the two outer knots while the X-ray emission is strongest in the inner jet region. We discuss the constraints implied by these data on the X-ray emission models and conclude that ``one-zone'' models fail and that at least a two-component model is needed to explain the jet's broadband emission. We propose that the X-ray emission originates in the jet proper while the bulk of the radio emission comes from a surrounding jet sheath. We also consider intermittent jet activity as a possible cause of the observed jet morphology.

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arXiv2007

Black-Hole Spin and Galactic Morphology

We investigate the conjecture by Sikora, Stawarz & Lasota (2007) that the observed AGN--radio-loudness bimodality can be explained by the morphology-related bimodality of black-hole spin distribution in the centers of galaxies: central black holes in giant elliptical galaxies may have (on average) much larger spins than black holes in spiral/disc galaxies. We study how accretion from a warped disc influences the evolution of black hole spins and conclude that within the cosmological framework, where the most massive BHs have grown in mass via merger driven accretion, one indeed expects most supermassive black holes in elliptical galaxies to have on average higher spin than black holes in spiral galaxies, where random, small accretion episodes (e.g. tidally disrupted stars, accretion of molecular clouds) might have played a more important role.

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arXiv2006

The 300 kpc Long X-ray Jet in PKS 1127-145, z=1.18 Quasar: Constraining X-ray Emission Models

We present a ~100 ksec Chandra X-ray observation and new VLA radio data of the large scale, 300 kpc long X-ray jet in PKS1127-145, a radio loud quasar at redshift z=1.18. With this deep X-ray observation we now clearly discern the complex X-ray jet morphology and see substructure within the knots. The X-ray and radio jet intensity profiles are seen to be strikingly different with the radio emission peaking strongly at the two outer knots while the X-ray emission is strongest in the inner jet region. The jet X-ray surface brightness gradually decreases by an order of magnitude going out from the core. The new X-ray data contain sufficient counts to do spectral analysis of the key jet features. The X-ray energy index of the inner jet is relatively flat with alpha_X = 0.66 +/-0.15 and steep in the outer jet with alpha_X = 1.0 +/-0.2. We discuss the constraints implied by the new data on the X-ray emission models and conclude that ``one-zone'' models fail and at least a two component model is needed to explain the jet's broad-band emission. We propose that the X-ray emission originates in the jet proper while the bulk of the radio emission comes from a surrounding jet sheath. We also consider intermittent jet activity as a possible cause of the observed jet morphology.

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arXiv2005

Are Quasar Jets Matter or Poynting Flux Dominated?

If quasar jets are accelerated by magnetic fields but terminate as matter dominated, where and how does the transition occur between the Poynting-dominated and matter-dominated regimes? To address this question, we study constraints which are imposed on the jet structure by observations at different spatial scales. We demonstrate that observational data are consistent with a scenario where the acceleration of a jet occurs within 10^{3-4} R_g. In this picture, the non-thermal flares -- important defining attributes of the blazar phenomenon - are produced by strong shocks formed in the region where the jet inertia becomes dominated by matter. Such shocks may be formed due to collisions between the portions of a jet accelerated to different velocities, and the acceleration differentiation is very likely to be related to global MHD instabilities.

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arXiv2005

Klein-Nishina Effects in the Spectra of Non-Thermal Sources Immersed in External Radiation Fields

We study Klein-Nishina (KN) effects in the spectrum produced by a steady state, non-thermal source where rapidly accelerated electrons cool by emitting synchrotron radiation and Compton upscattering ambient photons produced outside the source. We focus on the case where the radiation density inside the source exceeds that of the magnetic field. We show that the KN reduction in the electron Compton cooling rate causes the steady-state electron spectrum to harden at energies above γ_{KN}, where γ_{KN}= 1/4ε_0 and ε_0=hν_0/m_ec^2 is the characteristic ambient photon energy. The source synchrotron spectrum thus shows a high-energy ``bump'' or excess even though the electron acceleration spectrum has no such excess. In contrast, the low-energy Compton gamma-ray spectrum shows little distortion because the electron hardening compensates for the KN decline in the scattering rate. For sufficiently high electron energies, however, Compton cooling becomes so inefficient that synchrotron cooling dominates -- an effect omitted in most previous studies. The hardening of the electron distribution thus stops, leading to a rapid decline in Compton gamma-ray emission, i.e., a strong spectral break whose location does not depend on the maximum electron energy. This break can limit the importance of Compton gamma-ray pair production on ambient photons and implies that a source's synchrotron luminosity may exceed its Compton luminosity even though the source magnetic field energy density is smaller than the ambient radiation energy density. We discuss the importance of these KN effects in blazars, micro-quasars, and pulsar binaries.

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arXiv2005

Are quasar jets dominated by Poynting flux?

