7,072 research outputs found
Search for Non-Triggered Gamma Ray Bursts in the BATSE Continuous Records: Preliminary Results
We present preliminary results of an off-line search for non-triggered
gamma-ray bursts (GRBs) in the BATSE daily records for about 5.7 years of
observations. We found more GRB-like events than the yield of the similar
search of Kommers et al. (1998) and extended the Log N - log P distribution
down to 0.1 ph cm s. The indication of a turnover of the
log N - log P at a small P is not confirmed: the distribution is straight at
1.5 decades with the power law index -.6 and cannot be fitted with a standard
candle cosmological model.Comment: 4 pages, LaTeX, to appear in Proceedings "Gamma Ray Bursts in the
Afterglow Era", Rome, November 1998, A&AS, 199
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]
Power Density Spectra of GRBs
Power density spectra (PDSs) of long gamma-ray bursts (GRBs) provide useful
information on GRBs, indicating their self-similar temporal structure. The best
power-law PDSs are displayed by the longest bursts (T_90>100 s) in which the
range of self-similar time scales covers more than 2 decades. Shorter bursts
have apparent PDS slopes more strongly affected by statistical fluctuations.
The underlying power law can then be reproduced with high accuracy by averaging
the PDSs for a large sample of bursts. This power law has a slope approximately
equal to -5/3 and a sharp break at about 1 Hz.
The power-law PDS provides a new sensitive tool for studies of GRBs. In
particular, we calculate the PDSs of bright bursts in separate energy channels.
The PDS flattens in the hard channel (h\nu>300 keV) and steepens in the soft
channel (h\nu<50 keV), while the PDS of bolometric light curves approximately
follows the -5/3 law.
We then study dim bursts and compare them to the bright ones. We find a
strong correlation between the burst brightness and the PDS slope. This
correlation shows that the bursts are far from being standard candles and dim
bursts should be intrinsically weak. The time dilation of dim bursts is
probably related to physical processes occurring in the burst rather than to a
cosmological redshift.Comment: 9 pages, accepted to Ap
Model Data Fusion: developing Bayesian inversion to constrain equilibrium and mode structure
Recently, a new probabilistic "data fusion" framework based on Bayesian
principles has been developed on JET and W7-AS. The Bayesian analysis framework
folds in uncertainties and inter-dependencies in the diagnostic data and signal
forward-models, together with prior knowledge of the state of the plasma, to
yield predictions of internal magnetic structure. A feature of the framework,
known as MINERVA (J. Svensson, A. Werner, Plasma Physics and Controlled Fusion
50, 085022, 2008), is the inference of magnetic flux surfaces without the use
of a force balance model. We discuss results from a new project to develop
Bayesian inversion tools that aim to (1) distinguish between competing
equilibrium theories, which capture different physics, using the MAST spherical
tokamak; and (2) test the predictions of MHD theory, particularly mode
structure, using the H-1 Heliac.Comment: submitted to Journal of Plasma Fusion Research 10/11/200
Phonons in random alloys: the itinerant coherent-potential approximation
We present the itinerant coherent-potential approximation(ICPA), an analytic,
translationally invariant and tractable form of augmented-space-based,
multiple-scattering theory in a single-site approximation for harmonic phonons
in realistic random binary alloys with mass and force-constant disorder.
We provide expressions for quantities needed for comparison with experimental
structure factors such as partial and average spectral functions and derive the
sum rules associated with them. Numerical results are presented for Ni_{55}
Pd_{45} and Ni_{50} Pt_{50} alloys which serve as test cases, the former for
weak force-constant disorder and the latter for strong. We present results on
dispersion curves and disorder-induced widths. Direct comparisons with the
single-site coherent potential approximation(CPA) and experiment are made which
provide insight into the physics of force-constant changes in random alloys.
The CPA accounts well for the weak force-constant disorder case but fails for
strong force-constant disorder where the ICPA succeeds.Comment: 19 pages, 12 eps figures, uses RevTex
Kinetics of electron-positron pair plasmas using an adaptive Monte Carlo method
A new algorithm for implementing the adaptive Monte Carlo method is given. It
is used to solve the relativistic Boltzmann equations that describe the time
evolution of a nonequilibrium electron-positron pair plasma containing
high-energy photons and pairs. The collision kernels for the photons as well as
pairs are constructed for Compton scattering, pair annihilation and creation,
bremsstrahlung, and Bhabha & Moller scattering. For a homogeneous and isotropic
plasma, analytical equilibrium solutions are obtained in terms of the initial
conditions. For two non-equilibrium models, the time evolution of the photon
and pair spectra is determined using the new method. The asymptotic numerical
solutions are found to be in a good agreement with the analytical equilibrium
states. Astrophysical applications of this scheme are discussed.Comment: 43 pages, 7 postscript figures, to appear in the Astrophysical
Journa
Electron-Positron Pair Loading and the Origin of the Upstream Magnetic Field in GRB Shocks
We investigate here the effects of plasma instabilities driven by rapid
electron/positron pair cascades, which arise in the environment of GRB sources
as a result of back-scattering of a seed fraction of the original spectrum. The
injection of electron/positron pairs induces strong streaming motions in the
ambient medium. One therefore expects the pair-enriched medium ahead of the
forward shock to be strongly sheared on length scales comparable to the
radiation front thickness. Using three-dimensional particle-in-cell
simulations, we show that plasma instabilities driven by these streaming
electron/positron pairs are responsible for the excitation of
near-equipartition, turbulent magnetic fields. Our results reveal the
importance of the electromagnetic filamentation instability in ensuring an
effective coupling between electron/positron pairs and ions, and may help
explain the origin of large upstream fields in GRB shocks.Comment: ApJ in press, 13 pages, 9 fig
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