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GCN Circular 8530

Subject
GRB 081118: Swift-XRT refined analysis
Date
2008-11-19T04:09:56Z (16 years ago)
From
Loredana Vetere at PSU <vetere@astro.psu.edu>
L. Vetere, E. Hoversten (PSU) report on behalf of the Swift-XRT team.

The Swift-XRT started observing the field of GRB 081118 (trigger=334877,
Hoversten et al., GCN 8525) at 14:59:10.0 UT, 153.3 s after the BAT  
trigger.
The XRT observed the GRB in Windowed Timing (WT) mode until 216 s
after the trigger and Photon Counting (PC) mode thereafter.

The X-ray light curve from the first three orbits can be modelled with  
a broken
powerlaw with an initial rapid decay (slope of 4.8+/-1.0), a break  
time of 580 s,
followed by a shallower decay (slope of 0.6+/-0.3).

The spectrum of the WT data can be well fit by an absorbed powerlaw  
with a
photon index of 2.6 +/- 0.3 and an absorbing equivalent hydrogen column
density consistent with the Galactic one in the GRB direction (3.7e20  
cm^-2;
Kalberla et al.  2005). The observed (unabsorbed) 0.3-10 keV flux is  
1.79
(2.28) x 10^-10 erg cm^-2 s^-1.

The PC spectrum (T+250 s to T+19 ks) is best fit with a photon index of
2.5 +/- 0.3 and an absorbing equivalent hydrogen column density  
consistent
  with the Galactic one in the GRB direction (3.7e20 cm^-2; Kalberla  
et al.  2005).
The observed (unabsorbed) 0.3-10 keV flux over this interval is 7.6  
(9.4) x
10^-13 erg cm^-2 s^-1.

Assuming the X-ray emission from the burst continues to decline at the
same rate we predict an XRT count rate of 0.002 count s^-1 at T+24 hour,
or an observed 0.3-10 keV X-ray flux of 5.2 x 10^-14 erg cm^-2 s^-1.

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