GCN Circular 6360

A.P. Beardmore, K.L. Page, P.A. Evans, R.L.C. Starling (U. of Leicester),
C. Guidorzi (INAF-OAB, Univ. Bicocca), and C. Markwardt (GSFC/UMD) report 
on behalf of the Swift XRT team :

We have analysed the first three orbits of Swift XRT data from
GRB 070429B (trigger number 277582, Markwardt et al. GCN circular
6358
Not found
 
 
), which has a total photon counting mode exposure of 5.8ks.  Due
to a delayed slew because of an Earth-limb constraint, the XRT arrived 
on the source 244s after the BAT trigger. The tentative X-ray source
identified in GCN 6358
Not found
 
 
 is clearly detected in the first orbit of data,
and is absent thereafter.  The refined XRT position from 2.4ks of data 
in the first orbit is
   RA, DEC (J2000) = 328.0159, -38.8284
which is
   RA(J2000)  =  21d 52m 03.82s
   DEC(J2000) = -38d 49' 42.2"
with an uncertainty of 5.1 arcsec (radius, 90% containment).
This is 61 arcsec from the inital BAT position and 6.2 arcsec 
from the initial X-ray position reported in GCN 6358
Not found
 
 
.

An XRT light curve created from the first orbit of data, binned with
a minimum of 10 counts/bin, reveals a decaying source. A powerlaw
fit yields a poorly constrained decay slope of 0.94+/-0.47.

The X-ray spectrum from the first orbit (covering 254 to 2691 seconds
after the BAT trigger), modelled with an absorbed powerlaw and fit
using Cash statistics, gives a photon index of 2.5+1.3-1.2.  An upper
limit of 5.9e21 cm^-2 on the column density was found, compared with
the Galactic value of 1.8E+20 cm^-2 in this direction.  The absorbed
(unabsorbed) 0.3-10.0keV flux for this spectrum was 5.9E-13 (1.1E-12)
ergs cm^-2 s^-1.

Assuming the source continues to decay at the same rate, we predict an
XRT count rate of 3.0e-4 count/s at T+24 hours, which corresponds to an
observed (unabsorbed) flux of 1.5E-14 (2.8E-14) ergs cm^-2 s^-1.

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