GCN Circular 7460

C. Pagani, J. L. Racusin, J.A. Kennea, D. N. Burrows (PSU) and P. A. Evans (U
Leicester) report on behalf of the Swift-XRT team:

The Swift-XRT started observing GRB 080319C (trigger=306778, Pagani et al. GCN
7442
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 ) at 12:29:40 UT, 224 seconds after the BAT trigger. The Swift slew to the
burst was delayed because of an Earth constraint. The current dataset consist
of 2.2ks of Photon Counting mode data from the first orbit of observation.

Using 595 s of overlapping XRT Photon Counting mode and UVOT data, we find an
astrometrically corrected X-ray position (using the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 258.97980,
55.39197 which is equivalent to:

RA (J2000):  17 15 55.15
Dec (J2000): +55 23 31.1

with an uncertainty of 1.7 arcsec (radius, 90% confidence).

The 0.3-10 keV X-ray light curve starts at a count rate of ~7 counts/s and peaks
at ~18 counts/s at T+360 seconds, after which it shows a decline with a decay
slope of 0.9 +/- 0.1 with hints of superimposed flaring activity.

The spectrum of the PC data can be well fit by an absorbed powerlaw with photon
index 1.73 +/- 0.06 and column density of (1.5 +/- 0.1)e21 cm^-2, in excess of
the Galactic column density of 2.21e20 cm^-2 in this direction. The average
observed 0.3-10 keV flux is 1.0e-10 erg cm^-2 s^-1, which corresponds to an
unabsorbed flux of 1.3e-10 erg cm^-2 s^-1. The counts to observed flux
conversion factor at the time of this spectrum is 4.3e-11 erg cm^-2 count^-1.

If the underlying powerlaw decay continues as is, we predict an XRT count rate
of 0.11 counts/s at T+24hr, which corresponds to an observed 0.3-10keV flux of
4.7e-12 erg cm^-2 s^-1.

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