Prospects of hard X-ray polarimetry with Astrosat-CZTI

Chattopadhyay, Tanmoy; Vadawale, S.; Rao, A. R.; Sreekumar, S.; Bhattacharya, Dipankar

doi:10.1007/s10686-014-9386-1

Cited by 44 publications

(67 citation statements)

References 25 publications

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“…In this work, we have adopted the same analysis procedure as described in the above references, but have conducted the polarisation analysis of the two emission episodes separately. The Compton events are identified by first selecting the doublepixel events that occurred within a 40µs time window, and secondly, by filtering those double-pixel events against the various Compton kinematics criteria (Chattopadhyay et al 2014). The Compton lightcurve of GRB 160325A in 100-380 keV energy range is shown in Figure 7.…”

Section: Polarisation Measurementsmentioning

confidence: 99%

“…Despite the small number of Compton events, the second emission episode, on the other hand, is found to be highly polarised at > 3σ confidence level ( Figure 8) for two parameters of interest. This is because for on-axis GRB detection, the polarisation sensitivity of the instrument is high (Chattopadhyay et al 2014). In Figure 8, 2-D contours of confidence intervals of 68%, 90%, 99% (3 σ) and 99.9% (3.7 σ) of polarisation fraction and polarisation angle are shown 7 .…”

Section: Polarisation Measurementsmentioning

confidence: 99%

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Spectropolarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

Sharma

Iyyani

Bhattacharya

et al. 2020

Monthly Notices of the Royal Astronomical Society

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GRB 160325A is the only bright burst detected by AstroSat CZT Imager in its primary field of view to date. In this work, we present the spectral and polarimetric analysis of the prompt emission of the burst using AstroSat, Fermi and Niel Gehrels Swift observations. The prompt emission consists of two distinct emission episodes separated by a few seconds of quiescent/ mild activity period. The first emission episode shows a thermal component as well as a low polarisation fraction of PF < 37 % at 1.5 σ confidence level. On the other hand, the second emission episode shows a non-thermal spectrum and is found to be highly polarised with PF > 43 % at 1.5σ confidence level. We also study the afterglow properties of the jet using Swift/XRT data. The observed jet break suggests that the jet is pointed towards the observer and has an opening angle of 1.2 • for an assumed redshift, z = 2. With composite modelling of polarisation, spectrum of the prompt emission and the afterglow, we infer that the first episode of emission originates from the photosphere with localised dissipation happening below it, and the second from the optically thin region above the photosphere. The photospheric emission is generated mainly by inverse Compton scattering, whereas the emission in the optically thin region is produced by the synchrotron process. The low radiation efficiency of the burst suggests that the outflow remains baryonic dominated throughout the burst duration with only a subdominant Poynting flux component, and the kinetic energy of the jet is likely dissipated via internal shocks which evolves from an optically thick to optically thin environment within the jet.

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Section: Polarisation Measurementsmentioning

confidence: 99%

Section: Polarisation Measurementsmentioning

confidence: 99%

Spectropolarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

Sharma

Iyyani

Bhattacharya

et al. 2020

Monthly Notices of the Royal Astronomical Society

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“…The hard X-ray polarimetric capability of CZTI was extensively investigated by Chattopadhyay et al 7 , using Monte Carlo simulations. It was also experimentally verified before launch using polarized X-rays and more importantly, using unpolarized X-rays 8 .…”

Section: Hard X-ray Polarizationmentioning

confidence: 99%

Cadmium-Zinc-Telluride Imager On-Board Astrosat:A Multi-Faceted Hard X-Ray Instrument

Rao¹,

Bhattacharya²,

Bhalerao³

et al. 2017

Current Science

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The AstroSat satellite is designed to make multiwaveband observations of astronomical sources and the Cadmium-Zinc-Telluride Imager (CZTI) instrument of AstroSat covers the hard X-ray band. CZTI has a large area position-sensitive hard X-ray detector equipped with a coded aperture mask, thus enabling simultaneous background measurements. Ability to record simultaneous detection of ionizing interactions in multiple detector elements is a special feature of the instrument, and this is exploited to provide polarization information in the 100-380 keV region. CZTI provides sensitive spectroscopic measurements in the 20-100 keV region, and acts as an all-sky hard X-ray monitor and polarimeter above 100 keV. During the first year of operation, CZTI has recorded several gamma-ray bursts, measured the phase-resolved hard X-ray polarization of the Crab pulsar, and the hard X-ray spectra of many bright galactic X-ray binaries. The excellent timing capability of the instrument has been demonstrated with simultaneous observation of the Crab pulsar with radio telescopes like Gaint Metrewave Radio Telescope and Ooty Radio Telescope.

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“…The double pixel events within the photon tagging time window of 40 µs which satisfy the Compton kinematics are identified as valid Compton events. The validity of the Compton event selection and the polarization measurement capability of CZTI have been established by detailed simulation and experimental studies during ground calibration of CZTI (Chattopadhyay et al 2014;Vadawale et al 2015). The first onboard validation of CZTI polarimetry was obtained with the detection of polarization of GRB 160131A (Vadawale et al 2016) and GRB 151006A .…”

Section: Polarization Measurement -Further Evidence For the Second Pumentioning

confidence: 99%

Surprise in simplicity: an unusual spectral evolution of a single pulse GRB 151006A

Basak

Iyyani

Chand

et al. 2017

Monthly Notices of the Royal Astronomical Society

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We present a detailed analysis of GRB 151006A, the first GRB detected by Astrosat CZT Imager (CZTI). We study the long term spectral evolution by exploiting the capabilities of Fermi and Swift satellites at different phases, which is complemented by the polarization measurement with the CZTI. While the light curve of the GRB in different energy bands show a simple pulse profile, the spectrum shows an unusual evolution. The first phase exhibits a hard-to-soft (HTS) evolution until ∼ 16 − 20 s, followed by a sudden increase in the spectral peak reaching a few MeV. Such a dramatic change in the spectral evolution in case of a single pulse burst is reported for the first time. This is captured by all models we used namely, Band function, Blackbody+Band and two blackbodies+power law. Interestingly, the Fermi Large Area Telescope (LAT) also detects its first photon (> 100 MeV) during this time. This new injection of energy may be associated with either the beginning of afterglow phase, or a second hard pulse of the prompt emission itself which, however, is not seen in the otherwise smooth pulse profile. By constructing Bayesian blocks and studying the hardness evolution we find a good evidence for a second hard pulse. The Swift data at late epochs (> T 90 of the GRB) also shows a significant spectral evolution consistent with the early second phase. The CZTI data (100-350 keV), though having low significance (1σ), show high values of polarization in the two epochs (77% to 94%), in agreement with our interpretation.

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Prospects of hard X-ray polarimetry with Astrosat-CZTI

Cited by 44 publications

References 25 publications

Spectropolarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

Spectropolarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

Cadmium-Zinc-Telluride Imager On-Board Astrosat:A Multi-Faceted Hard X-Ray Instrument

Surprise in simplicity: an unusual spectral evolution of a single pulse GRB 151006A

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