Jets, black holes and disks in blazars

The Fermi and Swift satellites, together with ground based Cherenkov telescopes, has greatly improved our knowledge of blazars, namely Flat Spectrum Radio Quasars and BL Lac objects, since all but the most powerful emit most of their electro-magnetic output at gamma-ray energies, while the very powerful blazars emit mostly in the hard X-ray region of the spectrum. Often they show coordinated variability at different frequencies, suggesting that in these cases the same population of electrons is at work, in a single zone of the jet. The location of this region along the jet is a matter of debate. The jet power correlates with the mass accretion rate, with jets existing at all values of disk luminosities, measured in Eddington units, sampled so far. The most powerful blazars show clear evidence of the emission from their disks, and this has revived methods of finding the black hole mass and accretion rate by modelling a disk spectrum to the data. Being so luminous, blazars can be detected also at very high redshift, and therefore are a useful tool to explore the far universe. One interesting line of research concerns how heavy are their black holes at high redshifts. If we associate the presence of a relativistic jet with a fastly spinning black hole, then we naively expect that the accretion efficiency is larger than for non-spinning holes. As a consequence, the black hole mass in jetted systems should grow at a slower rate. In turn, this would imply that, at high redshifts, the heaviest black holes should be in radio-quiet quasars. We instead have evidences of the opposite, challenging our simple ideas of how a black hole grows.Comment: 12 pages, 11 figures. Invited review at the meeting "The Innermost Regions of Relativistic Jets and Their Magnetic Fields", Granada, Spai

Relativistic flows in blazars

The radiation we observe from blazars is most likely the product of the transformation of bulk kinetic energy into random energy. This process must have a relatively small efficiency (e.g. 10%) if jets are to power the extended radio-structures. Recent results suggest that the average power reaching the extended radio regions and lobes is of the same order of that produced by accretion and illuminating the emission line clouds. Most of the radiative power is produced in a well localized region of the jet, and, at least during flares, is mainly emitted in the gamma-ray band. A possible scenario qualitatively accounting for these facts is the internal shock model, in which the central engine produces a relativistic plasma flow in an intermittent way.Comment: 10 pages, 2 figures, invited talk at the meeting "Stellar Endpoints, AGN and the Diffuse Background", held in Bologna, Italy, Sept 199

GRB 060218 and the outliers with respect to the Ep-Eiso correlation

GRB 031203 and GRB 980425 are the two outliers with respect to the Ep-Eiso correlation of long GRBs. Recently Swift discovered a nearby extremely long GRB 060218 associated with a SN event. The spectral properties of this bursts are striking: on the one hand its broad band SED presents both thermal and non-thermal components which can be interpreted as due to the emission from the hot cocoon surrounding the GRB jet and as standard synchrotron self absorbed emission in the GRB prompt phase, respectively; on the other hand it is its long duration and its hard--to--soft spectral evolution which make this underluminous burst consistent with the Ep-Eiso correlation of long GRBs. By comparing the available spectral informations on the two major outliers we suggests that they might be twins of 060218 and, therefore, only apparent outliers with respect to the Ep_Eiso correlation. This interpretation also suggests that it is of primary importance the study the broad band spectra of GRBs in order to monitor their spectral evolution throughout their complete duration.Comment: To appear in the conference proceeding of the IV workshop on "Science with the new generation of high energy Gamma-Ray Experiment", 20-22 June 2006, Isola d'Elb

Blazars and Gamma Ray Bursts

Blazars and Gamma Ray Bursts (GRBs) are the fastest objects known so far. The radiation we see from these sources originates in a jet of similar aperture angle, and we think it is the result of the conversion of some of the jet kinetic energy into random motion of the emitting particles. Mechanisms for producing, collimating and accelerating the jets in these sources are uncertain, and it is fruitful to compare the characteristics of both class of sources in search of enlightening similarities. I discuss some general characteristics of blazars and GRBs such as the power of their jets compared with what they can extract through accretion, and the dissipation mechanism operating in the jets of both classes of sources. In both classes, there is a well defined trend between the bolometric power and the frequency at which this power is mainly emitted, but blazars are "redder when brighter", while GRBs are "bluer when brighter". Finally, I discuss some recent exciting prospects to use blazars to put constraints on the cosmic IR-Optical-UV backgrounds, and to use GRBs as standard candles to measure the Universe.Comment: 15 pages, 6 figures; invited talk at the VI Microquasar Workshop: " Microquasars and beyond" Como, September 200

What is the radiative process of the prompt phase of Gamma Ray Bursts?

Despite the dramatic improvement of our knowledge of the phenomenology of Gamma Ray Bursts, we still do not know several fundamental aspects of their physics. One of the puzzles concerns the nature of the radiative process originating the prompt phase radiation. Although the synchrotron process qualifies itself as a natural candidate, it faces severe problems, and many efforts have been done looking for alternatives. These, however, suffer from other problems, and there is no general consensus yet on a specific radiation mechanism.Comment: 6 pages, 3 figures, invited talk at the conference: X-Ray Astronomy 2009, Present Status, multiwavelength approach and future perspectives, September 2009, Bologn