1,219 research outputs found
Line emission from gamma-ray burst environments
The time and angle dependent line and continuum emission from a dense torus
around a cosmological gamma-ray burst source is simulated, taking into account
photoionization, collisional ionization, recombination, and electron heating
and cooling due to various processes. The importance of the hydrodynamical
interaction between the torus and the expanding blast wave is stressed. Due to
the rapid deceleration of the blast wave as it interacts with the dense torus,
the material in the torus will be illuminated by a drastically different photon
spectrum than observable through a low-column-density line of sight, and will
be heated by the hydrodynamical interaction between the blast wave and the
torus. A model calculation to reproduce the Fe K-alpha line emission observed
in the X-ray afterglow of GRB 970508 is presented. The results indicate that ~
10^{-4} solar masses of iron must be concentrated in a region of less than
10^{-3} pc. The illumination of the torus material due to the hydrodynamic
interaction of the blast wave with the torus is the dominant heating and
ionization mechanism leading to the formation of the iron line. These results
suggest that misaligned GRBs may be detectable as X-ray flashes with pronounced
iron emission line features.Comment: Accepted for publication in ApJ. Updated recombination rate data;
discussion on element abundances added; references update
X-ray spectral features from GRBs: Predictions of progenitor models
We investigate the potentially observable prompt or delayed X-ray spectral
features from the currently popular gamma-ray burst (GRB) models. During the
evolution of many GRB progenitors, a disk around the central GRB source is
produced. Shock heating as the GRB ejecta collide with the disk may produce
observable X-ray features. We first summarize predictions deduced from previous
calculations which invoke photoionization and relativistic blast waves. We then
calculate the quasi-thermal X-ray line features produced assuming the ejecta
are nonrelativistic (which is more likely for the disk interactions of many GRB
models). In the framework of the Hypernova/Collapsar model, delayed (a few days
- several months after the GRB) bursts of line-dominated, thermal X-ray
emission may be expected. The He-merger scenario predicts similar X-ray
emission line bursts <~ a few days after the GRB. These X-ray signatures should
be observable with Chandra and XMM-Newton out to at least z ~ 1. Weak emission
line features <~ a few days after the GRB may also result from the supranova
GRB scenario. In all three cases, significant X-ray absorption features, in
particular during the prompt GRB phase, are expected. No significant X-ray
spectral features might result from compact-object binary mergers.Comment: 20 pages, including 8 figures and 3 tables. Uses epsf.sty,
rotate.sty. Final version, accepted for publication in to ApJ. Revised
analytical estimate of maximum emission line luminosity. Numerical results
and conclusions unchange
Monte-Carlo simulations of thermal/nonthermal radiation from a neutron-star magnetospheric accretion shell
We discuss the space-and-time-dependent Monte Carlo code we have developed to
simulate the relativistic radiation output from compact astrophysical objects,
coupled to a Fokker-Planck code to determine the self-consistent lepton
populations. We have applied this code to model the emission from a magnetized
neutron star accretion shell near the Alfven radius, reprocessing the radiation
from the neutron sar surface. We explore the parameter space defined by the
accretion rate, stellar surface field and the level of wave turbulence in the
shell. Our results are relevant to the emission from atoll sources, soft-X-ray
transient X-ray binaries containing weakly magnetized neutron stars, and to
recently suggested models of accretion-powered emission from anomalous X-ray
pulsars.Comment: 24 pages, including 7 figures; uses epsf.sty. final version, accepted
for publication in ApJ. Extended introduction and discussio
Lamination And Microstructuring Technology for a Bio-Cell Multiwell array
Microtechnology becomes a versatile tool for biological and biomedical
applications. Microwells have been established long but remained
non-intelligent up to now. Merging new fabrication techniques and handling
concepts with microelectronics enables to realize intelligent microwells
suitable for future improved cancer treatment. The described technology depicts
the basis for the fabrication of a elecronically enhanced microwell. Thin
aluminium sheets are structured by laser micro machining and laminated
successively to obtain registration tolerances of the respective layers of
5..10\^Am. The microwells lasermachined into the laminate are with
50..80\^Am diameter, allowing to hold individual cells within the well.
