1,184 research outputs found
Dust sublimation by GRBs and its implications
The prompt optical flash recently detected accompanying GRB990123 suggests
that, for at least some GRBs, gamma-ray emission is accompanied by prompt
optical-UV emission with luminosity L(1-7.5eV)=10^{49}(\Delta\Omega/4\pi)erg/s,
where \Delta\Omega is the solid angle into which gamma-ray and optical-UV
emission is beamed. Such an optical-UV flash can destroy dust in the beam by
sublimation out to an appreciable distance, approximately 10 pc, and may clear
the dust out of as much as 10^7(\Delta\Omega/4\pi)M_sun of molecular cloud
material on an apparent time scale of 10 seconds. Detection of time dependent
extinction on this time scale would therefore provide strong constraints on the
GRB source environment. Dust destruction implies that existing, or future,
observations of not-heavily-reddened fireballs are not inconsistent with GRBs
being associated with star forming regions. In this case, however, if gamma-ray
emission is highly beamed, the expanding fireball would become reddened on a 1
week time scale.
If the optical depth due to dust beyond approximately 8 pc from the GRB is
0.2<\tau_V<2, most of the UV flash energy is converted to infra-red, \lambda
\sim 1 micron, radiation with luminosity \sim 10^{41} erg/s extending over an
apparent duration of \sim 20(1+z)(\Delta\Omega/0.01) day. Dust infra-red
emission may already have been observed in GRB970228 and GRB980326, and may
possibly explain their unusual late time behavior.Comment: 16 pages, including 1 figure, submitted to Ap
Multiwavelength Observations of GX 339-4 in 1996. II. Rapid X-ray Variability
As part of our multiwavelength campaign of GX 339-4 observations in 1996 we
present the rapid X-ray variability observed July 26 using the RXTE when the
source was in a hard state (= soft X-ray low state). We found that the source
was extremely variable, with many bright flares. The flares have relatively
symmetric time profiles. There are a few time intervals where the flux rises
steadily and then drops suddenly, sometimes to a level lower than the average
before the increase. Hardness ratios showed that the source was slightly softer
when the flux was brighter. The power density spectra (PDS) were also
complicated and we found that broken power laws do not provide adequate fits to
any of them. Instead a pair of zero-centered Lorentzians gives a good general
description of the shape of the PDS. We found several quasi-periodic
oscillations (QPO), including some that are harmonically spaced with the most
stable frequency at 0.35 Hz. While the overall rms variability of the source
was close to being constant throughout the observation (29% integrating between
0.01 and 50 Hz), there is a small but significant change in the PDS shape with
time. More importantly, we show that the soft 2-5 keV band is more variable
than the harder 5-10 and 10-40 keV bands, which is unusual for this source and
for other black hole candidates. Cross correlation functions (CCF) between
these bands show that the light curve for the 10-40 keV band lags that of the
2-5 keV band by 5 msec.Comment: Submitted to Astrophysical Journal. 20 pages. 8 figure
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
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
Early X-ray/UV Line Signatures of GRB Progenitors and Hypernovae
We calculate the X-ray/UV spectral line signatures expected from the
interaction of a gamma-ray burst afterglow and a dense pre-burst environment
produced by the progenitor. We explore the conditions under which Fe line and
edge equivalent widths of 1 keV can arise, and discuss the possibility
of gaining information about possible progenitor scenarios using X-ray metal
line spectra in the first few days of a burst. A wind or supernova shell around
the burst produces an X-ray absorption line spectrum and later emission lines,
while a hypernova funnel model produces mainly emission lines. The Fe \ked can
in some cases be more prominent than the Fe \kal line. Under simple assumptions
for the input continuum luminosity, current reports of observed Fe line
luminosities are compatible with an Fe-enriched funnel model, while lower
values are expected in shell models.Comment: revisions to ApJ ms first submitted 8/21/99; uses a higher and
flatter input spectrum, with modified implications suggesting preference for
funnel model
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
Time-Dependent Synchrotron and Compton Spectra from Jets of Microquasars
Jet models for the high-energy emission of Galactic X-ray binary sources have
regained significant interest with detailed spectral and timing studies of the
X-ray emission from microquasars, the recent detection by the HESS
collaboration of very-high-energy gamma-rays from the microquasar LS~5039, and
the earlier suggestion of jet models for ultraluminous X-ray sources observed
in many nearby galaxies. Here we study the synchrotron and Compton signatures
of time-dependent electron injection and acceleration, adiabatic and radiative
cooling, and different jet geometries in the jets of Galactic microquasars.
Synchrotron, synchrotron-self-Compton, and external-Compton radiation processes
with soft photons provided by the companion star and the accretion disk are
treated. An analytical solution is presented to the electron kinetic equation
for general power-law geometries of the jets for Compton scattering in the
Thomson regime. We pay particular attention to predictions concerning the rapid
flux and spectral variability signatures expected in a variety of scenarios,
making specific predictions concerning possible spectral hysteresis, similar to
what has been observed in several TeV blazars. Such predictions should be
testable with dedicated monitoring observations of Galactic microquasars and
ultraluminous X-ray sources using Chandra and/or XMM-Newton.Comment: Accepted for publication in ApJ; 37 manuscript pages, including 10
eps figures; uses AASTeX macro
Gamma Ray Burst Neutrinos Probing Quantum Gravity
Very high energy, short wavelength, neutrinos may interact with the
space-time foam predicted by theories of quantum gravity. They would propagate
like light through a crystal lattice and be delayed, with the delay depending
on the energy. This will appear to the observer as a violation of Lorenz
invariance. Back of the envelope calculations imply that observations of
neutrinos produced by gamma ray bursts may reach Planck-scale sensitivity. We
revisit the problem considering two essential complications: the imprecise
timing of the neutrinos associated with their poorly understood production
mechanism in the source and the indirect nature of their energy measurement
made by high energy neutrino telescopes.Comment: 14 pages, 2 figure
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