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 $\alpha-\Omega$ 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 $\gamma_b$ 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