[AAVSO-HEN] GRB 080516: Swift-XRT Team refined analysis

[grb at aavso.org]

The following is a GCN Circular circulated via the AAVSO network.
For an archive and more information on circulars visit:
http://lheawww.gsfc.nasa.gov/docs/gamcosray/legr/bacodine/gcn3_archive.html

TITLE:   GCN CIRCULAR
NUMBER:  7737
SUBJECT: GRB 080516: Swift-XRT Team refined analysis 
DATE:    08/05/16 11:08:12 GMT
FROM:    Kim Page at U.of Leicester  <kpa at star.le.ac.uk>

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

We have analysed the first five orbits of data obtained for GRB 080516 
(Holland et al., GCN Circ. 7733), covering 11.5 ks of Photon Counting (PC) 
mode data. The UVOT-enhanced XRT position was given by Beardmore et al. in 
GCN Circ. 7735.

The light-curve begins relatively flat, with a decay of alpha ~ 0.36 +/- 
0.17, until around 4800 seconds after the trigger, at which point the 
slope steepens to 1.04 +0.20/-0.18. 

The first five orbits of PC data can be modelled with an absorbed 
power-law, with Gamma = 2.7 +/- 0.4 and total NH = (1.3 +/- 0.3)x10^22 
cm^-2, considerably in excess of the Galactic value of 3.22x10^21 cm^-2. 
Using the relation in Grupe et al. (2007, ApJ, 133, 2216), this implies an 
estimated upper limit on the redshift of the burst of z<1.0. There is no 
evidence for spectral evolution across the temporal break. The observed 
(unabsorbed) flux over this time interval (90-2.5x10^4 s after the 
trigger) is 2.38x10^-12 (1.06x10^-11) erg cm^-2 s^-1. The counts to 
observed flux conversion is 5.8x10^-11 erg cm^-2 count^-1.

If the light-curve continues to decay at alpha ~ 1.04, the count rate at 
24 hours is predicted to be 3.7x10^-3 count s^-1, corresponding to an 
observed flux of 2.1x10^-13 erg cm^-2 s^-1.

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