arXiv subjects

M. Sikora

Publications and source records attributed to M. Sikora.

47 records · Page 3Linked to original sources

arXiv2001

Modeling the production of flares in $γ$-ray quasars

Theories of high energy radiation production in quasar jets can be verified by studies of both time-averaged spectra and variability patterns. While the former has been explored extensively, the latter is in its infancy. In this paper, we study the production of short-term flares in the shock-in-jet model. We examine how the flares' profiles depend on such parameters as shock/dissipation lifetime, electron-injection time profile, adiabaticity, and half-opening angle of the jet. In particular, we demonstrate the large difference between flare profiles produced in the radiative and adiabatic regimes. We apply our model to the $\sim$day timescale flares observed in optically violently variable (OVV) quasars, checking whether the external-radiation-Compton (ERC) model for $γ$-ray flares at energies $> 30$ MeV (EGRET range) can be reconciled with the flares observed at lower energies. Specifically, we show that the strict correlation between X-ray and $γ$-ray flares strongly supports the dominance of the synchrotron self-Compton mechanism in the X-ray band. We also derive conditions that must be satisfied by the ERC model in order to explain a lag of the $γ$-ray peak behind the optical one, as claimed to be observed in PKS 1406-076. Finally, we predict that in ERC models where the MeV peak is related to the break in electron distribution due to inefficient cooling of electrons below a certain energy, the flares should decay significantly more slowly in the soft $γ$-ray band than at energies greater than 30 MeV.

astro-ph↗

arXiv2001

Modeling Flares Produced in Blazars

We model light curves of flares observed in blazars, assuming that they are produced by relativistic electrons/positrons injected by forward-reverse shocks traveling down the jet with relativistic speeds. We approximate the radiating region as a thin shell enclosed between the two shock fronts. In addition to the conventional radiation processes such as synchrotron radiation and its Comptonization, we take into account Comptonization of light originating in the broad emission line region, and Comptonization of IR radiation from hot dust. We demonstrate that flares produced in the adiabatic regime decay much slower than flares produced in the radiative regime. The lack of such a difference between the X-ray and $γ$-ray flares in 3C279 strongly supports the synchrotron-self Compton (SSC) mechanism as responsible for X-ray production in this object. Finally, we discuss a possible scenario which can explain the `cooling' nature of the MeV peak in blazars by comparing the flares produced in soft $γ$-ray band and above 100 MeV.

astro-ph↗

arXiv2001

Jets in Quasars

In my review of jet phenomena in quasars, I focus on the following questions: How powerful are jets in radio-loud quasars? What is their composition? How are they launched? And why, in most quasars, are they so weak? I demonstrate the exceptional role that blazar studies can play in exploring the physics and structure of the innermost parts of quasar jets.

astro-ph↗

arXiv2001

Blazars

In our review of the blazar phenomenon, we discuss blazar models, with a focus on the following issues: sub-parsec jets and their environment; energy dissipation and particle acceleration; and radiative processes.

astro-ph↗

arXiv2000

Comptonization of Infrared Radiation from Hot Dust by Relativistic Jets in Quasars

We demonstrate the importance of near-infrared radiation from hot dust for Compton cooling of electrons/positrons in quasar jets. In our model, we assume that the non-thermal radiation spectra observed in OVV quasars are produced by relativistic electrons/positrons accelerated in thin shells which propagate down the jet with relativistic speeds. We show that the Comptonization of the near-IR flux is likely to dominate the radiative output of OVV quasars in the energy range from tens of keV up to hundreds of MeV, where it exceeds that produced by Comptonization of the UV radiation reprocessed and rescattered in the Broad Emission Line region. The main reason for this lies in the fact that the jet encounters the ambient IR radiation over a relatively large distance as compared to the distance where the energy density of the broad emission line light peaks. In the soft - to mid energy X-ray band, the spectral component resulting from Comptonization of the near-IR radiation joins smoothly with the synchrotron-self-Compton component, which may be responsible for the soft X-ray flux. At the highest observed gamma-ray energies, in the GeV range, Comptonization of broad emission lines dominates over other components.

astro-ph↗

arXiv1999

Modelling blazar radiation spectra Electron injection break and pair content of quasar jets

We study the dependence of nonthermal radiation spectra in OVV quasars on location of the low energy break in the electron/positron injection function. We show that the high energy spectra produced during the outbursts are presumably superposed from two components, one resulting from Comptonization of emission lines, which dominates at MeV-GeV energies, and the other resulting from Comptonization of infrared radiation, which dominates in the X-ray band.

