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GCN Circular 9914

Subject
GRB 090915: Swift-XRT detection of a fading X-ray counterpart
Date
2009-09-16T11:19:58Z (14 years ago)
From
Andy Beardmore at U Leicester <apb@star.le.ac.uk>
A. P. Beardmore, K. L. Page (U. Leicester) and T. Sakamoto (NASA/UMBC)
report on behalf of the Swift-XRT team :

The Swift-XRT started observing the field of GRB 090915 (Cummings and
Markwardt, GCN Circ. 9912) at 2009-09-16 03:59 UT, 12.4 hours after
the trigger. In 5ks of photon counting mode data, spanning two orbits,
we detect a source which fades from 0.0099 +/- 0.0022 count/s to
0.0033 +/- 0.0016 over a interval of 5.9ks.

Using 1678 s of XRT Photon Counting mode data and 1 UVOT image, 
we find an astrometrically corrected X-ray position (using the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue) for the source of RA, Dec (J2000) = 238.02021, 15.48775
which is equivalent to :

RA  (J2000):  15 52 4.85
Dec (J2000): +15 29 15.9

with an uncertainty of 3.3 arcsec (radius, 90% confidence).

This position may be improved as more data are received.  The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

The spectrum of the data from 44.6 ks to 52.8 ks after the trigger can
be fit by an absorbed powerlaw with a photon index of 1.20 +0.97/-0.54 
and a column density upper limit of 1.4 x 10^21 cm^-2, consistent with
the Galactic value of 3.2 x 10^20 cm^-2 in the direction of the
burst. The observed 0.3-10 keV flux for this spectrum is 4.9 +3.3/-4.9
x 10^-13 erg cm^-2 s^-1 which corresponds to an unabsorbed 0.3-10 keV
flux of 5.1 x 10^-13 erg cm^-2 s^-1.

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