GCN Circular 6646

A.P. Beardmore, K.L. Page, P.A. Evans (U. Leicester) and H. Ziaeepour 
(MSSL-UCL) report on behalf of the Swift-XRT team :

We have analysed the first three orbits of Swift-XRT data on GRB 070721B
(Ziaeepour et al., GCN 6640
Not found
 
 
). Using 5.78ks of Photon Counting mode data, 
we find a refined XRT position of  RA, Dec = 33.13710, -2.19462 which is

RA(J2000):   02:12:32.90
Dec(J2000): -02:11:40.6

with an uncertainty of 3.5 arcseconds (90% containment radius).  This
is 35 arcseconds from the refined BAT position reported by Palmer et
al. (GCN 6643
Not found
 
 
), 8.9 arcsec from the original XRT position reported in
Ziaeepour et al. (GCN 6640
Not found
 
 
), and 0.7 arcseconds from the UVOT position
reported by Schady (GCN 6641
Not found
 
 
).

The X-ray light curve shows an initial steep decay from T+100s to
T+144s, followed by a number of flares from T+255s to T+800s
which reached a maximum count rate of 50 count/s at T+315s.  The
underlying decay is a powerlaw with a slope of approximately -0.9,
though it is difficult to be precise with the data available so far.

The X-ray spectrum from the Windowed Timing mode data obtained during
the non-flare intervals from the 1st orbit (T+144s to T+220s and
T+400s to T+475s) can be fit with an absorbed powerlaw to give a photon
index of 1.48 +0.18-0.16 and a column density of 1.9 (+1.8-1.9) e20 cm^-2,
consistent with the Galactic value of 2.3e20 cm^-2 in this direction
(Kalberla et al. 2005).  The observed 0.3-10 keV flux is 2.37+/-0.25 e-10
ergs cm^-2 s^-1, which corresponds to an unabsorbed flux of 
2.45+/-0.25 e-10 ergs cm^-2 s^-1.

If the underlying decay continues at the same rate, we predict an XRT
count rate of 0.025 count/s at T+24hr, which corresponds to an observed
0.3-10keV flux of 1.4e-12 ergs cm^-2 s^-1.

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