GCN Circular 6421

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
Not found
 
 
) 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
Not found
 
 
)
and 57 arcsec from the BAT refined position (Krimm et al., GCN Circ. 6417
Not found
 
 
).

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.