21 research outputs found
AstroSat-CZTI detection of variable prompt emission polarization in GRB 171010A
We present spectro-polarimetric analysis of \thisgrb\ using data from \asat,
\fermi, and \swift, to provide insights into the physical mechanisms of the
prompt radiation and the jet geometry.
Prompt emission from \thisgrb\ was very bright (fluence
~ergs~cm) and had a complex structure composed of the
superimposition of several pulses. The energy spectra deviate from the typical
Band function to show a low energy peak ~keV --- which we interpret as
a power-law with two breaks, with a synchrotron origin. Alternately, the prompt
spectra can also be interpreted as Comptonized emission, or a blackbody
combined with a Band function. Time-resolved analysis confirms the presence of
the low energy component, while the peak energy is found to be confined in the
range of 100--200~keV.
Afterglow emission detected by \fermi-LAT is typical of an external shock
model, and we constrain the initial Lorentz factor using the peak time of the
emission. \swift-XRT measurements of the afterglow show an indication for a jet
break, allowing us to constrain the jet opening angle to 6\degr.
Detection of a large number of Compton scattered events by \asat-CZTI
provides an opportunity to study hard X-ray polarization of the prompt
emission. We find that the burst has high, time-variable polarization, with the
emission {\bf have higher polarization} at energies above the peak energy.
We discuss all observations in the context of GRB models and polarization
arising due to {\bf due to physical or geometric effects:} synchrotron emission
from multiple shocks with ordered or random magnetic fields, Poynting flux
dominated jet undergoing abrupt magnetic dissipation, sub-photospheric
dissipation, a jet consisting of fragmented fireballs, and the Comptonization
model.Comment: Accepted for publication in Ap
Prompt emission polarimetry of Gamma Ray Bursts with ASTROSAT CZT-Imager
X-ray and Gamma-ray polarization measurements of the prompt emission of
Gamma-ray bursts (GRBs) are believed to be extremely important for testing
various models of GRBs. So far, the available measurements of hard X-ray
polarization of GRB prompt emission have not significantly constrained the GRB
models, particularly because of the difficulty of measuring polarization in
these bands. The CZT Imager (CZTI) onboard {\em AstroSat} is primarily an X-ray
spectroscopic instrument that also works as a wide angle GRB monitor due to the
transparency of its support structure above 100 keV. It also has experimentally
verified polarization measurement capability in the 100 300 keV energy
range and thus provides a unique opportunity to attempt spectro-polarimetric
studies of GRBs. Here we present the polarization data for the brightest 11
GRBs detected by CZTI during its first year of operation. Among these, 5 GRBs
show polarization signatures with 3, and 1 GRB shows
2 detection significance. We place upper limits for the remaining 5
GRBs. We provide details of the various tests performed to validate our
polarization measurements. While it is difficult yet to discriminate between
various emission models with the current sample alone, the large number of
polarization measurements CZTI expects to gather in its minimum lifetime of
five years should help to significantly improve our understanding of the prompt
emission.Comment: Accepted for Publication in ApJ ; a figure has been update
Suzaku observation of the transient X-ray pulsar GRO J1008-57
We report the timing and broad-band spectral properties of the Be transient
high mass X-ray binary pulsar GRO J1008-57 using a Suzaku observation in the
declining phase of its 2007 November-December outburst. Pulsations with a
period of 93.737 s were clearly detected in the light curves of the pulsar up
to the 80-100 keV energy band. The pulse profile was found to be strongly
energy dependent, a double peaked profile at soft X-ray energy bands (< 8 keV)
and a single peaked smooth profile at hard X-rays. The broad-band energy
spectrum of the pulsar, reported for the first instance in this paper, is well
described with three different continuum models viz. (i) a high energy cut-off
power-law, (ii) a Negative and Positive power-law with EXponential cut-off
(NPEX), and (iii) a partial covering power-law with high energy cut-off.
Inspite of large value of absorption column density in the direction of the
pulsar, a blackbody component of temperature ~0.17 keV for the soft excess was
required for the first two continuum models. A narrow iron K_\alpha emission
line was detected in the pulsar spectrum. The partial covering model, however,
is found to explain the phase averaged and phase resolved spectra well. The dip
like feature in the pulse profile can be explained by the presence of an
additional absorption component with high column density and covering fraction
at the same pulse phase. The details of the results are described in the paper.Comment: 9 pages, 7 figures, Accepted for publication in MNRA