[vsnet-grb-info 2565] GRB 051227: XRT refined analysis

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4402
SUBJECT: GRB 051227: XRT refined analysis
DATE:    05/12/28 03:25:11 GMT
FROM:    David Burrows at PSU/Swift  <dxb15 at psu.edu>

A. Beardmore (U. Leicester), D. N. Burrows (PSU), K. Page (U. Leicester), 
F. Marshall (GSFC), M. Chester (PSU) report on behalf of the Swift XRT team:

We have analysed the first 3 orbits of XRT data from GRB 051227.
A 2.8ks photon counting mode image provides a refined XRT position:

RA(J2000) =  08h 20m 57.92s,
Dec(J2000) = +31d 55m 30.43s

with an uncertainty of 3.5 arcsec (90% containment). This position is 5.1 
arcsec away from the on-board XRT position quoted in Barbier et al. 
(GCN4397) and includes the latest XRT boresight correction.  This position 
is 106 arcseconds from the refined BAT position (Hullinger et al., GCN 4400).

The X-ray light curve displays multiple small flares, with the largest
peaking at a count rate of approximately 25 counts/s at T + 115s
(where T is the BAT trigger time). The underlying decay can be fit
with a broken power-law with an initial decay slope of -2.2+/-0.2, a
break at 475+235-80s, and a post-break slope of -0.95+/-0.15.

A power-law fit to the X-ray spectrum from T + 101s to T + 180s
gave a photon index of 1.4+/-0.2 and a column density of
(1.8+/-1.0)e21 cm**-2. We note the galactic hydrogen column density in the
direction of the burst is 4.3e20cm**-2. The 0.2-10.0keV observed flux was
4.2e-10 ergs cm**-2 s**-1, which corresponds to an unabsorbed flux of
5.0e-10 ergs cm**-2 s**-1.

If the burst continues decaying at the current rate we estimate an XRT
count rate of 0.0022 counts/s at T + 24hr, which corresponds to an
observed 0.2-10.keV flux of 1.1e-13 ergs cm**-2 s**-1.