GCN Circular 6737

P.A. Evans, K.L. Page (U. Leicester) and F. E. Marshall (NASA/GSFC) report on
behalf of the Swift-XRT team:

We have analysed the first 11 ks of Swift XRT data for GRB 070809
(Marshall et al. GCN Circ 6728), all of which is in Photon Counting (PC)
mode. We find a refined XRT position of RA, Dec = 203.7699, -22.1416
degrees, which is equivalent to:

RA (J2000) = 13h 35m 4.78s
Dec(J2000) = -22d 08' 29.8"

with an estimated uncertainty of 3.5 arcsec (radius, 90% containment
including systematics). This is 81.9" from the refined BAT position
(Krimm et al. GCN Circ 6732) and 4.2" from the XRT position reported in
GCN Circ 6728.

The XRT light curve shows fading behaviour which is well fitted by a
power-law with two breaks. The initial decay followed a slope of
alpha=2.3 (+0.7/-1.1), and then broke at ~T0+200 s to a shallow decay
with alpha=0.28 (+/-0.13). The shallow phase ended at approximately
T0+10000 s and the current decay follows a slope of alpha=1.35
(+0.80/-0.72).

The PC mode spectrum from T0+85-25000 s is well fitted by an absorbed
power-law with a column density of 6.7e20 (+7.0e20/-5.3e20) cm^-2,
consistent with the Galactic value of 6.40e20 cm^-2 (Kalberla et al.
2005). The spectral slope was gamma=1.49 (+0.25/-0.22), consistent with
the BAT value (GCN Circ 6732). The observed (unabsorbed) 0.3-10 keV flux
is 1.64e-12 (1.79e-12) erg cm^-2 s^-1.

If the source continues to decay with alpha=1.35, we predict a count
rate of 1.7e-3 counts s^-1 at T0+24 hours, which corresponds to an
observed  (unabsorbed) flux of 1.09e-13 (1.20e-13) erg cm^2 s^-1.

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