[vsnet-grb-info 4330] GRB 070220: Swift-XRT Refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6118
SUBJECT: GRB 070220: Swift-XRT Refined Analysis
DATE:    07/02/20 13:04:43 GMT
FROM:    Andy Beardmore at U Leicester  <apb at star.le.ac.uk>

A.P. Beardmore (U. Leicester), M. Stamatikos (NASA/ORAU), P.A. Evans,
M.R. Goad, O. Godet, K.L. Page, (U. Leicester), M. Capalbi (ASDC),
J.P. Osborne, R.L.C. Starling (U. Leicester) report on behalf of the 
Swift-XRT team:

We have analysed the first two orbits of Swift-XRT data available for
GRB 070220 (Stamatikos et al., GCN 6114), which were obtained prior to
the satellite entering the Malindi Gap.  The data consist of 217s
exposure in Windowed Timing (WT) mode and 4090s in Photon Counting
(PC) mode.

Using the PC data, we find a refined XRT position, which has been
astrometrically corrected by matching the UVOT images with the
USNO-B1 catalogue, of:

RA(J2000)  =  02h 19m 06.83s
DEC(J2000) = +68d 48' 16.1"

with an uncertainty of 2.0 arcseconds (90% confidence). This position
is 72.0 arcseconds and 3.0 arcseconds away from the original BAT and
XRT positions quoted in GCN 6114.

The 0.3-10.0 keV X-ray light curve out to T+7.6ks (where T is the BAT
trigger time) can be fit by a broken powerlaw, with an initial decay
slope of 1.76+/-0.06, a break time of 417+/-43s, and a post-break
slope of 0.99+/-0.04.

The WT spectrum from T+85s to T+294s can be modelled by an absorbed
powerlaw, with a photon index of 1.55+/-0.08 and a column density of
(4.9+/-0.5)e21 cm^-2 (compared with the Galactic value in this
direction of 3.8e21 cm^-2).  The 0.3-10.0 keV observed flux over this
time interval is (1.21+/-0.05)e-9 ergs cm^-2 s^-1, which corresponds
to an unabsorbed flux of (1.65+/-0.10)e-9 ergs cm^-2 s^-1.

While the temporal decay parameters are uncertain based on such
limited data, assuming the source continues to fade at the same rate
we predict an X-ray count rate of 0.019 count/s at T+24 hours, which
corresponds to an observed (unabsorbed) 0.3-10.0 keV flux of 1.4e-12
(1.9e-12) ergs cm^2 s^-1.

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