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GCN Circular 6779

GRB 070911: Swift XRT refined analysis
2007-09-12T17:21:52Z (17 years ago)
Milvia Capalbi at ISAC/ASDC <>
Giulia Stratta (ASDC), Matteo Perri (ASDC) and David
Burrows (PSU) report, on behalf of the Swift XRT team:

We have analyzed the first 9 orbits of Swift-XRT
data on GRB 070911 (GCN 6777, Krimm et al.) for a
total of 6.8 ks of exposure taken in Photon Counting
mode starting 14.5 hours after the burst.
This is the first BAT GRB after Swift safe-hold recovery.
Swift-XRT detected an uncatalogued, fading source
at the following coordinates
RA,Dec 25.8094,-33.4842 which is

  RA(J2000)  = 01h 43m 14.26s
  Dec(J2000) = -33d 29' 03.1"

with an estimated error radius of 6 arcseconds
(90% confidence). This error radius has been increased
to take into account the uncertainties in the pointing
calibration during Swift's reactivation phase.
This position is 0.5' from the BAT refined position
(GCN 6778, Palmer et al.). We note however that XRT position
uncertanties are currently being re-validated.

The 0.3-10 keV X-ray light curve from T0+14.5 h to T0+27.3 h
can be fit with a simple power law model.
The best fit decay index is alpha=-1.5+/-0.2

The 0.3-10.0 keV X-ray spectrum with 6.8 ks of integration
time, is well fit by an absorbed power law with a
photon index of 2.2 +/- 0.3 and a total column density of
NH=(1.3+/-0.5)e21 cm**-2 (chi2/dof=0.8/6).
The Galactic column density in the direction of the source
is 2.4e20 cm**-2 (Dickey & Lockmann 1990).
The 0.3-10 keV observed flux is 1.9e-12 erg cm**-2 s**-1
(unabsorbed: 2.8e-12 erg cm**-2 s**-1).
We note that the XRT CCD is now operating with a raised
substrate voltage, and thus has a different gain.
The new gain is not yet well calibrated, and so our spectral
results are only preliminary.

Assuming the X-ray emission continues to decline at the same
rate, we predict a 0.3-10 keV count rates of 0.02 cts/s 2 days
after the burst trigger, that corresponds to a flux of
1.3e-12 erg cm**-2 s**-1.

This is an official product of the Swift XRT team.
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