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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) on board 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 energy range 100–300 keV and thus provides a unique opportunity to attempt spectropolarimetric 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, five GRBs show polarization signatures with ⪆3σ, and one GRB shows 2σ detection significance. We place upper limits for the remaining five 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 that CZTI expects to gather in its minimum lifetime of five years should help to significantly improve our understanding of the prompt emission. [ABSTRACT FROM AUTHOR]

We present a spectropolarimetric analysis of GRB 171010A using data from AstroSat, Fermi, and Swift to provide insights into the physical mechanisms of the prompt radiation and jet geometry. The prompt emission from GRB 171010A was very bright (fluence >10−4 erg cm−2) and had a complex structure that was composed of the superimposition of several pulses. The energy spectra deviate from the typical Band function to show a low-energy peak ∼15 keV, which we interpret as a power law with two breaks, with a synchrotron origin. Alternatively, the prompt spectra can also be interpreted as Comptonized emission, or a blackbody combined with a Band function. A 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. The 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°. The detection of a large number of Compton-scattered events by AstroSat-CZTI provides an opportunity to study the hard X-ray polarization of the prompt emission. We find that the burst has a high time-variable polarization, and the emission has a higher polarization at energies above the peak energy. We discuss all observations in the context of GRB models and polarization arising as a result of physical or geometric effects: synchrotron emission from multiple shocks with ordered or random magnetic fields, a jet dominated by Poynting flux that undergoes abrupt magnetic dissipation, subphotospheric dissipation, a jet consisting of fragmented fireballs, and the Comptonization model. [ABSTRACT FROM AUTHOR]

We present the X-ray spectral and timing analysis of the transient black hole X-ray binary 4U 1630–47, observed with the AstroSat, Chandra, and MAXI space missions during its soft X-ray outburst in 2016. The outburst, from the rising phase until the peak, is detected neither in hard X-rays (15–50 keV) by the Swift/BAT nor in radio. Such nondetection, along with the source behavior in the hardness–intensity and color–color diagrams obtained using MAXI data, confirms that both Chandra and AstroSat observations were performed during the HS spectral state. The High Energy Grating (HEG) spectrum from the Chandra High-Energy Transmission Grating Spectrometer shows two strong, moderately blueshifted absorption lines at keV and keV, which are produced by Fe xxv and Fe xxvi in a low-velocity ionized disk wind. The corresponding outflow velocity is determined to be 366 ± 56 km s−1. Separate spectral fits of Chandra/HEG, AstroSat/SXT+LAXPC, and Chandra/HEG+AstroSat/SXT+LAXPC data show that the broadband continuum can be well described with a relativistic disk blackbody model, with a disk flux fraction of ∼0.97. Based on the best-fit continuum spectral modeling of Chandra, AstroSat, and Chandra+AstroSat joint spectra and using the Markov chain Monte Carlo simulations, we constrain the spectral hardening factor at and the dimensionless black hole spin parameter at 0.92 ± 0.04 within the 99.7% confidence interval. Our conclusion of a rapidly spinning black hole in 4U 1630–47 using the continuum spectrum method is in agreement with a previous finding applying the reflection spectral fitting method. [ABSTRACT FROM AUTHOR]

GRB 160802A is one of the brightest gamma-ray bursts (GRBs) observed by the Fermi Gamma-ray Burst Monitor (GBM) in the energy range of 10–1000 keV, while at the same time it is surprisingly faint at energies ≳2 MeV. An observation with the AstroSat/CZT Imager also provides the polarization that helps in constraining different prompt emission models using the novel joint spectra-polarimetric data. We analyze the Fermi/GBM data, and find two main bursting episodes that are clearly separated in time, one of which is particularly faint in higher energies and having certain differences in their spectra. The spectrum in general shows a hard-to-soft evolution in both the episodes. Only the later part of the first episode shows intensity tracking behavior corresponding to multiple pulses. The photon index of the spectrum is hard, and in over 90% cases, crosses even the slow cooling limit (α = −2/3) of an optically thin synchrotron shock model. Though such hard values are generally associated with a sub-dominant thermal emission, such a component is not statistically required in our analysis. In addition, the measured polarization in 100–300 keV is too high, π = 85 ± 29%, to be accommodated in such a scenario. Jitter radiation, which allows a much harder index up to α = +0.5, in principle can produce high polarization, but only beyond the spectral peak, which in our case lies close to 200–300 keV during the time when most of the polarization signal is obtained. The spectro-polarimetric data seems to be consistent with a subphotospheric dissipation process occurring within a narrow jet with a sharp drop in emissivity beyond the jet edge, and viewed along its boundary. [ABSTRACT FROM AUTHOR]