Abstract
Abstract GRB 160821A is the third most energetic gamma-ray burst observed by the Fermi gamma-ray space telescope. Based on the observations made by the Cadmium Zinc Telluride Imager on board AstroSat, here we report the most conclusive evidence to date of (i) high linear polarization ( detection), and (ii) variation of polarization angle with time, occurring twice during the rise and decay phase of the burst at 3.5σ and 3.1σ detections, respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines that are highly ordered on angular scales of 1/Γ, where Γ is the Lorentz factor of the outflow.
Highlights
Gamma-ray bursts (GRBs) are the most intense astrophysical outbursts in the Universe
In this Letter, for the first time, we present a conclusive evidence of polarization across the GRB 160821A in the energy range 100 − 300 keV using CZTI (Cadmium Zinc Telluride Imager) instrument aboard AstroSat (Singh et al 2014)
Based on the clear change in polarization angle (PA) where we could constrain the polarization at a higher significance (i.e the lower limit of 99% confidence interval of two parameters of interest of polarization fraction (PF) lie greater than zero) and the observed change in hardness ratio (HR) at high energies, we performed a relatively coarser time-resolved polarization analysis of the GRB by dividing the main episode into three non-overlapping time intervals: 115-129 s, 131 -139 s and 142 -155 s, which correspond to the rise, peak and decay phase of the burst respectively
Summary
Gamma-ray bursts (GRBs) are the most intense astrophysical outbursts in the Universe. In the last several decades, the spectra of prompt gamma-ray emission of GRBs have been extensively studied using various space observatories like BATSE onboard CGRO (Fishman 2013), Niel Gehrels Swift (Gehrels & Swift 2004), Fermi (Meegan et al 2009; Atwood et al 2009) etc. In this Letter, for the first time, we present a conclusive evidence of polarization across the GRB 160821A in the energy range 100 − 300 keV using CZTI (Cadmium Zinc Telluride Imager) instrument aboard AstroSat (Singh et al 2014). For the time interval T0 − 4.1 s to T0 + 194.6 s, an energy flux of 2.86 ± 0.007 × 10−6 erg/cm2/s is obtained in 10-1000 keV band This makes the burst the third brightest GRB observed by Fermi till date in terms of energy flux.
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