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

Swift for blazars

I will review recent advances in the field of blazars, highlighting the contribution of Swift. Together with other operating satellites (most notably Fermi, but also AGILE, WISE, Planck) and ground based facilities such as Cherenkov telescopes, Swift was (and is) crucial for improving our understanding of blazars. The main advances in the blazar field made possible by Swift includes the opening of the time domain investigation, since there are several sources with hundreds of simultaneous optical, UV and X-ray data taken at different times; the possibility to measure the black hole mass in very powerful blazars, that show clear signs of accretion disk emission; the possibility to classify blazar candidates, through X-ray observations; the finding of the most powerful and distant blazars, emitting strongly in the hard X-ray band accessible to Swift/BAT. All these improvements had and have a great impact on our understanding on how relativistic jets are formed and emit, on their power, and on how the heavy black holes in these systems first formed and grew.Comment: 12 pages, 10 figures, Journal of High Energy Astrophysics for the dedicated issue: "Swift: Ten Years of Discovery", in pres

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

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

Gamma Ray Bursts and Radio Loud Active Galactic Nuclei

We believe that the radiation we receive from Gamma-Ray Bursts (GRBs) and radio loud Active Galacti Nuclei (AGNs) originates from the transformation of bulk relativistic motion into random energy. Mechanisms to produce, collimate and accelerates the jets in these sources are uncertain, and it may be fruitful to compare the characteristics of both class of sources in search of enlightening similarities. I will present some general characteristics of radio loud AGNs and GRBs such as their bulk Lorentz factors and the power of their jets. I will also discuss the way in which the energy in bulk relativistic motion can be transformed into beamed radiation, and consider the possibility that both classes of sources can work in the same way, namely by an intermittent release of relativistic plasma at the base of the jet: shells ejected with slightly different velocities collide at some distance from the central engine, dissipating part of their kinetic energy, and keeping the rest to power the extended radio lobes (in AGNs) or to produce the afterglow (in GRBs).Comment: 8 pages, 2 figures invited talk at the Symposium: "Plasmas in the Laboratory and in the Universe: new insights and new challenges", September 16-19, 2003, Como, Ital

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