7,882 research outputs found
Steep Slopes and Preferred Breaks in GRB Spectra: the Role of Photospheres and Comptonization
The role of a photospheric component and of pair breakdown is examined in the
internal shock model of gamma-ray bursts. We discuss some of the mechanisms by
which they would produce anomalously steep low energy slopes, X-ray excesses
and preferred energy breaks. Sub-relativistic comptonization should dominate in
high comoving luminosity bursts with high baryon load, while synchrotron
radiation dominates the power law component in bursts which have lower comoving
luminosity or have moderate to low baryon loads. A photosphere leading to steep
low energy spectral slopes should be prominent in the lowest baryon loadComment: ApJ'00, in press; minor revs. 10/5/99; (uses aaspp4.sty), 15 pages, 3
figure
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]
Unstable GRB photospheres and electron-positron annihilation lines
We propose an emission mechanism of prompt gamma-ray bursts (GRBs) that can
reproduce the observed non-thermal spectra with high radiative efficiencies,
>50%. Internal dissipation below a photosphere can create a radiation-dominated
thermal fireball. If electron-positron pairs outnumber protons, radiative
acceleration of pairs drives the two-stream instabilities between pairs and
protons, leading to the ``proton sedimentation'' in the accelerating pair
frame. Pairs are continuously shock heated by proton clumps, scattering the
thermal photons into the broken power-law shape, with the non-thermal energy
that is comparable to the proton kinetic energy, consistent with observations.
Pair photospheres become unstable around the radius of the progenitor star
where strong thermalization occurs, if parameters satisfy the observed spectral
(Yonetoku) relation. Pair annihilation lines are predicted above continua,
which could be verified by GLAST.Comment: 4 pages, 2 figure
Optimal partial-arcs in VMAT treatment planning
Purpose: To improve the delivery efficiency of VMAT by extending the recently
published VMAT treatment planning algorithm vmerge to automatically generate
optimal partial-arc plans.
Methods and materials: A high-quality initial plan is created by solving a
convex multicriteria optimization problem using 180 equi-spaced beams. This
initial plan is used to form a set of dose constraints, and a set of
partial-arc plans is created by searching the space of all possible partial-arc
plans that satisfy these constraints. For each partial-arc, an iterative
fluence map merging and sequencing algorithm (vmerge) is used to improve the
delivery efficiency. Merging continues as long as the dose quality is
maintained above a user-defined threshold. The final plan is selected as the
partial arc with the lowest treatment time. The complete algorithm is called
pmerge.
Results: Partial-arc plans are created using pmerge for a lung, liver and
prostate case, with final treatment times of 127, 245 and 147 seconds.
Treatment times using full arcs with vmerge are 211, 357 and 178 seconds. Dose
quality is maintained across the initial, vmerge, and pmerge plans to within 5%
of the mean doses to the critical organs-at-risk and with target coverage above
98%. Additionally, we find that the angular distribution of fluence in the
initial plans is predictive of the start and end angles of the optimal
partial-arc.
Conclusions: The pmerge algorithm is an extension to vmerge that
automatically finds the partial-arc plan that minimizes the treatment time.
VMAT delivery efficiency can be improved by employing partial-arcs without
compromising dose quality. Partial arcs are most applicable to cases with
non-centralized targets, where the time savings is greatest
Gamma-rays from Galactic Black Hole Candidates with Stochastic Particle Acceleration
We consider stochastic particle acceleration in plasmas around stellar mass
black holes to explain the emissions above 1 MeV from Galactic black hole
candidates. We show that for certain parameter regimes, electrons can overcome
Coulomb losses and be accelerated beyond the thermal distribution to form a new
population, whose distribution is broad and usually not a power law; the peak
energy of the distribution is determined by the balance between acceleration
and cooling, with particles piling up around it. Radiation by inverse Compton
scattering off the thermal (from background) and non-thermal (produced by
acceleration) particles can in principle explain the hard X-ray to gamma-ray
emissions from black hole candidates. We present model fits of Cyg X-1 and GRO
J0422 in 50 keV -- 5 MeV region observed with OSSE and COMPTEL.Comment: 2 figures, to appear in March 20 of ApJ
On the interpretation of the multicolour disc model for black hole candidates
We present a critical analysis of the usual interpretation of the multicolour
disc model parameters for black hole candidates in terms of the inner radius
and temperature of the accretion disc. Using a self-consistent model for the
radiative transfer and the vertical temperature structure in a Shakura-Sunyaev
disc, we simulate the observed disc spectra, taking into account doppler
blurring and gravitational redshift, and fit them with multicolour models. We
show not only that such a model systematically underestimates the value of the
inner disc radius, but that when the accretion rate and/or the energy
dissipated in the corona are allowed to change the inner edge of the disc, as
inferred from the multicolour model, appears to move even when it is in fact
fixed at the innermost stable orbit.Comment: 4 pages including 2 figures, accepted for publication in MNRA
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
The population of deformed bands in Cr by emission of Be from the S + Mg reaction
Using particle- coincidences we have studied the population of final
states after the emission of 2 -particles and of Be in nuclei
formed in S+Mg reactions at an energy of . The data were obtained in a setup
consisting of the GASP -ray detection array and the multidetector array
ISIS. Particle identification is obtained from the E and E signals of
the ISIS silicon detector telescopes, the Be being identified by the
instantaneous pile up of the E and E pulses. -ray decays of the
Cr nucleus are identified with coincidences set on 2 -particles
and on Be. Some transitions of the side-band with show
stronger population for Be emission relative to that of 2
-particles (by a factor ). This observation is interpreted as
due to an enhanced emission of Be into a more deformed nucleus.
Calculations based on the extended Hauser-Feshbach compound decay formalism
confirm this observation quantitatively.Comment: 17 pages, 9 figures accepted for publication in J. Phys.
Self-Organized Criticality in Compact Plasmas
Compact plasmas, that exist near black-hole candidates and in gamma ray burst
sources, commonly exhibit self-organized non-linear behavior. A model that
simulates the non-linear behavior of compact radiative plasmas is constructed
directly from the observed luminosity and variability. The simulation shows
that such plasmas self organize, and that the degree of non-linearity as well
as the slope of the power density spectrum increase with compactness. The
simulation is based on a cellular automaton table that includes the properties
of the hot (relativistic) plasmas, and the magnitude of the energy
perturbations. The plasmas cool or heat up, depending on whether they release
more or less than the energy of a single perturbation. The energy release
depends on the plasmas densities and temperatures, and the perturbations
energy. Strong perturbations may cool the previously heated plasma through
shocks and/or pair creation.
New observations of some active galactic nuclei and gamma ray bursters are
consistent with the simulationComment: 9 pages, 5 figures, AASTeX, Submitted to ApJ
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