GCN Circular 9819

P.A. Evans (U. Leicester), C. Guidorzi (U Ferrara) and W.H. Baumgartner
(GSFC/UMBC) report on behalf of the Swift-XRT team:

The Swift-XRT observed the field of GRB 090817 (Beckmann et al. GCN
Circ. 9815
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 ) from T0+11 ks to T0+17.5 ks gathering 1.8 ks of Photon
Counting (PC) mode data.

Inside the INTEGRAL error circle we find a single, uncatalogued X-ray
source at RA, Dec = 63.96600, 44.12900 which is equivalent to:

RA (J2000):  04 15 51.85
Dec (J2000): +44 07 44.5

with an uncertainty of 2.0 arcsec (radius, 90% confidence). The optical
source USNO-B1.0 1341-0096763 lies only 1.3 arcsec from the XRT
position. This object is point-like in USNO-B1, with magnitudes I=18.5,
R=20.0, B=20.8.

The light curve of the X-ray source shows a rapid rise from 0.04 ct/sec
at T0+11 ks to 0.1 ct s^-1 at T0+12 ks; by T0+18 ks the count rate has
returned to 0.04 ct s^-1.  A spectrum formed from the PC mode data can
be modelled with an absorbed power law with a photon index of 2.5 (+0.7,
-0.6) with an absorption column of 9.5 (+4.9, -4.0) x 10^21 cm^-2, in
excess of the Galactic value of 4.0 x 10^21 cm^-2 (Kalberla et al.
2005).

Given the light curve of this source, it is not clear whether or not
this is a GRB afterglow. A further observation has therefore been
planned, and will take place today.

No other sources are detected in the INTEGRAL error circle, with a
3-sigma upper limit of 5 x 10^-3 ct s^-1. Using the typical
counts-to-observed-flux conversion factor from Evans et al (2009, MNRAS,
397, 1177) this gives a flux limit of 1.7 x 10^-13 erg cm^-2 s^1.

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



-- 
-------------------------

Phil Evans,
Swift Development Scientist
X-ray and Observational Astronomy Group,
University of Leicester

Tel: (0116) 252 5059
pae9@star.le.ac.uk
http://www.star.le.ac.uk/~pae9