The formation of relativistic astrophysical jets is presumably mediated by magnetic fields threading accretion disks and central, rapidly rotating objects. As it is accelerated by magnetic stresses, the jet's kinetic energy flux grows at the expense of its Poynting flux. However, it is unclear how efficient is the conversion from magnetic to kinetic energy and whether there are any observational signatures of this process. We address this issue in the context of jets in quasars. Using data from all spatial scales, we demonstrate that in these objects the conversion from Poynting-flux-dominated to matter-dominated jets is very likely to take place closer to the black hole than the region where most of the Doppler boosted radiation observed in blazars is produced. We briefly discuss the possibility that blazar activity can be induced by global MHD instabilities, e.g., via the production of localized velocity gradients that lead to dissipative events such as shocks or magnetic reconnection, where acceleration of relativistic particles and production of non-thermal flares is taking place.

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arXiv2003

X-Ray Emission from the Quasar PKS 1127-145: Comptonized IR Photons on Parsec Scales

We model the broad-band spectral energy distribution of the innermost "core-jet" region of the redshift z=1.187 quasar PKS 1127-145. We propose a scenario where the high energy photons are produced via the Compton scattering of thermal IR radiation by the relativistic particles in a parsec-scale jet. The high energy spectrum, together with the observed radio variability and superluminal expansion, suggest that PKS 1127-145 may be a blazar, despite the fact that its optical/UV component is likely dominated by thermal radiation from an accretion disk. The relation of PKS 1127-145 to MeV - blazars is discussed.

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arXiv2002

Learning about Jets from Observations of Blazars

Jets, paving their way outward through the inner regions of active nuclei, Compton-interact with the UV radiation from an accretion disc and broad emission line region. We calculate the predicted properties of the resulting spectral signatures of this bulk-Compton process, noting that they are independent on the fractional proton content or kinetic power of the jet, and use the presence or absence of such signatures to put constraints on the structure of jets near their bases.

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arXiv2002

On Radiation Drag and Neutrons in GRBs

As it was recently shown by Derishev etal (1999), initial GRB ejecta are likely to contain comparable number of protons and neutrons. During acceleration process, and/or later, due to interaction with external medium, such ejecta are likely to be split into spatially distinct shells of neutron and proton-electron plasmas. This leads to dynamical effects which can affect the afterglow light curves. In this paper we study these effects including, for the first time, radiation drag imposed on the neutron rich ejecta. In the presence of efficient radiation drag and for typical ejecta Lorentz factor (below 400), this is the neutron shell which moves faster and, after conversion to proton-electron plasma, powers the afterglow. After certain amount of deceleration, this shell is hit by the second one. Such collision will lead to reflaring, as observed in some afterglow light curves.

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arXiv2002

On the Nature of MeV-blazars

Broad-band spectra of the FSRQ (flat-spectrum-radio quasars) detected in the high energy gamma-ray band imply that there may be two types of such objects: those with steep gamma-ray spectra, hereafter called MeV-blazars, and those with flat gamma-ray spectra, GeV-blazars. We demonstrate that this difference can be explained in the context of the ERC (external-radiation-Compton) model using the same electron injection function. A satisfactory unification is reachable, provided that: (a) spectra of GeV-blazars are produced by internal shocks formed at the distances where cooling of relativistic electrons in a jet is dominated by Comptonization of broad emission lines, whereas spectra of MeV-blazars are produced at the distances where cooling of relativistic electrons is dominated by Comptonization of near-IR radiation from hot dust; (b) electrons are accelerated via a two step process and their injection function takes the form of a double power-law, with the break corresponding to the threshold energy for the diffusive shock acceleration. Direct predictions of our model are that, on average, variability time scales of the MeV-blazars should be longer than variability time scales of the GeV-blazars, and that both types of the blazar phenomenon can appear in the same object.

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arXiv2001

On Energetics and Structure of Sub-parsec Jets in Quasars

This paper reviews the topic of sub-parsec - scale jets in quasars, and covers the the following issues: observations of parsec and sub-parsec scale jets; energy dissipation and particle acceleration; radiative processes; magnetic fields, pair content, and energetics; and variability and its relation to the central engine activity. In particular, we describe how internal shocks can explain properties of gamma-ray flares and demonstrate that MeV blazars (those with luminosity peak in the 1-30 MeV range) can be unified with GeV blazars (those with luminosity peak at GeV energies) assuming that in GeV blazars the gamma-ray flares are produced via Comptonization of broad emission lines, whereas in the MeV blazars they result from Comptonization of infrared radiation of hot dust. We also make predictions about the radiative effects of bulk Compton process in the soft X-ray band and show how spectral and variability properties in that band can be used to constrain structure of jets near their bases.

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arXiv2001

Synergy Between Observations of AGN with GLAST and MAXI - Detecting X-ray Precursors of Gamma-ray Flares in Blazars

The EGRET instrument onboard of CGRO detected a number of gamma-ray flares from blazars, and it is expected that GLAST, to be launched in 2006, should detect many more such flares. These flares should be preceded by X-ray precursors, and the MAXI All-Sky Monitor, to be deployed around the time of launch of GLAST, is likely to detect such precursors. This paper highlights the interesting new insights into the structure of jets in blazars that may result from simultaneous X-ray and gamma-ray observations. In particular, it addresses the expected temporal relationship between the precursors and flares, and the expected luminosities of the precursors.

