GCN Circular 7875

J. Mao, C. Guidorzi, F. Pasotti (INAF-OAB) and C. B. Markwardt 
(CRESST/GSFC/UMD),
report on behalf of the Swift-XRT team:

The Swift-XRT began observing GRB 080613B (trigger=313954, Markwardt et
al., GCN Circ. 7873
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 ) in Windowed Timing (WT) mode, 76 s after the BAT
trigger. The data of the first orbit consist of 149 s in Windowed Timing
mode and 794 s in Photon Counting mode.

Using 757 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 = 173.79773, -7.10501 which is equivalent to:

RA (J2000): 11 35 11.46
Dec (J2000): -07 06 18.0

with an uncertainty of 2.3 arcsec (radius, 90% confidence).

The light curve from 76 to 219 s can be modelled  as a power law with a 
photon index
of 2.15 +/- 0.11 (90%), followed by a sudden drop of about 2 orders of 
magnitudes up to ~400 s,
after which the decay resumes a value around 1.5.

The WT mode spectrum spanning from 76 to 219 s can be fit by a
power-law model, with a spectral photon index of 1.17 +/- 0.07 and an 
absorbing column
density of 3.7 (-2.0, +2.3)e20 cm^-2 , consistent with the average Galactic
one, 3.2e20 cm^-2.

The corresponding observed (unabsorbed) 0.3-10 keV flux is 1.05e-9 (1.09e-9)
erg cm^-2 s^-1.

If the burst continues to decay at the same rate, we predict an
XRT count rate of 6e-5 count/s at T+24 hours, which corresponds
to an observed 0.3-10 keV flux of approximately 3.5e-15 erg cm^-2 s^-1.
However, we caution about this prediction due to the presently large
uncertainty on the decay rate.

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