A detailed time-resolved and energy-resolved spectro-polarimetric study of bright GRBs detected by AstroSat CZTI in its first year of operation
arxiv(2024)
摘要
The radiation mechanism underlying the prompt emission remains unresolved and
can be resolved using a systematic and uniform time-resolved
spectro-polarimetric study. In this paper, we investigated the spectral,
temporal, and polarimetric characteristics of five bright GRBs using archival
data from AstroSat CZTI, Swift BAT, and Fermi GBM. These bright GRBs were
detected by CZTI in its first year of operation, and their average polarization
characteristics have been published in Chattopadhyay et al. (2022). In the
present work, we examined the time-resolved (in 100-600 keV) and
energy-resolved polarization measurements of these GRBs with an improved
polarimetric technique such as increasing the effective area and bandwidth (by
using data from low-gain pixels), using an improved event selection logic to
reduce noise in the double events and extend the spectral bandwidth. In
addition, we also separately carried out detailed time-resolved spectral
analyses of these GRBs using empirical and physical synchrotron models. By
these improved time-resolved and energy-resolved spectral and polarimetric
studies (not fully coupled spectro-polarimetric fitting), we could pin down the
elusive prompt emission mechanism of these GRBs. Our spectro-polarimetric
analysis reveals that GRB 160623A, GRB 160703A, and GRB 160821A have Poynting
flux-dominated jets. On the other hand, GRB 160325A and GRB 160802A have
baryonic-dominated jets with mild magnetization. Furthermore, we observe a
rapid change in polarization angle by ∼ 90 degrees within the main pulse
of very bright GRB 160821A, consistent with our previous results. Our study
suggests that the jet composition of GRBs may exhibit a wide range of
magnetization, which can be revealed by utilizing spectro-polarimetric
investigations of the bright GRBs.
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