1,905 research outputs found
Quasi-Thermal Comptonization and GRBs
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
Testing the FR I/BL Lac unifying model with HST observations
Hubble Space Telescope (HST) observations provide a novel way of testing
unified models for FR I radio sources and BL Lac objects. The detection of
extended dust discs in some radio galaxies provides information on their jet
orientation. Given this, the strength of the compact nuclear sources of FR I
and BL Lacs can be compared with model predictions. As a pilot project, we
selected five radio galaxies which show extended nuclear discs in the HST
images. The relative orientation of the projected radio-jets and of the
extended nuclear discs indicates that they are not perpendicular, as the
simplest geometrical model would suggest, but that they form an angle of ~ 20 -
40 degrees with the symmetry axis of the disc: a significant change of
orientation occurs between the innermost AGN structure and the kpc-scale.
Nevertheless, the discs appear to be useful indicators of the radio sources
orientation since the angles formed by the disc axis and the jet with the line
of sight differ by only ~ 10 - 20 degrees. At the center of each disc an
unresolved nuclear source is present. We compared its luminosity with the
optical core luminosity of BL Lacs selected for having similar host galaxy
magnitude and extended radio luminosity. The BL Lac cores are between 2 E2 and
3 E5 times brighter than the corresponding radio galaxies ones. The FR I/BL Lac
core luminosity ratio shows a suggestive correlation with the orientation of
the radio galaxies with respect to the line of sight. The behavior of this
ratio is quantitatively consistent with a scenario in which the emission in the
FR I and BL Lac is dominated by the beamed radiation from a relativistic jet
with Doppler factor ~ 5 - 10, thus supporting the basic features of the
proposed unification schemes.Comment: 11 pages, 10 figures, submitted to MNRAS, revised versio
Relativistic large scale jets and minimum power requirements
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
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
The BL Lac heart of Centaurus A
Emission from the nucleus of the closest radio galaxy, Centaurus A, is
observed from the radio to the gamma ray band. We build, for the first time,
its overall Spectral Energy Distribution (SED) that appears to be intriguingly
similar to those of blazars, showing two broad peaks located in the
far-infrared band and at ~0.1 MeV respectively. The whole nuclear emission of
Centaurus A is successfully reproduced with a synchrotron self-Compton model.
The estimated physical parameters of the emitting source are similar to those
of BL Lacs, except for a much smaller beaming factor, as qualitatively expected
when a relativistic jet is orientated at a large angle to the line of sight.
These results represent strong evidence that Centaurus A is indeed a
misoriented BL Lac and provide strong support in favour of the unification
scheme for low luminosity radio-loud AGNs. Modeling of the SED of Centaurus A
also provides further and independent indications of the presence of velocity
structures in sub-pc scale jets.Comment: 5 pages, 2 figures, accepted for publication in MNRAS, pink page
Evidence for anisotropy in the distribution of short-lived gamma-ray bursts
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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