The individual process steps are described and results on the microstructuring
are given.Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/EDA-Publishing
Fireballs Loading and the Blast Wave Model of Gamma Ray Bursts
A simple function for the spectral power
is proposed to model, with 9 parameters, the spectral and temporal evolution of
the observed nonthermal synchrotron power flux from GRBs in the blast wave
model. Here mc is the observed dimensionless photon
energy and is the observing time. Assumptions and an issue of lack of
self-consistency are spelled out. The spectra are found to be most sensitive to
the baryon loading, expressed in terms of the initial bulk Lorentz factor
, and an equipartition term which is assumed to be constant in
time and independent of . Expressions are given for the peak spectral
power at the photon energy of the spectral power peak. A general rule is that the total
fireball particle kinetic energy , where is the deceleration time scale and is the maximum measured bolometric
power output in radiation, during which it is carried primarily by photons with
energy .Comment: 26 pages, including 4 figures, uses epsf.sty, rotate.sty; submitted
to ApJ; revised version with extended introduction, redrawn figures, and
correction
X-ray Spectral Signatures of the Photon Bubble Model for Ultraluminous X-ray Sources
The nature of ultraluminous X-ray sources in nearby galaxies is one of the
major open questions in modern X-ray astrophysics. One possible explanation for
these objects is an inhomogeneous, radiation dominated accretion disk around a
black hole -- the so-called ``photon bubble'' model. While
previous studies of this model have focused primarily on its
radiation-hydrodynamics aspects, in this paper, we provide an analysis of its
X-ray spectral (continuum and possible edge and line) characteristics. Compton
reflection between high and low density regions in the disk may provide the key
to distinguishing this model from others, such as accretion onto an
intermediate mass black hole. We couple a Monte Carlo/Fokker-Planck radiation
transport code with the XSTAR code for reflection to simulate the photon
spectra produced in a photon bubble model for ULXs. We find that reflection
components tend to be very weak and in most cases not observable, and make
predictions for the shape of the high-energy Comptonizing spectra. In many
cases the Comptonization dominates the spectra even down to a few keV.
In one simulation, a \sim 9 \kev feature was found, which may be considered a
signature of photon bubbles in ULXs; furthermore, we make predictions of high
energy power-laws which may be observed by future instruments.Comment: Accepted for publication in the Astrophysical Journa
Effects of geometric anisotropy on local field distribution: Ewald-Kornfeld formulation
We have applied the Ewald-Kornfeld formulation to a tetragonal lattice of
point dipoles, in an attempt to examine the effects of geometric anisotropy on
the local field distribution. The various problems encountered in the
computation of the conditionally convergent summation of the near field are
addressed and the methods of overcoming them are discussed. The results show
that the geometric anisotropy has a significant impact on the local field
distribution. The change in the local field can lead to a generalized
Clausius-Mossotti equation for the anisotropic case.Comment: Accepted for publications, Journal of Physics: Condensed Matte
H_2 Absorption and Fluorescence for Gamma Ray Bursts in Molecular Clouds
If a gamma ray burst with strong UV emission occurs in a molecular cloud,
there will be observable consequences resulting from excitation of the
surrounding H2. The UV pulse from the GRB will pump H2 into
vibrationally-excited levels which produce strong absorption at wavelengths <
1650 A. As a result, both the prompt flash and later afterglow will exhibit
strong absorption shortward of 1650 A, with specific spectroscopic features.
Such a cutoff in the emission from GRB 980329 may already have been observed by
Fruchter et al.; if so, GRB 980329 was at redshift 3.0 < z < 4.4 . BVRI
photometry of GRB 990510 could also be explained by H2 absorption if GRB 990510
is at redshift 1.6 < z < 2.3. The fluorescence accompanying the UV pumping of
the H2 will result in UV emission from the GRB which can extend over days or
months, depending on parameters of the ambient medium and beaming of the GRB
flash. The 7.5-13.6 eV fluorescent luminosity is \sim 10^{41.7} erg/s for
standard estimates of the parameters of the GRB and the ambient medium.
Spectroscopy can distinguish this fluorescent emission from other possible
sources of transient optical emission, such as a supernova.Comment: 13 pages, including 4 figures. submitted to Ap.J.(Letters
Redshift determination in the X-ray band of gamma-ray bursts
If gamma-ray bursts originate in dense stellar forming regions, the
interstellar material can imprint detectable absorption features on the
observed X-ray spectrum. Such features can be detected by existing and planned
X-ray satellites, as long as the X-ray afterglow is observed after a few
minutes from the burst. If the column density of the interstellar material
exceeds ~10^{23} cm^{-2} there exists the possibility to detect the K_alpha
fluorescent iron line, which should be visible for more than one year, long
after the X-ray afterglow continuum has faded away. Detection of these X-ray
features will make possible the determination of the redshift of gamma-ray
bursts even when their optical afterglow is severely dimmed by extinction.Comment: 15 pages with 5 figures. Submitted to Ap
Broadband Spectral Analysis of PKS 0528+134: A Report on Six Years of EGRET Observations
The multiwavelength spectra of PKS 0528+134 during six years of observations
by EGRET have been analyzed using synchrotron self-Compton (SSC) and external
radiation Compton (ERC) models. We find that a two-component model, in which
the target photons are produced externally to the gamma-ray emitting region,
but also including an SSC component, is required to suitably reproduce the
spectral energy distributions of the source. Our analysis indicates that there
is a trend in the observed properties of PKS 0528+134, as the source goes from
a gamma-ray low state to a flaring state. We observe that during the higher
gamma-ray states, the bulk Lorentz factor of the jet increases and the ERC
component dominates the high-energy emission. Our model calculations indicate
the trend that the energies of the electrons giving rise to the synchrotron
peak decreases, and the power-ratio of the gamma-ray and low energy spectral
components increases, as the source goes from a low to a high gamma-ray state.Comment: 36 pages, 13 figures, final version accepted for publication in ApJ;
includes minor modification
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