GCN Circular 8178

Dirk Grupe (PSU) and Phil Evans (U Leicester) report on behalf of the 
Swift-XRT team:


The XRT began observing the field of GRB 080903 (trigger=323542; Grupe 
et al.,
GCN Circ 8169
Not found
 
 
) at 01:13:21.8 UT, 58.5 seconds after
the BAT trigger.

The enhanced XRT position given in GCN Circ 8177
Not found
 
 
 (Goad et al)
is 30.3" away from the ground-processed BAT position given in GCN Circ. 
8176
Not found
 
 

(Tueller et al.),  consistent with that position.

The 0.3-10 keV X-ray light curve starts at a level of 2e-9 ergs/s/cm2 (40
counts/s) in the XRT with a rather flat decay slope at these early times of
alpha = 1.00+/-0.04. The light curve breaks at about 1 ks after the 
burst and
steepens dramatically to alpha = 2.47+/-0.14. If this decay slope 
continues as is,
we predict an XRT count rate at T0+24 hours of 3e-5 counts/s or 2e-15
ergs/s/cm2, so undetectable for XRT.


The spectrum of the Windowed Timing data of the first orbit
can be fitted by an absorbed single powerlaw with an unusual flat photon
index Gamma = 0.94 +/- 0.06 and column density fixed to the Galactic 
value of
1.67e21 cm-2 in the direction of the burst. This photon index is 
consitent with
that found by the BAT in the 15-150 keV band (Gamma=0.84+/-0.52; Tueller 
et al.,
GCN Circ. 8176
Not found
 
 
). The spectrum becomes slightly steeper at later times.
The Photon Counting mode data show an photon spectral slope of 
Gamma=1.23+/-0.16
with the absorption column density fixed to the Galactic value.


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