339 research outputs found
ASTROSAT: Some Key Science Prospects
ASTROSAT is an astronomy satellite designed for simultaneous multi-wavelength
studies in the Optical/UV and a broad X-ray energy range. With four X-ray
instruments and a pair of UV-Optical telescopes, ASTROSAT will provide
unprecendented opportunity for simultaneous multi-wavelength observations,
which is of immense value in study of highly variable sources, especially X-ray
binaries and Active Galactic Nuclei. The Large Area X-ray Proportional Counters
(LAXPC) of ASTROSAT, which has the largest effective area in the hard X-ray
band compared to all previous X-ray missions, will enable high time resolution
X-ray measurements in the 2-80 keV band with moderate energy resolution. Here
we give a brief summary of the payload characteristics of ASTROSAT and discuss
some of the main science topics that will be addressed with the LAXPC, and with
simultaneous observations with the UVIT telescopes, with particular emphasis on
X-ray binaries and compact objects. The possibility of aiding gravitational
wave experiments is also briefly mentioned.Comment: Invited Review to International Journal of Modern Physics D (based on
a talk in ASTROD-5 symposium at RRI, Bangalore
Transient High Mass X-ray Binaries
High Mass X-ray Binaries (HMXBs) are interesting objects that provide a wide
range of observational probes to the nature of the two stellar components,
accretion process, stellar wind and orbital parameters of the systems. A large
fraction of the transient HMXBs are found to be Be/X-ray binaries in which the
companion Be star with its circumstellar disk governs the outburst. These
outbursts are understood to be due to the sudden enhanced mass accretion to the
neutron star and is likely to be associated with changes in the circumstellar
disk of the companion. In the recent years, another class of transient HMXBs
have been found which have supergiant companions and show shorter bursts.
X-ray, infrared and optical observations of these objects provide vital
information regarding these systems. Here we review some key observational
properties of the transient HMXBs and also discuss some important recent
developments from studies of this class of sources. The X-ray properties of
these objects are discussed in some detail whereas the optical and infrared
properties are briefly discussed.Comment: 21 Pages, 8 Figures, To appear in the special issue of the Bulletin
of the Astronomical Society of India on Transients at different wavelengths,
eds D.J. Saikia and D.A. Gree
Prospect of polarisation measurements from black hole binaries in their thermal state with a scattering polarimeter
X-ray polarisation measurement is a unique tool which may provide crucial
information regarding the emission mechanism and the geometry of various
astrophysical sources, like neutron stars, accreting black holes, pulsar wind
nebulae, AGNs, Supernova Remnants etc. and can help us to probe matter under
extreme magnetic fields and extreme gravitational fields. Although the three
other domains of X-ray Astronomy i.e. timing, spectral and imaging are well
developed, there has been very little progress in X-ray Polarimetry with only
one definitive polarisation measurement and a few upper limits available so
far. Radiation from accreting black holes in their thermal dominated (High
Soft) state is expected to be polarised due to scattering in the plane parallel
atmosphere of the disk. Also, special and general relativistic effects in the
innermost parts of the disk predicts energy dependent rotation in the plane of
polarisation and some distinct signatures which can be used as a probe for
measuring the parameters of the black hole like its spin, emissivity profile
and the angle of inclination of the system. We present the results from an
analysis of expected minimum detectable polarisation from some of the galactic
black hole binaries GRO J1655-40, GX339-4, H1743-322, Cyg X-1 and XTE J1817-330
in their thermal dominated state with a proposed Thomson X-ray Polarimeter. A
proposal for a scattering polarimeter has been submitted to the Indian Space
Research Organization (ISRO) for a dedicated small satellite mission and a
laboratory unit has been built. Along with the measurement of the degree of
polarisation, the polarisation angle measurement is also important, hence the
error in the polarisation angle measurement for a range of detection
significance is also obtained.Comment: 12 pages, 13 figures, 2 tables, accepted for publication by MNRA
BRST symmetry for Regge-Teitelboim based minisuperspace models
The Einstein-Hilbert action in the context of Higher derivative theories is
considered for finding out their BRST symmetries. Being a constraint system,
the model is transformed in the minisuperspace language with the FRLW
background and the gauge symmetries are explored. Exploiting the first order
formalism developed by Banerjee et. al. the diffeomorphism symmetry is
extracted. From the general form of the gauge transformations of the field, the
analogous BRST transformations are calculated. The effective Lagrangian is
constructed by considering two gauge fixing conditions. Further, the BRST
(conserved) charge is computed which plays an important role in defining the
physical states from the total Hilbert space of states. The finite field
dependent BRST (FFBRST) formulation is also studied in this context where the
Jacobian for functional measure is illustrated specifically.Comment: 16 pages, title changed, revised version, to appear in EPJ
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