[vsnet-grb-info 7382] GRB 090307: Swift-XRT afterglow confirmation and analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8946
SUBJECT: GRB 090307: Swift-XRT afterglow confirmation and analysis
DATE:    09/03/08 13:54:58 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at star.le.ac.uk>

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

Swift-XRT observed the field of GRB 090307 (BAT trigger 345551; GCN
Circs. 8943, 8945) for 4.6 ks, from 135 s to 80 ks after the BAT
trigger. All of the data were taken in Photon Counting (PC) mode.  The
X-ray source reported by Evans et al. (GCN Circ. 8943) is seen to fade,
and is therefore identified as the afterglow, confirming the GRB nature
of this BAT trigger. The source has an enhanced XRT position: RA, Dec =
244.9944, -28.6333 which is equivalent to:

RA (J2000): 16 19 58.64
Dec (J2000): -28 37 59.9

with an uncertainty of 2.1 arcsec (radius, 90% confidence). The light
curve shows a power-law decay, with an index 0.7 (+0.2, -0.1).

The PC mode spectrum can be fitted with an absorbed power-law, with a
photon index of 2.21 (+0.72, -0.60) and an absorbing column of 3.5
(+2.7,-1.1) e21 cm-2, in excess of the Galactic value of 1.3e21 cm-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.1 x 10-11 (7.6 x 10-11) erg cm-2
count-1.

If the light curve continues to decay with alpha=0.7, the XRT count-rate
at T+48 hrs will be 0.002 counts s-1, corresponding to an observed
(unabsorbed) 0.3-10 keV flux of 7.38e-14 (1.4e-13) erg cm-2 s-1.

The results of the automatic XRT analysis are available online at
http://www.swift.ac.uk/xrt_products/00345551.

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