TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5428
SUBJECT: GRB 060805B: XRT Team Analysis
DATE:    06/08/09 13:50:00 GMT
FROM:    Olivier Godet at U.of Leicester  <og19@star.le.ac.uk>

O. Godet, K. L. Page, M. R. Goad, E. Rol, P.T. O'Brien (U Leicester),
P. Schady (UCL-MSSL), and K. Hurley (UCB) report on behalf of the
Swift-XRT team:

GRB 060805B was detected by IPN (GNC Circ 5407). XRT started to observe
35.1 hours after the IPN trigger for a present 25ks of data. The full
IPN error box was within the XRT field of view. Using the ftool
xrtcentroid, the first 25ks of the XRT data revealed an X-ray source in
the IPN error box at (J2000):

RA =  18 10 05.56
Dec = +58 09 16.3

with an uncertainty of 3.9 arcseconds (90% containment). This position
consistent with that given by Butler 2006 (GCN Circ. 5414) does not
correspond to any catalogued X-ray sources. We note that the X-ray
position is consistent with the position of two optical sources S1 and
S2 in the USNO-B1.0 (Monet et al. 2003) and APM-North (McMahon et al.
2000) catalogues, repectively.  The source S1 (RA(J2000) = 18 10 05.56
and Dec = +58 09 19.3) is 2.9 arcseconds away from the XRT position. The
source S2 (RA(J2000) = 18 10 05.60 and Dec = +58 09 19.1) is 2.8
arcseconds away from the XRT position. The positions of the optical
sources are consistent with those reported by Butler 2006 (GCN Circ.
5414).

The spectrum from the first 19ks of the PC data can be well fitted by an
absorbed power-law with Gamma = 1.78 +0.36/-0.33. No clear evidence of
absorption in excess over the Galactic absorption (nH(Gal) = 3.7e20
cm^-2 from Dickey & Lockman 1990) is seen in the data. The unabsorbed
flux in the 0.5-10 keV energy band is 3.9 +1.8/-1.1e-13 erg cm^-2 s^-1.
These results are consistent with those given by Butler (2006; GCN Circ
5414).

Using the initial 19ks of data did not allow us to determine if the
X-source was really fading. Indeed, fitting the light-curve with a
constant (C) gives a value of chi^2/dof=6.5/7 with C = 0.0087+/-0.0012,
while using a power-law gives a value of chi^2/dof=4.0/6 with alpha =
1.62 +1.70/-1.56 (http://www.swift.ac.uk/grb060805b.shtml). Using a
power-law allows an improvement of delta chi^2 = 2.5 for 1 dof i.e. less
than 90% confidence. 

Adding 6 ks of data obtained at later time allows us to confirm that the
X-ray source is actually fading with a decay slope of alpha = 1.20
+0.72/-0.53 (http://www.swift.ac.uk/grb060805b.shtml) and is likely to
be the afterglow of GRB 060805B.

We would like to encourage ground observations in order to determine the
redshift of this burst.

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