GCN Circular 22322

S. J. LaPorte (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU),
A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), S.L. Gibson
(U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'Ai
(INAF-IASFPA) and A.Y. Lien report on behalf of the Swift-XRT team:

We have analysed 6.1 ks of XRT data for GRB 180111A (Lien et al. GCN
Circ. 22318
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 ), from 94 s to 23.6 ks after the  BAT trigger. The data
comprise 117 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN Circ. 22319
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 ).

The late-time light curve (from T0+4.0 ks) can be modelled with a
power-law decay with a decay index of alpha=1.46 (+0.38, -0.29).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.48 (+/-0.09). The
best-fitting absorption column is  4.0 (+3.0, -2.6) x 10^20 cm^-2, in
excess of the Galactic value of 1.1 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.66 (+0.21, -0.12)
and a best-fitting absorption column of 1.17 (+4.88, -0.09) x 10^20
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.8 x 10^-11 (3.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.17 (+4.88, -0.09) x 10^20 cm^-2
Galactic foreground: 1.1 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.66 (+0.21, -0.12)

If the light curve continues to decay with a power-law decay index of
1.46, the count rate at T+24 hours will be 9.2 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.5 x
10^-14 (3.6 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00804692.

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