[vsnet-grb-info 6640] GRB 080916A: Swift XRT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8244
SUBJECT: GRB 080916A: Swift XRT refined analysis
DATE:    08/09/16 17:19:57 GMT
FROM:    Rhaana Starling at U of Leicester  <rlcs1 at star.le.ac.uk>

R.L.C. Starling (U. Leicester) and H. Ziaeepour (MSSL-UCL) report on 
behalf of the Swift-XRT team:

We have analysed the first two orbits of Swift XRT data for GRB 080916A 
(Ziaeepour et al., GCN Circ. 8237) from T0+70 s to T0+7440 s with total 
exposure time 3.4 ks. The enhanced X-ray position is given in Evans et 
al., GCN Circ. 8241. 

The light curve can be represented by a 5-segment broken power law with an 
initial decay alpha1=1.3+0.4/-0.6 breaking at Tbk1=102+4/-6 s to a steep 
decay of alpha2=3.5+/-0.3 until Tbk2=190+25/-13 s. The light curve then 
flattens to alpha3=1.07+0.06/-0.3 and breaks at Tbk3=690+150/-140 s to a 
further steep decay of alpha4=2.9+2/-0.8. From Tbk4=1270+240/-220 s the 
decay becomes shallow again with alpha5=0.3+/-0.2. 

The spectrum from 3.2 ks of Photon Counting mode data can be fit with an 
absorbed power law with photon index Gamma=2.1+/-0.2 and intrinsic 
absorption (in excess of the Galactic value of 1.8e20 cm-2) of 
nH=(2.6+0.7/-0.6)e21 cm-2. This is consistent with the spectrum formed 
from the initial 170 s of Windowed Timing mode data. The observed 
(unabsorbed) count rate to flux conversion is 1 count/s = 4.0e-11 
(6.8e-11) erg/cm2/s. 

The predicted count rate at T+24hr would be approximately 0.1 count/s if 
the decay continues on the current plateau-like phase.

This circular is an official product of the Swift-XRT team.