TITLE:   GCN CIRCULAR
NUMBER:  6682
SUBJECT: GRB 070729: Swift-XRT refined analysis
DATE:    07/07/29 14:43:33 GMT
FROM:    Cristiano Guidorzi at INAF-OAB  <cristiano.guidorzi@brera.inaf.it>

C. Guidorzi, P. Romano (U.Bicocca & INAF-OAB), A. Moretti (INAF-OAB)
report on behalf of the Swift-XRT team:

we have analysed the first five orbits of Swift-XRT data obtained for
GRB 070729 (Guidorzi et al., GCN Circ. 6678), which include 9.1 ks of
Photon Counting (PC) data.

We find a refined X-ray position of RA, Dec (J2000) =
56.31385, -39.32400 which is equivalent to:

RA (J2000):  03 45 15.32
Dec (J2000): -39 19 26.4

with an uncertainty of 4.5 arcsec (radius, 90% confidence). This is 3.2
arcsec from the initial XRT position (GCN Circ. 6678) and 53 arcsec from
the BAT ground-calculated position (Sato et al., GCN Circ. 6681).

The XRT light-curve of the first orbit, in the time interval from
T+95s to T+600s, shows a fading behaviour with a power-law index
of 1.5+/-0.6. The upper limit on the count rate derived from the
data of the following orbits is consistent with it.

The spectrum (35 photons) can be fit using Cash statistics
with an absorbed power law with a photon index of 1.8+/-0.4
and a column density consistent with the Galactic value
(2.6e20 cm^-2; Kalberla et al. 2005).
The absorbed (unabsorbed) 0.3-10.0keV flux for the spectrum is
(2.2+/-0.7)e-13 ((2.4+/-0.7)e-13) ergs cm^-2 s^-1, at mean time of
T+0.13d.

If the decay continues with a slope of ~ 1.5, the count rate at 24
hours is predicted to be 1e-5 count s^-1; this ranges between 4e-4 and
3e-7 count s^-1, taking into account the uncertainties on the slope. Using
the spectral fit detailed above, this corresponds to an observed flux
<2e-14 erg cm^-2 s^-1 (over 0.3-10 keV).

This circular is an official product of the XRT team.