TITLE:   GCN CIRCULAR
NUMBER:  7111
SUBJECT: GRB071118: Swift XRT refined analysis
DATE:    07/11/19 09:59:03 GMT
FROM:    Alberto Moretti at Obs Brera Merate  <alberto.moretti@brera.inaf.it>

A. Moretti (INAF-OAB), J. R. Cummings (NASA/UMBC), C. Guidorzi
(Univ. Bicocca & INAF-OAB), P. Romano (Univ. Bicocca & INAF-OAB),
P. Evans (U Leicester)report on behalf of the Swift-XRT team:

We have analysed the first 4 orbits of Swift-XRT data obtained for GRB
071118 (trigger number 296856, Cummings et al., GCN Circ. 7106).  The
data consist of 122 s in Windowed Timing (WT) mode, starting 128
s after the BAT trigger and 8.8 ks in Photon Counting (PC) mode.
Using 1298 s of overlapping XRT Photon Counting mode and UVOT V-band
data, we find an astrometrically corrected X-ray position (using the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue): RA, Dec = 299.71558, 70.12449 which is equivalent to:

RA (J2000):  19 58 51.74
Dec (J2000): +70 07 28.2

with an uncertainty of 1.6 arcsec (radius, 90% confidence).  This
location is 2.7 arcseconds from the XRT position measured using prompt
downlinked data (Cummings et al., GCN Circ. 7106) and 152 arcseconds
from the BAT refined position (Markwardt et al.  GCN Circ. 7109).

The light-curve, in the first orbit (up to 2 ks from the trigger)
shows a steep decay accompanied by flaring activity Since the second
orbit, the afterglow shows a more regular decay with a shallow phase
(alpha1=0.6+/-0.2) until 11 ks and a steeper decay (alpha2=1.6+/-0.2).

The spectrum formed from all the WT data can be modelled with a
power-law of photon index Gamma = 1.64 +/- 0.13, with an absorbing
column of NH = (2.7 +/- 0.6)e21 cm^-2 (in excess with respect to the
Galactic value of 9.7e20 cm^-2). The spectrum formed from the PC data
can be modelled with a power-law of photon index Gamma = 1.62 +/-
0.06, with an absorbing column of NH = (2.6 +/- 0.4)e21 cm^-2, which
is consistent with the previous one.

Assuming the light-curve continues to decay with alpha2=1.6, the count
rate at 48 hours is predicted to be 0.002 count s^-1. This corresponds
to an observed (unabsorbed) flux of 1.3e-13 (1.7e-13) erg cm^-2 s^-1.

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