2,298 research outputs found
Precursor Plerionic Activity and High Energy Gamma-Ray Emission in the Supranova Model of Gamma-Ray Bursts
The supranova model of gamma-ray bursts (GRBs), in which the GRB event is
preceded by a supernova (SN) explosion by a few months to years, has recently
gained support from Fe line detections in X-ray afterglows. A crucial
ingredient of this model yet to be studied is the fast-rotating pulsar that
should be active during the time interval between the SN and the GRB, driving a
powerful wind and a luminous plerionic nebula. We discuss some observational
consequences of this precursor plerion, which should provide important tests
for the supranova model: 1) the fragmentation of the outlying SN ejecta
material by the plerion and its implications for Fe line emission; and 2) the
effect of inverse Compton cooling and emission in the GRB external shock due to
the plerion radiation field. The plerion-induced inverse Compton emission can
dominate in the GeV-TeV energy range during the afterglow, being detectable by
GLAST from redshifts and distinguishable from self-Compton
emission by its spectrum and light curve. The prospects for direct detection
and identification of the precursor plerion emission are also briefly
considered.Comment: ApJ vol.583, in pres
ON THE GEOMETRY OF THE X-RAY EMITTING REGION IN SEYFERT GALAXIES
For the first time, detailed radiative transfer calculations of Comptonized
X-ray and gamma-ray radiation in a hot pair plasma above a cold accretion disk
are performed using two independent codes and methods. The simulations include
both energy and pair balance as well as reprocessing of the X- and gamma-rays
by the cold disk. We study both plane-parallel coronae as well as active
dissipation regions having shapes of hemispheres and pill boxes located on the
disk surface. It is shown, contrary to earlier claims, that plane-parallel
coronae in pair balance have difficulties in selfconsistently reproducing the
ranges of 2-20 keV spectral slopes, high energy cutoffs, and compactnesses
inferred from observations of type 1 Seyfert galaxies. Instead, the
observations are consistent with the X-rays coming from a number of individual
active regions located on the surface of the disk.
A number of effects such as anisotropic Compton scattering, the reflection
hump, feedback to the soft photon source by reprocessing, and an active region
in pair equilibrium all conspire to produce the observed ranges of X-ray
slopes, high energy cutoffs, and compactnesses. The spread in spectral X-ray
slopes can be due to a spread in the properties of the active regions such as
their compactnesses and their elevations above the disk surface. Simplified
models invoking isotropic Comptonization in spherical clouds are no longer
sufficient when interpreting the data.Comment: 9 pages, 3 postscript figures, figures can be obtained from the
authors via e-mail: [email protected]
Image sequence analysis for emerging interactive multimedia services - The European COST 211 framework
Cataloged from PDF version of article.Flexibility and efficiency of coding, content extraction,
and content-based search are key research topics in
the field of interactive multimedia. Ongoing ISO MPEG-4 and
MPEG-7 activities are targeting standardization to facilitate such
services. European COST Telecommunications activities provide
a framework for research collaboration. COST 211bis and COST
211ter activities have been instrumental in the definition and
development of the ITU-T H.261 and H.263 standards for videoconferencing
over ISDN and videophony over regular phone
lines, respectively. The group has also contributed significantly
to the ISO MPEG-4 activities. At present a significant effort
of the COST 211ter group activities is dedicated toward image
and video sequence analysis and segmentationâan important
technological aspect for the success of emerging object-based
MPEG-4 and MPEG-7 multimedia applications. The current
work of COST 211 is centered around the test model, called
the Analysis Model (AM). The essential feature of the AM is
its ability to fuse information from different sources to achieve
a high-quality object segmentation. The current information
sources are the intermediate results from frame-based (still) color
segmentation, motion vector based segmentation, and changedetection-based
segmentation. Motion vectors, which form the
basis for the motion vector based intermediate segmentation, are
estimated from consecutive frames. A recursive shortest spanning
tree (RSST) algorithm is used to obtain intermediate color and
motion vector based segmentation results. A rule-based region
processor fuses the intermediate results; a postprocessor further
refines the final segmentation output. The results of the current
AM are satisfactory; it is expected that there will be further
improvements of the AM within the COST 211 project
Plasma Ejection from Magnetic Flares and the X-ray Spectrum of Cygnus X-1
The hard X-rays in Cyg X-1 and similar black hole sources are possibly
produced in an active corona atop an accretion disk. We suggest that the
observed weakness of X-ray reflection from the disk is due to bulk motion of
the emitting hot plasma away from the reflector. A mildly relativistic motion
causes aberration reducing X-ray emission towards the disk. This in turn
reduces the reprocessed radiation from the disk and leads to a hard spectrum of
the X-ray source. The resulting spectral index is Gamma=1.9B^{1/2} where
B=gamma(1+beta) is the aberration factor for a bulk velocity beta=v/c. The
observed Gamma=1.6 and the amount of reflection, R=0.3, in Cyg X-1 in the hard
state can both be explained assuming a bulk velocity beta=0.3. We discuss one
possible scenario: the compact magnetic flares are dominated by e+- pairs which
are ejected away from the reflector by the pressure of the reflected radiation.
