TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4796
SUBJECT: GRB 051221B: Swift-XRT Team Refined Analysis
DATE:    06/02/20 09:35:13 GMT
FROM:    Kim Page at U.of Leicester  <kpa@star.le.ac.uk>

K.L. Page (U. Leicester), D.N. Burrows (PSU) and E. Rol (U. Leicester)
report on behalf of the Swift-XRT team:
 
It has recently been pointed out (Halpern 2006, GCN 4614) that the nature
of GRB 051221B has still not been definitively settled (GRB vs hard X-ray
transient). Prompted by this report, we are providing further analysis
details of the Swift-XRT observation of this source.

The XRT began observing the field of the possible GRB 051221B (GCN 4376, 
Boyd et al. 2005) 281 seconds after the trigger. 7 seconds of data in
Windowed Timing (WT) mode were obtained, before switching into Photon
Counting (PC) mode.
 
The X-ray source was only detected during the first orbit, with 14 counts
in the WT data and 51 counts in 385 seconds of PC data (starting 290
seconds after the trigger). The second orbit, covering 1319 seconds of
exposure time (starting 3.6 ks after the trigger) shows only 1 count at
the position of the source. These later data give a 3-sigma upper limit of
1.5e-3 ct s^-1.
 
The start of the light-curve falls extremely steeply (alpha ~ 9,
calculated with respect to the trigger time). There is an indication of
flattening at 400 - 500 seconds after the trigger, though this is based on
only 2 bins of data, each containing 5 counts.
 
Using Cash statistics because of the low number of counts, the spectrum
for the first orbit of PC data (290 - 680 seconds after the trigger) can
be modelled with a power-law of Gamma = 1.27 +/- 0.37, with the estimated
Galactic column fixed at 5.6e21 cm^-2 (Dickey & Lockman 1990). This
spectrum has a mean count rate of 0.15 ct s^-1 and an observed
(unabsorbed) flux of 1.43e-11 (1.77e-11) erg cm^-2 s^-1.

The XRT observations of this source were terminated early in order to
allow followup of the short GRB 051221A.  Therefore, no additional data
are available.  However, the rapid decline in X-ray flux appears to
support a GRB origin.

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