53 research outputs found
High Energy Phenomena in Blazars
Advances in the capabilities of X-ray, gamma-ray and TeV telescopes have
brought new information on the physics of relativistic jets, which are
responsible for the blazar "phenomenon". In particular the broad band
sensitivity of the BeppoSAX satellite, extending up to 100 KeV has allowed
unprecedented studies of their hard X-ray spectra. I summarize here some basic
results and present a unified view of the blazar population, whereby all
sources contain essentially similar jets despite diversities in other
properties, like the presence or absence of emission lines in their optical
spectra. Blazars with emission lines are of particular interest in that it is
possible to estimate both the luminosity of the jet and the luminosity of the
accretion disk. Implications for the origin of the power carried by
relativistic jets, possibly involving rapidly spinning supermassive black holes
are discussed. We suggest that emission line blazars are accreting at near
critical rates, while BL lacs, where emission lines are weak or absent are
highly subcritical.Comment: 11 pages, 7 figures, presented at the 20th Texas Symposium, 10-15
Dec. 200
A Model for the X-Ray and UV Emission from Seyfert Galaxies and Galactic Black Holes
We propose that the X--ray emission from radio quiet AGN and galactic black
holes is due to Comptonization of soft thermal photons emitted by the
underlying accretion disk in localized structures (blobs). The power per unit
area produced by the blobs, impinging on the disk, can easily dominate the
radiation internally produced by the disk. In this case the electron
temperature and the high energy spectrum can be determined in a similar way as
in the previously studied homogeneous model (Haardt \& Maraschi 1991). However
in the present model: a) the emitted spectrum is largely independent of the
{\it fraction} of gravitational power dissipated in the blobs; b) the X--ray
spectrum can be harder depending on a form factor of the blobs; c) the UV (or
soft X--ray for galactic objects) luminosity that is not intercepted by the
blobs can be larger than the X--ray luminosity. In the framework of a
simplified accretion disk dynamo model, we make order of
magnitude estimates of the number of active blobs, their size, luminosity and
hence their compactness, finding values in agreement with what is observed. The
expected UV to X--ray spectra and correlations of X--ray and UV light curves
are discussed.Comment: 10 pages. Plain Tex. Accepted in ApJ Letter
Blazars distance indications from Fermi and TeV data
A new method to constrain the distance of blazars with unknown redshift using
combined observations in the GeV and TeV regimes will be presented. The
underlying assumption is that the Very High Energy (VHE) spectrum corrected for
the absorption of TeV photons by the Extragalactic Background Light (EBL) via
photon-photon interaction should still be softer than the extrapolation of the
gamma-ray spectrum observed by Fermi/LAT. Starting from the observed spectral
data at VHE, the EBL-corrected spectra are derived as a function of the
redshift z and fitted with power laws. Comparing the redshift dependent VHE
slopes with the power law fits to the LAT data an upper limit to the source
redshift can be derived. The method is applied to all TeV blazars detected by
LAT with known distance and an empirical law describing the relation between
the upper limits and the true redshifts is derived. This law can be used to
estimate the distance of unknown redshift blazars: as an example, the distance
of PKS 1424+240 is inferred.Comment: Contribution to SciNeGHE 2010, Trieste, Italy, September 2010; 4
pages, 2 figur
Constraints to the SSC model for Mkn 501
We fit the SEDs of the TeV blazar Mkn 501 adopting the homogeneous
Synchrotron-Self Compton model to simultaneous X-ray and TeV spectra recently
become available. We present detailed model spectra calculated with the above
constraints and taking into account the absorption of TeV photons by the IR
background. We found that the curved TeV spectra can be naturally reproduced
even without IRB absorption. Taking IRB absorption into account changes the
required parameter values only slightly.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference
"X-Ray Astronomy '99", Bologna, Italy, September 199
TeV blazars and their distance
Recently, a new method to constrain the distance of blazars with unknown
redshift using combined observations in the GeV and TeV regimes has been
developed, with the underlying assumption that the Very High Energy (VHE)
spectrum corrected for the absorption of TeV photons by the Extragalactic
Background Light (EBL) via photon-photon interaction should still be softer
than the gamma-ray spectrum observed by Fermi/LAT. The constraints found are
related to the real redshifts by a simple linear relation, that has been used
to infer the unknown distance of blazars. The sample will be revised with the
up-to-date spectra in both TeV and GeV bands, the method tested with the more
recent EBL models and finally applied to the unknown distance blazars detected
at VHE.Comment: Contribution to "Cosmic Radiation Fields: Sources in the early
Universe", Desy, Germany, November 9-12, 2010; 6 pages, 3 figures (revised
version
X-ray variability and correlations in the two phase disk-corona model for Seyfert galaxies
We discuss in detail the broad band X-ray variability expected in Seyfert
galaxies in the framework of a Compton cooled corona model. Variations in the
optical depth tau of the corona cause spectral changes in the Comptonized
emission in the sense that the spectrum steepens and the temperature decreases
with increasing tau. If the corona is pair dominated, tau is determined by the
compactness (proportional to luminosity for fixed size). This yields a definite
relation between spectral shape and intensity implying a variation in intensity
of a factor 10 for a change of 0.2 in the 2-10 keV spectral index. For low pair
density coronae, no definite prediction is possible. In the absence of
substantial changes in the structure of the corona, the soft thermal component
correlates with the medium-hard Comptonized component. Examples are computed
using the ROSAT response matrix, showing that current observations of NGC 5548
and Mrk 766 showing different behaviours can be reconciled with the model. The
spectral behaviour of Mrk 766 strongly suggests that the coronal plasma in this
source is not pair dominated.Comment: 24 pages, AAS latex [11pt,aaspptwo,flushrt,tighten], + 9.ps figures
Accepted for pubblication in ApJ, 09 05 96 Also available at
http://fy.chalmers.se/~haardt/personal/curr.htm
Constraints on the Physical Parameters of TeV Blazars
We consider the constraints on the physical parameters of a homogeneous SSC
model that can be derived from the spectral shape and variability of TeV
blazars. Assuming that the relativistic electron spectrum is a broken power
law, where the break energy is a free parameter, we write the
analytical formulae that allow to connect the physical parameters of the model
to observable quantities. The constraints can be summarized in a plane where
the coordinates are the Doppler factor and the magnetic field. The consistency
between the break energy and the balance between cooling and escape and the
interpretation of the soft photon lags measured in some sources as radiative
cooling times are treated as additional independent constraints.
We apply themethod to the case of three well known blazars, PKS 2155-304, Mrk
421 and Mrk 501.Comment: 36 pages, incl. 6 figures in PS format, AAS LaTeX, to be published in
ApJ, Dec 199
- âŠ