We also discuss physical constraints on the disk-corona model and argue that
the magnetic flares are related to magneto-rotational instabilities in the
accretion disk.Comment: The final version, accepted for publication in ApJ Letter
Radiative acceleration and transient, radiation-induced electric fields
The radiative acceleration of particles and the electrostatic potential
fields that arise in low density plasmas hit by radiation produced by a
transient, compact source are investigated. We calculate the dynamical
evolution and asymptotic energy of the charged particles accelerated by the
photons and the radiation-induced electric double layer in the full
relativistic, Klein-Nishina regime. For fluxes in excess of , the radiative force on a diluted plasma
(n\la 10^{11} cm) is so strong that electrons are accelerated rapidly
to relativistic speeds while ions lag behind owing to their larger inertia. The
ions are later effectively accelerated by the strong radiation-induced double
layer electric field up to Lorentz factors , attainable in the
case of negligible Compton drag. The asymptotic energies achieved by both ions
and electrons are larger by a factor 2--4 with respect to what one could
naively expect assuming that the electron-ion assembly is a rigidly coupled
system. The regime we investigate may be relevant within the framework of giant
flares from soft gamma-repeaters.Comment: 14 pages, 7 figures, ApJ, in press (tentatively scheduled for the v.
592, 2003 issue
Electron-Positron Pairs in Hot Accretion Flows and Thin Disk Coronae
We investigate equilibrium accretion flows dominated by pairs. We
consider one- and two-temperature accretion disk coronae above a thin disk, as
well as hot optically thin two-temperature accretion flows without an
underlying thin disk; we model the latter in the framework of
advection-dominated accretion flows (ADAFs). In all three cases we include
equipartition magnetic fields. We confirm the previous result that the
equilibrium density of pairs in two-temperature ADAFs is negligible; and show
that the inclusion of magnetic fields and the corresponding synchrotron cooling
reduces the pair density even further. Similarly, we find that pairs are
unimportant in two-temperature coronae. Even when the corona has significantly
enhanced heating by direct transfer of viscous dissipation in the thin disk to
the corona, the inefficient Coulomb coupling between protons and electrons acts
as a bottleneck and prevents the high compactness required for pair-dominated
solutions. Only in the case of a one-temperature corona model do we find
pair-dominated thermal equilibria. These pair-dominated solutions occur over a
limited range of optical depth and temperature.Comment: 38 pages, including 10 figures, LaTeX; to appear in Ap
The broad-band X-ray spectrum of the blazar PKS B1830-211 by Chandra and INTEGRAL
In this paper we present a broad-band study of the X-ray emission of the
blazar PKS1830-211 based on Chandra and Integral observations. Notwithstanding
the high redshift (z=2.507), it is a bright X-ray source (F(2-10 keV)~10^{-11}
erg cm^{-2} s^{-1}), due to gravitational lensing by an intervening galaxy at
z=0.89. Previous X-ray observations attribute the observed absorption at E<2
keV to the lensing galaxy. Our analysis, although not in contrast with this
hypothesis, suggests also the possibility of an intrinsic (ionized) absorption,
taking place at the front side of the jet. This scenario is also supported by
some evidence, in the same data, of a feature observed at 2.15 keV which can be
interpreted as a blueshifted iron line (v/c ~ 0.18). The SED of PKS1830-211 can
be well modelled by combining a Synchrotron Self-Compton component and an
external source of photons to be scattered up to \gamma-ray energies by
relativistic electrons moving outward in the jet. The main source of low energy
photons is a dust torus at the temperature of 10^3 K as expected in MeV
blazars.Comment: Accepted for publication in A&
- âŠ