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GCN Circular 19011

GRB160131A: detection of polarisation by Astrosat CZTI
2016-02-12T13:23:06Z (8 years ago)
Dipankar Bhattacharya at IUCAA <>
S. V. Vadawale (PRL), T. Chattopadhyay  (PRL),  N. P. S. Mithun (PRL),
A. R. Rao (TIFR),  D. Bhattacharya (IUCAA), Varun Bhalerao (IUCAA)
report on behalf of the Astrosat CZTI collaboration:

Astrosat CZT Imager detected GRB 160131A (Page and Barthelmy, GCN 18951)
at an incident angle of 63 degrees from the telescope pointing axis.
The T90 was measured to be 106 s and 97 s in 30 - 100 keV and 100 - 300
keV respectively. CZTI being sensitive to polarimetry [Chattopadhyay et
al, 2014, EXPA, Vadawale et al 2015, A&A], we have analysed the double
events due to Compton scattering and detect a clear azimuthal modulation
of 0.30 +/- 0.12 %, with a detection significance of 2.5 sigma. Based on a
preliminary mass modelling, we estimate that the source polarisation
fraction lies between ~ 50 - 60 % in 100-300 keV band.  A detailed mass 
modeling to refine this measurement and its estimated uncertainty is in 
progress.  In an earlier event, GRB 151006A (Bhalerao et al. GCN 18422) 
CZT Imager data showed a marginal evidence for polarisation signal.  From 
the higher detection confidence for this brighter GRB160131A, we estimate 
that CZT Imager can provide time resolved spectro-polarimetry for GRBs 
with fluence larger than 10^-4 erg per cm^2.

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