TITLE:   GCN CIRCULAR
NUMBER:  6421
SUBJECT: GRB 070518: Swift-XRT refined analysis
DATE:    07/05/19 07:50:58 GMT
FROM:    Cristiano Guidorzi at INAF-OAB  <cristiano.guidorzi@brera.inaf.it>

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

We have analysed the first two orbits of GRB 070518 (Guidorzi
et al., GCN Circ. 6415) with total observing times of 186 s in
Windowed Timing mode and 4.2 ks in Photon Counting mode in the Swift XRT.
The Photon Counting mode image provides a refined XRT
position at RA,DEC(J200) = 254.19800, +55.29444 which is

RA(J2000)  = 16h 56m 47.52s
Dec(J2000) = +55d 17' 40.0"

with an uncertainty of 3.9 arcsec (radius, 90% containment).
This is  2.5 arcsec from the initial X-ray position,
2.8 arcsec from the UVOT position (GCN Circ. 6415)
and 57 arcsec from the BAT refined position (Krimm et al., GCN Circ. 6417).

The XRT light curve exhibits an initial flaring behaviour up to
~T+200 s, after which a steep decay (4.7 ± 0.4) follows after the
last flare, with a break at T+460 (+/- 40) s to a shallower
decay (1.11 ± 0.13) up to T+11ks.

We extracted two spectra from the WT data during the flaring
activity, due to a strong spectral evolution.
The first spectrum, from 77 s to 158 s, can be fit with an
absorbed power law with a photon index of 2.3+/-0.1 and
column density of (9.6+/-1.6)E20 cm^-2 significantly in excess
of the Galactic value (2.2E20 cm^-2; Dickey & Lockman, 1990).
The second spectrum, from 165 to 253 s, has a photon index
of 2.9+/-0.1 and same column density as that of the first spectrum.

The absorbed (unabsorbed) 0.3-10.0keV flux for the first spectrum
is 5.1E-10 (7.2E-10) ergs cm^-2 s^-1.

Assuming the source continues to decay at the same rate, we predict
an XRT count rate of 6.3E-4 counts/s at T+24 hours, which corresponds
to an observed (unabsorbed) flux of 2.0E-14 (2.8E-14) ergs cm^-2 s^-1.

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