GCN Circular 9368

A.P. Beardmore and P.A. Evans (U. Leicester) report on behalf of the
Swift-XRT team :

We have analysed 4.5 ks of XRT data for GRB 090515 (Beardmore et
al. GCN Circ. 9356
Not found
 
 
), from 70 s to 11.5 ks after the BAT trigger. The
data comprise 132 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode.  The enhanced XRT position for this
burst was given by Osborne et al. (GCN Circ. 9367
Not found
 
 
).

Initially, the light curve follows a shallow decay from a rate of 30
count s^-1 (at T+70 s), with a decay index of 0.59, out to
approximately T+245 s. The count rate then drops by a factor of
~250 in the next ~70 s (i.e. suggesting a decay index in excess of 10
at this time). The burst is no longer detected beyond the first orbit
to a 3 sigma count rate upper limit of 0.0058 count s^-1 (from T+5.7
to T+11.5 ks).

A spectrum formed from the WT mode data (from T+70 to T+195 s) can be
fitted with an absorbed power-law with a photon spectral index of
1.88 +/- 0.14. The best-fitting absorption column is (6.1 +3.0 -2.8) x
10^20 cm^-2, in excess of the Galactic value of 1.9 x 10^20 cm^-2
(Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV
flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.1
x 10^-11) erg cm^-2 count^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00352108.

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