GCN Circular 7399

A.P. Beardmore, J.P. Osborne, R.L.C. Starling, K.L. Page, P.A. Evans 
(U. Leicester) and J.R. Cummings (NASA/UMBC) report on behalf of 
the Swift-XRT team:

The Swift-XRT started observing GRB 080310 (trigger no. 305288,
Cummings et al. GCN 7382
Not found
 
 
) at 2008-03-10 08:39:27.85 UT, 89s after the
trigger. The observation so far spans 3 snapshots, with Windowed Timing
mode data from T+95s to T+799s, and Photon Counting mode thereafter.

The best XRT position is the UVOT-enhanced position from Osborne et al.
(GCN 7394
Not found
 
 
).

At first, the 0.3-10keV X-ray light curve decays from 39 count s^-1 at
T+95s to 18 count s^-1 at T+130s but then shows strong flaring
activity. An initial rebrightening occurs from about T+135s to T+420s,
reaching ~135 count s^-1 at approximate times of 200s, 250s, 280s and
350s after the trigger.  This is followed by a second flare from T+500
to T+620s, where it reaches a maximum of ~115 count s^-1 at T+565s.
Following the flaring activity, the X-ray light curve, thus far, shows
a slow decline out to the end of the third snapshot (T+1.0ks to
T+18.7ks), where it reaches a count rate of 0.1 count s^-1 with an
underlying decay slope of alpha~0.5.

The X-ray data also show strong spectral evolution during the flaring
intervals.  A 1.5-10keV/0.3-1.5keV hardness ratio reveals the X-ray
emission initially hardens from T+135s to T+200s as the light curve
rebrightens, at which point it remains approximately constant until
T+360s, before softening as the light curve decays.  Similarly, the
source hardens again during the second flare from T+500s to T+565s,
then softens as this flare declines.

A spectrum of the WT data from T+200s to T+360s, where the hardness
ratio is approximately constant, can be well fit by an absorbed
powerlaw with photon index 1.45 +/- 0.02 and column density of (7.0
+/- 1.0)e21 cm^-2 at the redshift of the burst (z=2.43, Prochaska et
al. GCN 7388
Not found
 
 
; Berger and Rauch GCN 7389
Not found
 
 
; Vreeswijk et al. GCN 7391
Not found
 
 
),
in addition to the Galactic column density of 3.3e20 cm^2 in this
direction. The observed 0.3-10keV flux is (6.1 +/- 0.1)e-9 erg cm^-2 s^-1,
which corresponds to an unabsorbed flux of 6.8e-9 erg cm^-2 s^-1. 
The counts to observed flux conversion factor at the time of
this spectrum is 4.9e-11 erg cm^-2 count^-1.

The PC mode data from the second snapshot (T+5.1ks to T+7.3ks) show a 
softer spectrum, with a photon index of 1.9 +/- 0.2 and column density 
consistent with the value determined above. The observed 0.3-10keV flux at 
this time is (1.1 +/- 0.1) e-11 erg cm^-2 s^-1, and corresponds to a flux 
conversion factor of 4.7e-11 erg cm^-2 count^-1.

If the underlying powerlaw decay continues as is, we predict an XRT count 
rate of 0.05 count s^-1 at T+24hr, which corresponds to an observed 
0.3-10keV flux of 2.5e-12 erg cm^-2 s^-1.

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