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arXiv1999

On Pair Content and Variability of Sub-Parsec Jets in Quasars

X-ray observations of blazars associated with the OVV (Optically Violently Variable) quasars put strong constraints on the electron - positron pair content of radio-loud quasar jets. From those observations, we infer that jets in quasars contain many more electron - positron pairs than protons, but dynamically are still dominated by protons. In particular, we show that pure electron - positron jet models can be excluded, as they overpredict soft X-ray radiation; likewise, pure proton - electron jets can be excluded, as they predict too weak nonthermal X-ray radiation. An intermediate case is viable. We demonstrate that jets which are initially proton-electron ("proto-jets") can be pair-loaded via interaction with 100 - 300 keV photons produced in hot accretion disc coronae, likely to exist in active galactic nuclei in general. If the coronal radiation is powered by magnetic flares, the pair loading is expected to be non-uniform and non-axisymmetric. Together with radiation drag, this leads to velocity and density perturbations in a jet and formation of shocks, where the pairs are accelerated. Such a scenario can explain rapid (time scale of about a day) variability observed in OVV quasars.

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arXiv1997

Variability, Power, and Pair Content of AGN Jets

Simultaneous observations of blazars in different spectral regimes imply that the amplitude of variability depends on the observational band. Both in the low energy spectral component and in the high energy spectral component, the amplitude increases with the photon energy. We show that such behavior can be explained in terms of a two-component model, where the spectra observed during flares are superpositions of spectra arising from at least two distinct, spatially separated sources. Using the two-component model for blazar variability we derive constraints on jet physics in quasars. Particularly, we demonstrate that if production of X-rays during rapid flares is dominated by Comptonization of external radiation, then the upper limits for the total power of jets imply that the jet plasma is strongly dominated by pairs.

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arXiv1997

Radiation Processes in Blazars

We present an overview of the current theoretical models attempting to describe the structure and radiative processes operating in blazars and discuss the observational constraints that these models must confront.

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arXiv1997

On Black Hole Spins and Dichotomy of Quasars

Most quasars are known to be radio quiet and according to the ``spin paradigm'', which connects their radio-loudness with the value of the black hole spin, they must harbor very slowly rotating black holes. On the other hand, quasars are powered by accretion which tends to increase the spin of the black hole. We show that the Blandford-Znajek mechanism is not efficient enough to counteract the spinning up of black holes by an accretion disk, regardless of the accretion rate. We establish conditions under which it is possible to obtain low values of the black hole spin, in the scenario involving a sequence of many accretion events with random angular momenta. Our results are used to select possible evolutionary scenarios which can explain radio bimodality of quasars in terms of the ``spin paradigm''.

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arXiv1996

The Compton Mirror in NGC 4151

We show that the sharp cutoff in the hard X-ray spectrum of NGC 4151, unusual for Seyfert 1 galaxies, can be reconciled with the average Seyfert 1 spectrum if we assume that the central source is completely hidden from our line of sight by the thick part of the accretion disk or by the broad emission line clouds. The observed X-ray radiation is produced by scattering of the Seyfert 1-type spectrum in the higher, cooler parts of the accretion disk corona, or in a wind. A sharp cutoff appears as a result of the Compton recoil effect. This model naturally explains a discrepancy regarding the inclination of the central source, inferred to be low (face-on) from observations of the iron $Kα$ emission line, but inferred to be high on the basis of optical and UV observations.

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arXiv1995

ON THE GEOMETRY OF THE X-RAY EMITTING REGION IN SEYFERT GALAXIES

For the first time, detailed radiative transfer calculations of Comptonized X-ray and gamma-ray radiation in a hot pair plasma above a cold accretion disk are performed using two independent codes and methods. The simulations include both energy and pair balance as well as reprocessing of the X- and gamma-rays by the cold disk. We study both plane-parallel coronae as well as active dissipation regions having shapes of hemispheres and pill boxes located on the disk surface. It is shown, contrary to earlier claims, that plane-parallel coronae in pair balance have difficulties in selfconsistently reproducing the ranges of 2-20 keV spectral slopes, high energy cutoffs, and compactnesses inferred from observations of type 1 Seyfert galaxies. Instead, the observations are consistent with the X-rays coming from a number of individual active regions located on the surface of the disk. A number of effects such as anisotropic Compton scattering, the reflection hump, feedback to the soft photon source by reprocessing, and an active region in pair equilibrium all conspire to produce the observed ranges of X-ray slopes, high energy cutoffs, and compactnesses. The spread in spectral X-ray slopes can be due to a spread in the properties of the active regions such as their compactnesses and their elevations above the disk surface. Simplified models invoking isotropic Comptonization in spherical clouds are no longer sufficient when interpreting the data.

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