GCN Circular 12283

A. Rowlinson (U. Leicester) and C. B. Markwardt (NASA/GSFC) report on
behalf of the Swift-XRT team:

We have analysed 7.8 ks of XRT data for GRB 110818A (Markwardt	et al.
GCN Circ. 12279
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 ), from 384 s to 18.3 ks after the  BAT trigger. The
data comprise 61 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The best available XRT position  (using the
promptly downlinked event data, the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 317.3378,
-63.9813 which is equivalent to:

RA (J2000): 21 09 21.07
Dec(J2000): -63 58 52.7

with an uncertainty of 2.0 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=5.8 (+0.6, -0.7). At T+461 s  the decay
flattens to an alpha of 1.57 (+0.14, -0.11) before breaking again at
T+2167 s to a final decay with index alpha=0.95 (+0.10, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.97 (+0.12, -0.11). The
best-fitting absorption column is  7.8 (+2.7, -2.4) x 10^20 cm^-2, in
excess of the Galactic value of 2.6 x 10^20 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.6 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.8 (+2.7, -2.4) x 10^20 cm^-2
Galactic foreground: 2.6 x 10^20 cm^-2
Excess significance: 5.3 sigma
Photon index:	     1.97 (+0.12, -0.11)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00500914.

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