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Search Results: 1 - 10 of 189952 matches for " G. Ghisellini "
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What is the radiative process of the prompt phase of Gamma Ray Bursts?
G. Ghisellini
Physics , 2010, DOI: 10.1063/1.3475299
Abstract: 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.
The blazar's divide and the properties of Fermi blazars
G. Ghisellini
Physics , 2009,
Abstract: The LAT instrument, onboard the Fermi satellite, in its first three months of operation detected more than 100 blazars at more than the 10 sigma level. This is already a great improvement with respect to its predecessor, the instrument EGRET onboard the Compton Gamma Ray Observatory. Observationally, the new detections follow and confirm the so-called blazar sequence, relating the bolometric observed non-thermal luminosity to the overall shape of the spectal energy distribution. We have studied the general physical properties of all these bright Fermi blazars, and found that their jets are matter dominated, carrying a large total power that correlates with the luminosity of their accretion disks. We suggest that the division of blazars into the two subclasses of broad line emitting objects (Flat Spectrum Radio Quasars) and line-less BL Lacs is a consequence of a rather drastic change of the accretion mode, becoming radiatively inefficient below a critical value of the accretion rate, corresponding to a disk luminosity of ~1 per cent of the Eddington one. The reduction of the ionizing photons below this limit implies that the broad line clouds, even if present, cannot produce significant broad lines, and the object becomes a BL Lac.
Electron positron pairs in blazar jets and gamma-ray loud radio-galaxies
G. Ghisellini
Physics , 2012, DOI: 10.1111/j.1745-3933.2012.01280.x
Abstract: The matter content of extragalactic relativistic jets is still an unsolved issue. There are strong arguments against pure electron-positron pair jets, but pairs could outnumber the electrons associated with protons by a factor 10-20. This impacts on the estimate of the jet kinetic power, by reducing it by the same factor, and on the total energy delivered to leptons by the particle acceleration mechanism. Pairs cannot be created in the same jet-zone responsible for the high energy gamma-ray emission we see in blazars, because the reprocessing of the created pairs would overproduce the X-ray flux. Copious pair creation could occur in the inner zone of the still accelerating jet, where the bulk Lorentz factor is small. It is found that the inner zone can produce a sufficient number of pairs to replenish the zone of the jet where most of the luminosity is emitted, but only if the gamma-ray luminosity of the inner jet is above 1e44 erg/s at ~1 MeV. Since the beaming is modest, this emission can be observed at large viewing angles, and detected in radio-galaxies and lobe dominated quasars at the flux level of 1e-12 - 1e-11 erg/cm2/s for a source at a redshift z=0.1.
Spectra and power of relativistic jets
G. Ghisellini
Physics , 2002, DOI: 10.1016/S1387-6473(03)00061-7
Abstract: The power of blazar jets rivals the power that gravity can extract from accreting matter. The mechanism launching and accelerating jets can be considered as the most efficient engine operating in radio--loud sources. It is still a matter of debate if the jet carries this power to the radio lobes, hundreds of kpc away, in the form of Poynting flux or bulk kinetic energy, or both, and if these two ingredients have relative weights changing along the way. Accordingly, there are two (or more) possible general scenarios for how the jet can dissipate part of its power into radiation. It can be through e.g. reconnection of the magnetic field in the purely electromagnetic scenario, or through internal shocks in the matter dominated picture. Ways to discriminate these ideas are welcome.
Gamma Ray Bursts: open problems
G. Ghisellini
Physics , 2003,
Abstract: The internal/external synchrotron shock scenario has proved very successful in interpreting the key observations about gamma ray bursts. There still remains, however, some big uncertainties. The hottest issue concerns the nature of the progenitor, but there are also other problems concerning the global energetics, coupled with the issue of the degree of the collimation of the fireball. To be efficient, internal shocks within the relativistic wind must occur with large contrasts of their bulk Lorentz factors, and it is not clear yet the role of the Compton drag process in limiting the velocity differences. The fireball itself can be "hot" or "cold" according to what accelerates it to ultrarelativistic bulk speeds. In this respect the recent observations of a black body shape of the early phases of a few bursts shed new light on this issue. The most popular radiation process thought to explain the prompt emission is synchrotron, but it faces severe problems when comparing the expected spectrum with observations. Alternatives are called for. Emission features in the X-ray afterglow and absorption features in the prompt spectra are a powerful diagnostical tool. Besides shedding light on the nature of the progenitor, they can constrain the total energy release in a beaming-independent way.
Jetted Active Galactic Nuclei
G. Ghisellini
Physics , 2011, DOI: 10.1142/S2010194512004345
Abstract: Most of the electromagnetic output of blazars (BL Lac objects and Flat Spectrum Radio Quasars) comes out in the gamma-ray band, making the Large Area Telescope [0.1-100 GeV] onboard the Fermi satellite and the Cherenkov telescopes crucial for gather crucial data and thus to understand their physics. These data are complemented by the observations of the Swift satellite in the X-ray and optical-UV bands, and by ground based optical and radio telescopes. This rich coverage of the spectrum allows a robust modelling, from which important trends start to emerge. In powerful sources we see the contribution of the accretion disk that, once modeled, give us the black hole mass and the accretion rate. Even when not directly visible, the disk luminosity can be derived through the broad emission lines. Therefore we start to know the jet power, the disk luminosity, and the black hole mass, 3 crucial ingredients if we want to draw a general scenario. At the start, jets are believed to be magnetically dominated. And yet, on the scale where they emit most of their luminosity, their power is already in the form of kinetic energy of particles. Relativistic jets are formed for a very broad range of the disk luminosity, from close to Eddington down to at least 1e-4 Eddington. Their power correlates with the accretion rate, and can be even more powerful than the accretion disk luminosity.
Jets, black holes and disks in blazars
G. Ghisellini
Physics , 2013, DOI: 10.1051/epjconf/20136105001
Abstract: 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.
Extreme synchrotron blazars: the case of Mkn 501
G. Ghisellini
Physics , 1997, DOI: 10.1016/S0920-5632(98)00248-5
Abstract: BeppoSAX observations of Mkn501 in April 1997 (Pian et al. 1998), have revealed an extraordinary X-ray emission from this BL Lac object, during a phase of high activity at TeV energies, as monitored with the Whipple, HEGRA and CAT Cerenkov telescopes. The 0.1-200 keV spectrum was hard, and the X-ray power output peaked at or above 100 keV, 2 or 3 orders of magnitude more than what indicated by previuos observations, while the luminosity increased by at least a factor 20. The X-ray spectrum hardens when the source is brighter, but variations seem limited to energies greater than 0.5 keV. All these unprecedented spectral information pose severe constraints to all models, and we discuss in particular how the homogenous, one-zone synchrotron self Compton model must be modified to account for the observed properties. Other sources, besides Mkn 501, could undergo similar flares.
Swift for blazars
G. Ghisellini
Physics , 2015,
Abstract: 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.
Relativistic flows in blazars
G. Ghisellini
Physics , 2000, DOI: 10.1063/1.1434625
Abstract: 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.
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