Skip to main content
Testing. You are viewing the public testing version of GCN. For the production version, go to
New Announcement Feature, Code of Conduct, Circular Revisions. See news and announcements

GCN Circular 10201

GRB 091127: Swift-XRT afterglow detection and analysis
2009-11-28T08:50:28Z (15 years ago)
Phil Evans at U of Leicester <>
P.A. Evans, K.L. Page (U. Leicester) and E. Troja (NASA/GSFC/ORAU) report on
behalf of the Swift-XRT team:

We have analysed 2.4 ks of XRT data for GRB 091127 (Troja et al. GCN
Circ. 10191), from 3.2 ks to 15.8 ks after the BAT trigger. The data
comprise 761 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. Using 1066 s of PC mode data and 4 UVOT images, we
find an enhanced XRT position (using the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue): RA, Dec = 36.58290,
-18.95250 which is equivalent to:

RA (J2000): 02 26 19.89
Dec(J2000): -18 57 08.9

with an uncertainty of 1.4 arcsec (radius, 90% confidence). This is
consistent with the optical/UV/IR position reported in GCN Circs

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.09 (+/-0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.97 (+/-0.08). The
best-fitting absorption column is 7.9 (+1.7, -1.6) x 10^20 cm^-2, in
excess of the Galactic value of 2.8 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.98 (+0.15, -0.14)
and a best-fitting absorption column of 9.8 (+3.3, -3.1) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 3.9 x 10^-11 (4.9 x 10^-11) erg cm^-2

If the light curve continues to decay with a power-law decay index of
1.09, the count rate at T+24 hours will be 0.24 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.3 x
10^-12 (1.2 x 10^-11) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at

This circular is an official product of the Swift-XRT team.
Looking for U.S. government information and services? Visit