GCN Circular 6192

C. Guidorzi, P. Romano, S. Vergani and A. Moretti (Uni-Bicocca and OAB)
report on behalf of the Swift-XRT team:

We have analysed the first two orbits of Swift-XRT data obtained for the
GRB 070311 detected by INTEGRAL (Mereghetti et al., GCN Circ. 6189
Not found
 
 
),
which includes 4.2 ks of Photon Counting (PC) mode data, beginning
7000 s after the burst trigger time.
Using the PC data we found a fading source at the position of
RA, Dec = 87.5351, 3.3750, which is equivalent to

RA(J2000)  =  05 50 08.43
Dec(J2000) = +03 22 30.0

with an estimated error radius of 3.8 arcsec (90% confidence). This lies
3.3 arcsec from the optical afterglow discovered by REM (Covino et al.,
GCN Circ. 6190
Not found
 
 
) and confirmed by PAIRITEL (Bloom et al., GCN Circ. 6191
Not found
 
 
).

The XRT light-curve can be fit with a single power law with index of
1.9 � 0.4 from 7 ks to 14 ks after the burst.

The PC photon spectrum can be modelled by a power-law of Gamma=1.6 +/- 0.2,
with a total absorbing column fixed to the Galactic value of 2.6x10^^21 
cm^-2
in this direction. Alternatively, if the absorbing column is left free 
to vary,
we find Gamma=1.9 +/- 0.4 and a column density of (3.8 +/- 1.6)x1021 cm^-2
(90% confidence).

The mean observed (unabsorbed) flux over 0.3-10 keV over this period
is 3.9e-12 (4.9e-12) erg cm^-2 s^-1.

If the decay continues with the same rate as above, the count rate is
predicted to be 1.1e-3 count s^-1 at 24 hours. This corresponds to an
observed flux of 6.5e-14 erg cm^-2 s^-1 (over 0.3-10 keV), with the
unabsorbed value being 7.9e-14 erg cm^-2 s^-1, using the spectral fit to
the PC data.

This circular is an official product of the XRT team.