GCN Circular 22338

J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), P.A. Evans (U.
Leicester), S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), V.
D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), D.N.
Burrows (PSU) and J.K. Cannizzo report on behalf of the Swift-XRT team:

We have analysed 6.6 ks of XRT data for GRB 180115A (Cannizzo et al.
GCN Circ. 22335
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 ), from 137 s to 13.1 ks after the  BAT trigger. The
data comprise 86 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Goad et al. (GCN Circ. 22337
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.6 (+0.6, -0.5), followed by a break at T+269 s to an
alpha of 0.71 (+/-0.04).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.26 (+0.13, -0.12). The
best-fitting absorption column is  consistent with the Galactic value
of 1.7 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.94 (+0.12, -0.11) and a best-fitting absorption
column of 3.6 (+2.4, -2.0) x 10^20 cm^-2. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.3 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.6 (+2.4, -2.0) x 10^20 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.94 (+0.12, -0.11)

If the light curve continues to decay with a power-law decay index of
0.71, the count rate at T+24 hours will be 0.025 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.3 x
10^-13 (9.0 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/00805318.

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