669 research outputs found
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
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
Dense, thin clouds and reprocessed radiation in the central regions of Active Galactic Nuclei
The primary radiation generated in the central continuum-forming region of
Active Galactic Nuclei can be reprocessed by very dense, small-scale clouds
that are optically-thin to Thomson scattering. In spite of the extreme
conditions expected to prevail in this innermost, central environment, the
radiative clouds can survive and maintain cool temperatures relative to the
ambient emitting region by means of magnetic confinement. Motivated by these
ideas, we present a detailed quantitative study of such clouds, explicitly
describing the physical properties they can attain under thermal and radiative
equilibrium conditions. We also discuss the thermal stability of the gas in
comparison to that of other reprocessing material thought to reside at larger
distances from the central source. We construct a model to predict the emergent
spectra from a source region containing dense clouds which absorb and reemit
the primary radiation generated therein. Our predicted spectra show the
following two important results: (i) the reprocessed flux emitted at optical/UV
energies is insufficient to account for the blue bump component in the observed
spectra; and (ii) the amount of line radiation that is emitted is at least
comparable to (and in many cases dominates) the continuum radiation. The lines
are extremely broad and tend to accumulate in the extreme ultraviolet, where
they form a peak much more prominent than that which is observed in the
optical/UV. This result is supported by current observations, which indicate
that the spectral energy distribution of radio-quiet AGN may indeed reach a
maximum in the EUV band.Comment: 14 pages, 5 figures, latex, uses epsf and rotate, accepted for
publication in M
Limits from rapid TeV variability of Mrk 421
The extreme variability event in the TeV emission of Mrk 421, recently
reported by the Whipple team, imposes the tightest limits on the typical size
of the TeV emitting regions in Active Galactic Nuclei (AGN). We examine the
consequences that this imposes on the bulk Lorentz factor of the emitting
plasma and on the radiation fields present in the central region of this Active
Nucleus. No strong evidence is found for extreme Lorentz factors. However,
energetics arguments suggest that any accretion in Mrk 421 has to take place at
small rates, compatible with an advection-dominated regime.Comment: 5 pages (Latex MNRAS style), revised version, submitted to MNRA
Hydrodynamics of Internal Shocks in Relativistic Outflows
We study the hydrodynamical effects of two colliding shells, adopted to model
internal shocks in various relativistic outflows such as gamma-ray bursts and
blazars. We find that the density profiles are significantly affected by the
propagation of rarefaction waves. A split-feature appears at the contact
discontinuity of the two shells. The shell spreading with a few ten percent of
the speed of light is also shown to be a notable aspect. The conversion
efficiency of the bulk kinetic energy to internal one shows deviations from the
widely-used inelastic two-point-mass-collision model. Observational
implications are also shortly discussed.Comment: 6 pages, 4 figures, Proceeding of International Symposium on High
Energy Gamma-ray Astronomy (July 26-30, 2004, Heidelberg, Germany
Radiative pressure feedback by a quasar in a galactic bulge
We show that Eddington-limited black hole luminosities can be sufficient to
deplete a galaxy bulge of gas through radiation pressure, when the ionization
state of the gas and the presence of dust are properly taken into account. Once
feedback starts to be effective it can consistently drive all the gas out of
the whole galaxy. We estimate the amount by which the effect of radiation
pressure on dusty gas boosts the mass involved in the Eddington limit and
discuss the expected column density at which the gas is ejected. An example is
shown of the predicted observed nuclear spectrum of the system at the end of an
early, obscured phase of growth when the remaining column density NH ~ f * 1e24
cm^-2 where f is the gas fraction in the bulge.Comment: correct corrupted figures in pdf version, MNRAS accepted, 5 pages, 3
figure
BeppoSAX Observations of Mkn 421: clues on the particle acceleration ?
Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. We present
highlights of the results of the thorough temporal and spectral analysis
discussed by Fossati et al. (1999) and Maraschi et al. (1999), focusing on the
flare of April 1998, which was simultaneously observed also at TeV energies.
The detailed study of the flare in different energy bands reveals a few very
important new results: (a) hard photons lag the soft ones by 2-3 ks *a behavior
opposite to what is normally found in High energy peak BL Lacs X-ray spectra*;
(b) the flux decay of the flare can be intrinsically achromatic if a stationary
underlying emission component is present. Moreover the spectral evolution
during the flare has been followed by extracting X-ray spectra on few ks
intervals, allowing to detect for the first time the peak of the synchrotron
component shifting to higher energies during the rising phase, and then
receding. The spectral analysis confirms the delay in the flare at the higher
energies, as above a few keV the spectrum changes only after the peak of the
outburst has occurred. The spectral and temporal information obtained challenge
the simplest models currently adopted for the (synchrotron) emission and most
importantly provide clues on the particle acceleration process. A theoretical
picture accounting for all the observational constraints is discussed, where
electrons are injected at low energies and then progressively accelerated
during the development of the flare.Comment: 5 pages, LaTeX, uses aipproc.sty, 3 PostScript figures, to appear in
the Proceedings of the "Fifth Compton Symposium", held in Portsmouth (NH),
1999 Septembe
Compton dragged gamma-ray bursts: the spectrum
We calculate the spectrum resulting from the interaction of a fireball with
ambient soft photons. These photons are assumed to be produced by the walls of
a funnel in a massive star. By parameterizing the radial dependence of the
funnel temperature we calculate the deceleration of the fireball
self-consistently, taking into account the absorption of high energy gamma-rays
due to interaction with the softer ambient photons. The resulting spectrum is
peaked at energies in agreement with observations, has a nu^2 slope in the
X-ray band and a steep power-law high energy tail.Comment: 5 pages, 3 figures, accepted for publication in MNRAS, pink page
Events in the life of a cocoon surrounding a light, collapsar jet
According to the collapsar model, gamma-ray bursts are thought to be produced
in shocks that occur after the relativistic jet has broken free from the
stellar envelope. If the mass density of the collimated outflow is less than
that of the stellar envelope, the jet will then be surrounded by a cocoon of
relativistic plasma. This material would itself be able to escape along the
direction of least resistance, which is likely to be the rotation axis of the
stellar progenitor, and accelerate in approximately the same way as an
impulsive fireball. We discuss how the properties of the stellar envelope have
a decisive effect on the appearance of a cocoon propagating through it. The
relativistic material that accumulated in the cocoon would have enough kinetic
energy to substantially alter the structure of the relativistic outflow, if not
in fact provide much of the observed explosive power. Shock waves within this
plasma can produce gamma-ray and X-ray transients, in addition to the standard
afterglow emission that would arise from the deceleration shock of the cocoon
fireball.Comment: 16 pages, 5 figures, slightly revised version, accepted for
publication in MNRA
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