astro-ph↗

arXiv1999

On Beaming Effects in Afterglow Light Curves

The most luminous GRBs can be explained in terms of models involving stellar mass central engines only if the ejecta are beamed. As was pointed out by Rhoads, the dynamics of the blast wave, formed by the beamed ejecta sweeping the external gas, can be significantly modified by the sideways expansion. This is because in this case the surface of the blast wave increases faster than just due to the radial divergence and so the blast wave deceleration rate increases faster. According to analytical estimates, the effect becomes important shortly after the bulk Lorentz factor of the blast wave drops below the inverse of the initial opening angle of the beamed ejecta and is accompanied by a sharp break in the afterglow light curve. However, our numerical studies, which follow the dynamical evolution of the blast wave, the evolution of the electron energy distribution, and take into account the light travel effects related to the lateral size of the source, show that the break of the light curve is weaker and much smoother than the one analytically predicted. A prominent break emerges only for a model without sideways expansion.

astro-ph↗

arXiv1999

X-ray Observations of BL Lacertae During 1997 Outburst and its Association with Quasar-like Characteristics

This paper reports on the X-ray emission from BL Lacertae during its July 1997 outburst as observed with the Rossi X-ray Timing Explorer (RXTE), compares the RXTE data to previous measurements, and interprets the overall electromagnetic emission in the context of the currently popular theoretical models. The source is bright and variable, with the 2 - 10 keV flux approximately two to three and a half times higher than measured in November 1995 by {\sl Asca}. The spectrum is also harder, with power law energy indices of $\sim 0.4 - 0.6$, compared to $\sim 0.9$ in Nov. 1995. Both in the optical band, where BL Lacertae now shows broad emission lines, and in the X-ray band, where the spectrum is hard, the overall electromagnetic distribution of BL Lacertae is similar to that observed in blazars associated with quasars rather than to that seen in the more common High-energy - peaked BL Lac - type objects (HBLs). We argue that the high energy (X-ray and $γ$-ray) emission from BL Lacertae consists of two spectral components: X-rays are produced by Comptonization of synchrotron radiation, while the $γ$-rays produced by Comptonization of the broad emission line flux.

astro-ph↗

arXiv1998

On the spin paradigm and the radio dichotomy of quasars

We investigate whether models based on the assumption that jets in quasars are powered by rotating black holes can explain the observed radio dichotomy of quasars. We show that in terms of the ``spin paradigm'' models, radio-loud quasars could be objects in which the black hole's rotation rate corresponds to an equilibrium between spin-up by accretion and spin-down by the Blandford-Znajek mechanism. Radio-quiet quasars could be hosting black holes with an average spin much smaller than the equilibrium one. We discuss possible accretion scenarios which can lead to such a bimodal distribution of black hole spins.

astro-ph↗

arXiv1997

Bending relativistic jets in AGNs

We present simulations of relativistic jets propagating in a nonuniform medium. Specifically, we study the bending of jets propagating obliquely to the vector of the density-gradient. Our results are applied to the NGC 4258, where such a medium is assumed to be provided by the atmosphere of the sub-parsec accretion disk tilted with respect to the original direction of the jet propagation. As a result, the jet is bent on a scale comparable to the density scaleheight of the disk atmosphere. The magnitude of the bending effect is found to be largest for light jets with low Lorentz factors. The predicted direction of bending is consistent with the observations.

astro-ph↗

arXiv1997

Implication of the discovery of the proper motion of the optical counterpart of GRB 970228 for the models of gamma-ray bursts

The accurate position determination of GRB 970228 by the Beppo/SAX Satellite led to the discovery of a fading X-ray and optical counterparts of the burst. About a month after the GRB event the proper motion of the ejecta was detected and the extended optical source has faded below the Keck detection level. We analyze these observations in the framework of the most popular gamma ray burst models. We find a lower limit on the distance to GRB 970228 d>6.6 pc. We estimate the amount of energy required to explain the motion of a point source and the variability of the extended source. We find that the cosmological models are ruled out. The Galactic corona models suffer difficulties if the distance to the burst is > 100 kpc or the variability of the extended source is directly connected with the burst No constraints are found for GRBs originating between 6.6 pc and $\sim 10$ kpc.

astro-ph↗