GCN Circular 23106

V. D'Elia (SSDC), S.L. Gibson (U. Leicester), Z. Liu (NAOC / U.
Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo
(INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J. LaPorte
(PSU), J.P. Osborne (U. Leicester) and P.A. Evans (U. Leicester) report
on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the MAXI-detected
burst GRB 180809A (Yatabe et al. GCN Circ. 23103
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 ) in a series of
observations tiled on the sky. The total exposure time is 777 s,
distributed over 6 tiles; the maximum exposure at a single sky location
was 898 s. The data were collected between T0+13.7 ks and T0+22.0 ks,
and are entirely in Photon Counting (PC) mode. 

An uncatalogued X-ray source is detected and is above the RASS limit,
and is therefore likely the GRB afterglow. Using 731 s of PC mode data
and 3 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 = 297.90271, -34.78546 which is equivalent to:

RA (J2000): 19h 51m 36.65s
Dec(J2000): -34d 47' 07.6"

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This
position is 9.1 arcmin from the MAXI position.	The light curve is
currently consistent with a constant source of mean count rate 2.2e-01
ct/sec. A power-law fit gives an index of 0.3 (+/-0.9). More data are
necessary to determine potential fading.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.4 (+0.7, -0.6). The
best-fitting absorption column is  8.4 (+5.4, -3.8) x 10^21 cm^-2, in
excess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.2 x 10^-11 (8.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.4 (+5.4, -3.8) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: 3.1 sigma
Photon index:	     2.4 (+0.7, -0.6)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020814.
The results of the full analysis of the tiled XRT observations are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00072.

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