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
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 ), 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
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 ).

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.