TITLE:   GCN CIRCULAR
NUMBER:  6452
SUBJECT: GRB 070521: Swift-XRT refined analysis
DATE:    07/05/22 09:07:39 GMT
FROM:    Cristiano Guidorzi at INAF-OAB  <cristiano.guidorzi@brera.inaf.it>

C. Guidorzi, P. Romano (Univ Bicocca&INAF-OAB), J. Hill (GSFC),
A. Beardmore, P.A. Evans (U. Leicester), A. Moretti (INAF-OAB),
report on behalf of the Swift team:

We have analysed the first five orbits of GRB 070521 (Guidorzi
et al., GCN Circ. 6431) with total observing times of 10.2 ks in
Photon Counting mode in the Swift XRT.
The XRT position, using the UVOT to astrometrically correct the XRT field,
assuming a fixed mapping between the XRT and UVOT instruments, is
RA(J2000)=242.6608 deg, Dec(J2000)=+30.2561 deg, corresponding to:

RA(J2000)  = 16h 10m 38.60s
Dec(J2000) = +30d 15' 21.8"

with an uncertainty of 1.7 arcsec (radius, 90% containment).
This was based on 3 intervals of overlapping XRT/UVOT V-band data
totalling 866 s. The UVOT astrometry is performed relative to USNOB1.

This is 6.6 arcsec from the initial X-ray position,
4.0 arcsec from the XRT position notice,
and 17 arcsec from the BAT refined position (Palmer et al., GCN Circ. 6440).

The XRT light curve exhibits an initial flaring behaviour superposed
to a power-law decay (index of 0.5 +/- 0.1) up to ~T+600 s.
A break in the power-law decay is observed at T+(6.8 +/- 0.9) ks,
after which it steepens with a power-law index of 1.7 +/- 0.1
up to T+34 ks (90% confidence intervals).

We extracted a spectrum of the PC data from T+4.7 ks to T+13 ks.
This can be fit with an absorbed power law with a photon index
of 2.11 +/- 0.16 and column density of (7+/-1)E21 cm^-2
significantly in excess of the Galactic value
(2.8E20 cm^-2; Dickey & Lockman, 1990).

The absorbed (unabsorbed) 0.3-10.0keV flux for that spectrum
is 1.6E-11 (3.2E-11) ergs cm^-2 s^-1.

Assuming the source continues to decay at the same rate, we predict
an XRT count rate of 7.9E-3 counts/s at T+24 hours, which corresponds
to an observed (unabsorbed) flux of 4.6E-13 (9.3E-13) ergs cm^-2 s^-1.

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