1,476 research outputs found

    Quasi-Thermal Comptonization and GRBs

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    Quasi-thermal Comptonization is an attractive alternative to the synchrotron process to explain the spectra of GRBs, even if we maintain other important properties of the internal shock scenario, implying a compact emitting region and an equipartition magnetic field. Photon-photon absorption and electron-positron pairs can play a crucial role: this process may lock the effective temperature in a narrow range and may be the reason why burst spectra have high energy cut-offs close to the rest mass-energy of the electron. If the progenitors of GRB are hypernovae, the circum-burst matter is dominated by the wind of the pre-hypernova star. The presence of this dense material has strong effects on the generation of the radiation of the burst and its afterglow.Comment: 7 pages, contributed paper for the meeting: "Gamma-ray bursts: the first three minutes", Editors: J. Poutanen and R. Svensso

    Relativistic large scale jets and minimum power requirements

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    The recent discovery, by the Chandra satellite, that jets of blazars are strong X-ray emitters at large scales (0.1-1 Mpc) bears support to the hypothesis that (also) on these scales the emitting plasma is moving at highly relativistic speeds. In this case in fact the emission via inverse Compton scattering off cosmic background photons is enhanced and the resulting predicted X-ray spectrum well accounts for the otherwise puzzling observations. Here we point out another reason to favor relativistic large scale jets, based on a minimum power argument: by estimating the Poynting flux and bulk kinetic powers corresponding to, at least, the relativistic particles and magnetic field responsible for the emission, one can derive the value of the bulk Lorentz factor for which the total power is minimized. It is found that both the inner and extended parts of the jet of PKS 0637-752 satisfy such condition.Comment: 5 pages, 3 figures; revised version accepted for publication in MNRA

    Reprocessing of radiation by multi-phase gas in Low Luminosity Accretion Flows

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    We discuss the role that magnetic fields in low luminosity accretion flows can play in creating and maintaining a multi-phase medium, and show that small magnetically-confined clouds or filaments of dense cold gas can dramatically reprocess the `primary' radiation from tori. In particular, radio emission would be suppressed by free-free absorption, and an extra (weak) component would appear at optical wavelengths. This is expected to be a common process in various environments in the central regions of Active Galaxies, such as broad line regions, accretion disk coronae and jets.Comment: submitted to MNRAS; 4 pages, 1 figure (MNRAS LaTex style

    Evidence for anisotropy in the distribution of short-lived gamma-ray bursts

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    Measurements of the two-point angular correlation function w(\theta) for 407 short gamma-ray bursts collected in the Current BATSE Catalogue reveal a ~2 \sigma deviation from isotropy on angular scales \theta ~ 2-4 degrees. Such an anisotropy is not observed in the distribution of long gamma-ray bursts and hints to the presence of repeated bursts for up to ~13% of the sources under exam. However, the available data cannot exclude the signal as due to the presence of large-scale structure. Under this assumption, the amplitude of the observed w(\theta) is compatible with those derived for different populations of galaxies up to redshifts ~0.5, result that suggests short gamma-ray bursts to be relatively local sources.Comment: 5 pages, 4 figures, submitted to MNRA
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