514 research outputs found
Models of Accretion Disks
An accretion flow onto a supermassive black hole is the primary process
powering quasars. However, a geometry of this flow is not well constrained.
Both global MHD simulations and observations suggest that there are several
emission components present in the nucleus: an accretion disk, hot plasma
(corona or sphere) with electrons scattering the optical and UV photons, and an
outflow (wind/jet). The relative location and size of these emission
components, as well as their "interplay" affect the emerging quasar spectrum. I
review briefly standard accretion disk models and the recent progress, point
out discrepancies between the predicted and observed spectra and discuss some
issues in fitting these models to the broad-band spectral energy distribution
of quasars. I present examples of models fitted simultaneously to the
optical-UV-X-ray data and possible constraints on the parameters.Comment: Invited review presented at the conference on "The Multicolored
Landscape of Compact Objects and Their Explosive Origins" held on June 11-24,
2006 in Cefalu, Ital
X-ray emission from GPS and CSS sources
Many X-ray observations of GigaHertz Peaked Spectrum and Compact Steep
Spectrum sources have been made with Chandra X-ray Observatory and XMM-Newton
Observatory over the last few years. The X-ray spectra contribute the important
information to the total energy distribution of the compact radio sources. In
addition the spatial resolution of Chandra allows for studies of the X-ray
morphology of these sources on arcsec scales and provide a direct view of their
environments. This paper gives a review of the current status of the X-ray
observations and their contribution to our understanding of the nature of these
compact radio sources. It also describes primary physical processes that lead
to the observed X-ray emission and summarize X-ray emission properties expected
from interactions between an expanding radio source and the intergalactic
environment.Comment: 6 pages, 9 figures, to appear in the proceedings of the '4th Workshop
on Compact Steep Spectrum and Gigahertz-Peaked Spectrum Radio Sources', AN in
pres
Broad-band Jet Emission in Young and Powerful Radio Sources: the Case of the CSS Quasar 3C 186
We present the X-ray analysis of a deep ~200 ksec Chandra observation of the
compact steep spectrum radio-loud quasar 3C 186 (z=1.06) and investigate the
contribution of the unresolved radio jet to the total X-ray emission. The
spectral analysis is not conclusive on the origin of the bulk of the X-ray
emission. In order to examine the jet contribution to the X-ray flux, we model
the quasar spectral energy distribution (SED), adopting several scenarios for
the jet emission. For the values of the main physical parameters favored by the
observables, a dominant role of the jet emission in the X-ray band is ruled out
when a single zone (leptonic) scenario is adopted, even including the
contribution of the external photon fields as seed photons for inverse Compton
emission. We then consider a structured jet, with the blazar component that-
although not directly visible in the X-ray band - provides an intense field of
seed synchrotron photons Compton-scattered by electrons in a mildly
relativistic knot. In this case the whole X-ray emission can be accounted for
if we assume a blazar luminosity within the range observed from flat spectrum
radio quasars. The X-ray radiative efficiency of such (structured) jet is
intimately related to the presence of a complex velocity structure. The jet
emission can provide a significant contribution in X-rays if it decelerates
within the host galaxy, on kiloparsec scales. We discuss the implications of
this model in terms of jet dynamics and interaction with the ambient medium.Comment: 17 pages, 5 figures, 4 tables. Accepted for publication in Ap
Probing the accretion disk - jet connection via instabilities in the inner accretion flow. From microquasars to quasars
We present various instability mechanisms in the accreting black hole systems
which might indicate at the connection between the accretion disk and jet. The
jets observed in microquasars can have a persistent or blobby morphology.
Correlated with the accretion luminosity, this might provide a link to the
cyclic outbursts of the disk. Such duty-cycle type of behavior on short
timescales results from the thermal instability caused by the radiation
pressure domination. The same type of instability may explain the cyclic
radioactivity of the supermassive black hole systems. The somewhat longer
timescales are characteristic for the instability caused by the partial
hydrogen ionization. The distortions of the jet direction and complex
morphology of the sources can be caused by precession of the disk-jet axis.Comment: 2 pages, 2 figures; Proceedings of the 275 IAU Symposium "Jets at all
scales", Buenos Aires, 13-17.09.2010; eds. G. Romero, R. Sunyaev, T. Bellon
Non-linear shock acceleration and high energy gamma rays from clusters of galaxies
Merger and accretion shocks in clusters of galaxies can accelerate particles
via first order Fermi process. Since this mechanism is believed to be
intrinsically efficient, shocks are expected to be modified by the backreaction
of the accelerated particles. Such a modification might induce appreciable
effects on the non--thermal emission from clusters and a suppression of the
heating of the gas at strong shocks. Here we consider in particular the gamma
ray emission and we discuss the capability of Cherenkov telescopes such as HESS
to detect clusters at TeV energies.Comment: 6 pages, to appear in the proceedings of the Gamma 2004 Symposium on
High Energy Gamma Ray Astronomy, Heidelberg, July 2004 (AIP Proceedings
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