TITLE:   GCN CIRCULAR
NUMBER:  6644
SUBJECT: GRB 070721A: Swift/XRT refined analysis
DATE:    07/07/21 14:31:11 GMT
FROM:    Phil Evans at U of Leicester  <pae9@star.le.ac.uk>

P.A. Evans (U. Leicester) and H. Ziaeepour (UCL-MSSL) report on behalf of 
the Swift/XRT team:

We have analyzed the first 1.3 ks of Swift XRT data for GRB 070721A 
(Ziaeepour et al. GCN Circ. 6639), comprising 35 s in Windowed Timing (WT) 
mode, and 1.3 ks in Photon Counting (PC) mode. The burst is clearly 
detected. Using 799 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 = 3.16348, -28.55017 which is equivalent to:

RA (J2000):  00 12 39.24
Dec (J2000): -28 33 00.6

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This position 
is 3.2" from the initial XRT position reported in GCN Circ. 6639, and 95" 
from the BAT-refined position reported by Sakamoto et al. (GCN Circ. 
6642).

The XRT light curve shows a fading behaviour, which is well fitted by a 
broken power-law. The initial decay followed a slope of alpha=2.97 
(+0.57/-0.42). At T0+281 (+112/-71) s the decay shallows to an alpha of 
0.75 (+0.39/-0.49).

The Photon Counting mode spectrum is well fitted by an absorbed power-law, 
with a column density of 1.61e20 cm^-2 (the Galactic value) and a gamma of 
2.24 (+0.33/-0.26). This is consistent with the value derived for the BAT 
spectrum (GCN Circ 6642). The observed (unabsorbed) 0.3-10 keV flux is 
6.53e-12 (8.23e-12) erg cm^-2 s^-1.

If the source continues to decay at the same rate, we predict a count rate 
of 2.7e-3 counts/s at T0+24 hours, which corresponds to an observed 
(unabsorbed) flux of 1.87e-13 (2.36e-13) erg cm^-2 s^-1.

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