GCN Circular 6902

B. Sbarufatti, V. Mangano, V. La Parola, E. Troja (INAF-IASFPA),
P. Evans, A. Beardmore (U Leicester), D. N. Burrows (PSU),
F. E. Marshall (NASA/GSFC) report on behalf of the Swift-XRT team

We have analysed the first orbits of Swift-XRT data obtained for
GRB 071011 (trigger=293924, GCN 6882
Not found
 
 
). The observation consists
of 6.6 ks exposure in Photon Counting (PC) mode, starting 2.7 ks
after the trigger. The GRB afterglow is clearly detected inside
the XRT field of view (FoV) at a large off axis angle only in the
initial 1.6 ks. During the following 5 ks the source was outside
the FoV because of an incorrect spacecraft pointing.
Using an improved algorithm that takes into account the presence
of the underlying hot column we obtain a refined position of:

RA, Dec= 8.38816, 61.13327 (degrees)

    RA(J2000)=    00h 33m 33.1s
    Dec(J2000)=  +61d 07' 59.77"


with an estimated uncertainty radius of 4.1 arcsec (90% containment
radius). This is 2.7 arcsec away from the previous XRT position
(GCN 6891
Not found
 
 
) and 5.5 arcsec away from the optical afterglow reported
by Cenko (GCN 6895
Not found
 
 
) and Perley et al. (GCN 6898
Not found
 
 
). We note that the
position uncertainty of 4.1 arcseconds includes both systematic and
statistical uncertainty, but the systematic component may be
underestimated for a source this close to the edge of the field of
view.

The light curve shows a behavior consistent with a powerlaw decay
with slope -1 +/- 0.7. Due to the large uncertainty we need more
data to constrain the afterglow evolution.

The spectrum extracted from the initial 1.6 ks PC data can be
modelled with an absorbed  power-law with photon index
Gamma = 2.6 +/- 0.5, and an absorbing column of
NH = (1.5 +/- 0.7)E22 cm-2, in excess with respect to the Galactic
value of 5.18E21 cm-2. The observed (unabsorbed) flux is
1.26 (6.00)E-11 ergs cm-2 s-1.

All errors are quoted at 90% confidence level.

Additional X-ray observations planned for Oct 14-15 should help
improve both the counterpart position accuracy and the measurement
of the decay rate and spectral parameters.

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