GCN Circular 9876

G. Stratta, V. D'Elia, M. Perri (ASDC) report on behalf of the
Swift XRT team:

At 23:36 UT, September 2nd, 2009, Swift began a Target of Opportunity
observation of the Fermi GBM/LAT discovered burst GRB 090902B (de
Palma et al., GCN 9867
Not found
 
 
, Bissaldi et al., GCN 9866
Not found
 
 
) about 12.5 hours
after the trigger.

We confirm that the X-ray source reported by Kennea et al. (GCN
Circ. 9868
Not found
 
 
) shows a decaying behavior and is likely the afterglow of
GRB 090902B.

The enhanced XRT position for this burst was given by Evans
(GCN. Circ 9871).

The X-ray light curve from T+12.53 hr to T+17.38 hr can be fit with
a single power-law model with a decay index of -1.3+/-0.9 (90% confidence
level).

With 4.8 ks of integration time, the X-ray spectrum from T+12.53 hr to T+17.38 hr
after the Fermi/GBM trigger can be fit by an absorbed power-law model
with a photon index of 2.1 (+0.3,-0.3) and a rest frame column density of
(3.4+/-0.9) x 10^22 cm^-2 at z=1.822 (Cucchiara et al., GCN 9873
Not found
 
 
)
in addition to the Galactic column density
in the direction of the burst (3.88 x 10^20 cm^-2, Kalberla et al. 2005).
Errors are at 90% confidence level.
The observed 0.3-10.0 keV flux is 3.2 x 10^-12 erg cm^-2 s^-1 which
corresponds to an unabsorbed flux of 4.1 x 10^-12  erg cm^-2 s^-1.

Providing the source continues to decay at the same rate, we predict an
observed flux of about 8.7 x 10^-13 erg cm^-2 s^-1 at T+2 days.

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

[GCN OPS NOTE(03sep09): Per author's request, the missing Cucchiara
reference was added.]