[vsnet-grb-info 9703] GRB 100901A: Swift-XRT Team refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  11171
SUBJECT: GRB 100901A: Swift-XRT Team refined analysis
DATE:    10/09/02 06:00:48 GMT
FROM:    Kim Page at U.of Leicester  <kpa at star.le.ac.uk>

K.L. Page (U. Leicester) and S. Immler (CRESST/GSFC/UMD) report on behalf 
of the Swift-XRT team:

We have analysed 13 ks of XRT data for GRB 100901A (Immler et al., GCN 
Circ. 11159), from 128 s to 33.2 ks after the BAT trigger. The data 
comprise 448 s in Windowed Timing (WT) mode (the first 9 s were taken 
while Swift was slewing) with the remainder in Photon Counting (PC) mode. 
The enhanced XRT position for this burst was given by Osborne et al. (GCN. 
Circ 11167).

The first snapshot (data collected until ~620 s after the trigger) 
consists of a number of superimposed flares, with the main peak around 410 
s; this peak was also seen in the BAT data (Sakamoto et al., GCN Circ. 
11169). The light curve then shows a decaying trend with alpha = 0.74 +/- 
0.12, until around 12 ks, after which there is a small rebrightening, to a 
plateau level of around 0.3 count s^-1.

A spectrum formed from the WT mode data can be fitted with an absorbed 
power-law with a photon spectral index of 1.68 (+/-0.04). The best-fitting 
absorption column is 3.6 (+0.7, -0.6) x 10^21 cm^-2, at a redshift of 
1.408 (Chornock et al., GCN Circ 11164), in addition to the Galactic value 
of 7.1 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a 
photon index of 2.19 (+/-0.07) and a best-fitting absorption column of 3.9 
(+/-0.9) x 10^21 cm^-2 (in addition to the Galactic vale). The counts to 
observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this 
spectrum is 3.5 x 10^-11 (5.1 x 10^-11) erg cm^-2 count^-1.

Because of the rebrightening/plateau shape of the light curve at this time, we 
cannot sensibly predict the count rate at 24 hours.

The results of the XRT-team automatic analysis are available at 
http://www.swift.ac.uk/xrt_products/00433065.

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