TITLE:   GCN CIRCULAR
NUMBER:  6427
SUBJECT: GRB 070520A: Swift-XRT refined analysis
DATE:    07/05/20 21:45:39 GMT
FROM:    Pat Romano at OAB-Swift  <patrizia.romano@brera.inaf.it>

P. Romano (Univ Bicocca&INAF-OAB), A. Moretti (INAF-OAB),
C. Guidorzi (Univ Bicocca&INAF-OAB), S.D. Vergani (DIAS-DCU)
report on behalf of the Swift team:

We have analysed the first four orbits of XRT data on
GRB 070520A (Romano et al., GCN Circ. 6423) with total observing
times of 79 s in Windowed Timing mode (from T+165 to T+244 s)
and 6.5 ks in Photon Counting mode (from T+3565 s).
The Photon Counting mode image provides a refined XRT
position at RA,DEC(J2000) = 193.36131,+74.99038 which is

RA(J2000)  = 12h 53m 26.72s
Dec(J2000) = +74d 59' 25.4"

with an uncertainty of 4.1 arcsec (radius, 90% containment).
This is  2.6 arcsec from the initial X-ray position,
and 43 arcsec from the initial BAT position (Romano et al.,
GCN Circ. 6423).

The XRT light curve exhibits an initial flaring behaviour throughout
the first orbit (in the WT data, i.e., up to T+244 s) with an
underlying power-law with slope 4.4+/-0.5.
The PC data start at  T+3565 s, when the count rate had decreased by
three orders of magnitude. The PC data power-law slope is 0.1+/-0.4.

The mean WT spectrum, extracted during the flaring activity,
can be fit with an absorbed power law with a photon index of 3.1+/-0.2
and a column density of (2.2+/-0.4)E21 cm^-2 significantly in excess
of the Galactic value (2.3E20 cm^-2; Dickey & Lockman, 1990).
The absorbed (unabsorbed) 0.3-10.0keV flux for the WT spectrum is
5.2E-10 (1.5E-9) ergs cm^-2 s^-1.

Assuming the source continues to decay at the present rate, we
predict an XRT count rate of 1E-4 counts/s at T+24 hours,
which corresponds to an unabsorbed flux of 5E-15 ergs cm^-2 s^-1.

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

[GCN OPS NOTE(21may07):  Per author's request, the sentence starting
"The absorbed (unabsorbed) 0.3-10.0keV flux ..." was added.]