GCN Circular 22376

J.P. Osborne (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), D.N. Burrows
(PSU), A. Tohuvavohu (PSU), S. J. LaPorte (PSU) and K.L. Page report on
behalf of the Swift-XRT team:

We have analysed 8.0 ks of XRT data for GRB 180204A (Page et al. GCN
Circ. 22375
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 ), from 90 s to 21.2 ks after the  BAT trigger. The data
comprise 32 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 = 330.1333,
30.8381 which is equivalent to:

RA (J2000): 22 00 32.00
Dec(J2000): +30 50 17.1

with an uncertainty of 2.1 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.15 (+/-0.03).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.02 (+0.23, -0.22). The
best-fitting absorption column is  6.9 (+1.7, -1.5) x 10^21 cm^-2, in
excess of the Galactic value of 1.0 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 4.3 x 10^-11 (7.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.9 (+1.7, -1.5) x 10^21 cm^-2
Galactic foreground: 1.0 x 10^21 cm^-2
Excess significance: 6.5 sigma
Photon index:	     2.02 (+0.23, -0.22)

If the light curve continues to decay with a power-law decay index of
1.15, the count rate at T+24 hours will be 2.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (2.0 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00808483.

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