31,422 research outputs found
Comparative study of different scattering geometries for the proposed Indian X-ray polarization measurement experiment using Geant4
Polarization measurements in X-rays can provide unique opportunity to study
the behavior of matter and radiation under extreme magnetic fields and extreme
gravitational fields. Unfortunately, over past two decades, when X-ray
astronomy witnessed multiple order of magnitude improvement in temporal,
spatial and spectral sensitivities, there is no (or very little) progress in
the field of polarization measurements of astrophysical X-rays. Recently, a
proposal has been submitted to ISRO for a dedicated small satellite based
experiment to carry out X-ray polarization measurement, which aims to provide
the first X-ray polarization measurements since 1976. This experiment will be
based on the well known principle of polarization measurement by Thomson
scattering and employs the baseline design of a central low Z scatterer
surrounded by X-ray detectors to measure the angular intensity distribution of
the scattered X-rays. The sensitivity of such experiment is determined by the
collecting area, scattering and detection efficiency, X-ray detector
background, and the modulation factor. Therefore, it is necessary to carefully
select the scattering geometry which can provide the highest modulation factor
and thus highest sensitivity within the specified experimental constraints. The
effective way to determine optimum scattering geometry is by studying various
possible scattering geometries by means of Monte Carlo simulations. Here we
present results of our detailed comparative study based on Geant4 simulations
of five different scattering geometries which can be considered within the
weight and size constraints of the proposed small satellite based X-ray
polarization measurement experiment.Comment: 14 pages, 6 figures, accepted for publication in "Nuclear Inst. and
Methods in Physics Research, A
A method for detecting gravitational waves coincident with gamma ray bursts
The mechanism for gamma ray bursters and the detection of gravitational waves
(GWs) are two outstanding problems facing modern physics. Many models of gamma
ray bursters predict copious GW emission, so the assumption of an association
between GWs and GRBs may be testable with existing bar GW detector data. We
consider Weber bar data streams in the vicinity of known GRB times and present
calculations of the expected signal after co-addition of 1000 GW/GRBs that have
been shifted to a common zero time. Our calculations are based on assumptions
concerning the GW spectrum and the redshift distribution of GW/GRB sources
which are consistent with current GW/GRB models. We discuss further
possibilities of GW detection associated with GRBs in light of future bar
detector improvements and suggest that co-addition of data from several
improved bar detectors may result in detection of GWs (if the GW/GRB assumption
is correct) on a time scale comparable with the LIGO projects.Comment: Accepted by MNRAS. 9 pages, 6 ps figures, MNRAS style. Proof
corrections made, accepted versio
Probing the low-luminosity GRB population with new generation satellite detectors
We compare the detection rates and redshift distributions of low-luminosity
(LL) GRBs localized by Swift with those expected to be observed by the new
generation satellite detectors on GLAST (now Fermi) and, in future, EXIST.
Although the GLAST burst telescope will be less sensitive than Swift's in the
15--150 keV band, its large field-of-view implies that it will double Swift's
detection rate of LL bursts. We show that Swift, GLAST and EXIST should detect
about 1, 2 & 30 LL GRBs, respectively, over a 5-year operational period. The
burst telescope on EXIST should detect LL GRBs at a rate of more than an order
of magnitude greater than that of Swift's BAT. We show that the detection
horizon for LL GRBs will be extended from for Swift to in the EXIST era. Also, the contribution of LL bursts to the observed GRB
redshift distribution will contribute to an identifiable feature in the
distribution at .Comment: 6 pages, 4 figures, accepted by MNRA
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