TITLE:   GCN CIRCULAR
NUMBER:  9599
SUBJECT: GRB 090628: Swift XRT refined analysis
DATE:    09/06/30 15:35:22 GMT
FROM:    Vanessa Mangano at INAF-IASFPA  <vanessa@ifc.inaf.it>

V. Mangano, B. Sbarufatti (INAF-IASF PA) report on behalf
the Swift XRT Team:

We have analyzed 19 ks of PC data of the candidate X-ray
afterglow of GRB 090628 (Mangano et al, GCN Circ. 9585)
spanning the time interval from T+2796 s to T+101.7 ks.

The best position available for the source is the
enhanced Swift-XRT position from Goad et al. (GCN Circ. 9590):
RA, Dec = 237.05272, -15.98489 which is equivalent to

RA (J2000): 15h 48m 12.65s
Dec (J2000): -15d 59' 05.6"

with an uncertainty of 1.9 arcsec (radius, 90% confidence).

The 0.3-10 keV XRT light curve shows an initial flat or rising
phase followed by a steep decay. The best fit with a broken
power-law model is insensitive to the initial rise slope,
of the order of 0.4, possibly consistent with zero,
and provides a break at 14+/-4 ks after the trigger
and a post-break decay slope of -1.3+/-0.3.
The source is rather faint, and at this rate it will
reach a level of 8.0e-4 counts/s at T+48 hours.
All quoted errors are at the 90% confidence level.

The average spectrum can be fitted by an absorbed
power-law model with Cash statistics. Best fit spectral
parameters are NH < 2.0e21 cm^-2 (consistent with Galactic
absorption of 8.7e20 cm^-2, Kalberla et al. 2005)
and photon index = 1.4 +/- 0.3 (Cstat(dof): 87.6(113)).
The 0.3-10 keV observed(unabsorbed) average flux is
5.0e-13(5.5e-13) erg cm^-2 s^-1.

The rate to flux conversion factor is 8.0e-11.

The results of the XRT team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00355942

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