Gabriele Ghisellini

The Collimation-corrected Gamma-Ray Burst Energies Correlate with the Peak Energy of Their νFν Spectrum

We consider all bursts with known redshift and νFν peak energy, E. For a good fraction of them an estimate of the jet opening angle is available from the achromatic break of their afterglow light curve. This allows the derivation of the collimation-corrected energy of the bursts, Eγ. The distribution of the values of Eγ is more spread out than in previous findings, covering about 2 orders of magnitude. We find a surprisingly tight correlation between Eγ and the source frame Epeak: E(1 + z) ∝ E. This correlation can shed light on the still uncertain radiation processes for the prompt GRB emission. More importantly, if the small scatter of this newly found correlation could be confirmed by forthcoming data, it would be possible to use it for cosmological purposes.

Constraints on the Physical Parameters of TeV Blazars

We consider the constraints that can be derived from the spectral shape and variability of TeV blazars on the homogeneous synchrotron self-Compton (SSC) model. Assuming that the relativistic electron spectrum is a broken power law, in which the break energy is a free parameter, we write analytical formulae that allow us to connect observable quantities to the physical parameters of the model. We also give approximate analytic formulae for the case of Compton scattering occurring in the Klein-Nishina regime, which is of particular relevance for TeV Blazars. In particular we find that, even in the latter regime, a power-law component can be present at the highest energies. Further restrictions in the parameter space are set on the assumption that the break energy results from a balance between cooling and escape and that the soft photon lags measured in some sources derive from radiative cooling of high-energy particles. The constraints can be summarized as allowed regions in the Doppler factor-magnetic field parameter space and are in principle sufficient to determine the model parameters and their uncertainties unequivocally. We apply the method to three well-studied sources: Mrk 421, PKS 2155-304, and Mrk 501. For Mrk 421 the available data are sufficient to constrain the model fully. The additional restrictions are found to be consistent with the model parameters, supporting the proposed interpretation. In the case of PKS 2155-304, not yet detected in the TeV band, we estimate the peak frequency of the Compton component (40 GeV) and the expected TeV flux. The derived physical parameters are similar for the two sources, with a relatively large value of the Doppler factor (δ ~ 25) and a magnetic field of B 0.2 G. For Mrk 501 we consider both the historical low state and the flaring state observed in 1997 April. In the first case, consistency between the various assumptions is reached for δ 10 and B 0.3 G. For the high state, a similar value of the Doppler factor and a somewhat larger value of the magnetic field are indicated, while the extremely large frequency of the observed synchrotron peak requires continuous injection/reacceleration of relativistic particles.

General physical properties of bright Fermi blazars

We studied all blazars of known redshift detected by the Fermi satellite during its first 3-month survey. For the majority of them, pointed Swift observations ensure a good multiwavelength coverage, enabling us to reliably construct their spectral energy distributions (SEDs). We model the SEDs using a one-zone leptonic model and study the distributions of the derived interesting physical parameters as a function of the observed γ-ray luminosity. We confirm previous findings concerning the relation of the physical parameters with source luminosity which are at the origin of the blazar sequence. The SEDs allow to estimate the luminosity of the accretion disc for the majority of broad emitting line blazars, while for the lineless BL Lac objects in the sample upper limits can be derived. We find a positive correlation between the jet power and the luminosity of the accretion disc in broad-line blazars. In these objects, we argue that the jet must be proton dominated, and that the total jet power is of the same order of (or slightly larger than) the disc luminosity. We discuss two alternative scenarios to explain this result.

Structured jets in TeV BL Lac objects and radiogalaxies - Implications for the observed properties

TeV BL Lacertae objects require extreme relativistic bulk motions in the gamma-ray emission region, but at the VLBI scale their radio knots hardly move. The same sources show evidence, in radio, of a structure made of a fast spine plus a slow layer. We propose that this structure exists even on the spatial scale of regions responsible for the gamma-ray emission. One component sees the (beamed) radiation produced by the other, and this enhances the inverse Compton emission of both components. In addition, this allows the magnetic field to be nearly in equipartition with the emitting particles. The inverse Compton emission of the spine is anisotropic in its frame, possibly producing a deceleration of the spine by the Compton rocket effect. In this scenario, the slow layer is also a relatively strong high-energy emitter, and thus radiogalaxies become potentially detectable by GLAST.

The contribution of the obscuring torus to the X-ray spectrum of Seyfert galaxies: a test for the unification model

The presence of an obscuring torus around the nucleus of a Seyfert galaxy, as supposed in the popular unification scheme, can strongly modify the X-ray spectrum for both type 1 and type 2 Seyfert galaxies. We study this problem by means of Monte Carlo simulations, finding that, if the torus is Compton-thick, it can scatter at small angles a significant fraction of the nuclear radiation, and contribute to the continuum of Seyfert 1 galaxies above ∼10 keV, and to the fluorescence iron line at 6.4 keV. At large inclination angles and for large torus column densities, the spectrum is attenuated by photoabsorption and Compton scattering, while the iron fluorescence line produced by the torus can have large equivalent widths

A MODEL FOR THE X-RAY AND ULTRAVIOLET 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

The power of blazar jets

\alpha-\Omega

Internal shocks in the jets of radio-loud quasars

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.

Canonical high-power blazars

We estimate the power of relativistic, extragalactic jets by modelling the spectral energy distribution of a large number of blazars. We adopt a simple one-zone, homogeneous, leptonic synchrotron and inverse Compton model, taking into account seed photons originating both locally in the jet and externally. The blazars under study have an often dominant high-energy component which, if interpreted as due to inverse Compton radiation, limits the value of the magnetic field within the emission region. As a consequence, the corresponding Poynting flux cannot be energetically dominant. Also the bulk kinetic power in relativistic leptons is often smaller than the dissipated luminosity. This suggests that the typical jet should comprise an energetically dominant proton component. If there is one proton per relativistic electrons, jets radiate around 2‐10 per cent of their power in high-power blazars and 3‐30 per cent in less powerful BL Lacs.

BeppoSAX Observations of Unprecedented Synchrotron Activity in the BL Lacertae Object Markarian 501

The central engine causing the production of jets in radio sources may work intermittently, accelerating shells of plasma with different mass, energy and velocity. Faster but later shells can then catch up slower earlier ones. In the resulting collisions shocks develop, converting some of the ordered bulk kinetic energy into magnetic field and random energy of the electrons which then radiate. We propose that this internal shock scenario, which is the scenario generally thought to explain the observed gamma-ray burst radiation, can also work for radio sources in general, and for blazars in particular. We investigate in detail this idea, simulating the birth, propagation and collision of shells, calculating the spectrum produced in each collision, and summing the locally produced spectra from those regions of the jet which are simultaneously active in the observers frame. We can thus construct snapshots of the overall spectral energy distribution, time-dependent spectra and light curves. This allows us to characterize the predicted variability at any frequency, study correlations between the emission at different frequencies, specify the contribution of each region of the jet to the total emission, and find correlations between flares at high energies and the birth of superluminal radio knots and/or radio flares. The model has been applied to reproduce qualitatively the observed properties of 3C 279. Global agreement in terms of both spectra and temporal evolution is found. In a forthcoming work, we will explore the constraints that this scenario sets on the initial conditions of the plasma injected in the jet and the shock dissipation for different classes of blazars.