GCN Circular 20918

J.A. Kennea (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), B. Sbarufatti (INAF-OAB/PSU),
D.N. Burrows (PSU) and	report on behalf of the Swift-XRT team: report
on behalf of the Swift-XRT team:

We have analysed 7.2 ks of XRT data for GRB 170318B, from 64 s to 41.1
ks after the  BAT trigger. The data comprise 95 s in Windowed Timing
(WT) mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial rise, with a power-law
index of alpha=-0.5 (+0.4, -0.5), followed by a break at T+134 s  to an
alpha of 2.96 (+0.16, -0.15).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.6 (+/-0.4). The
best-fitting absorption column is  6.4 (+1.7, -1.4) x 10^22 cm^-2, in
excess of the Galactic value of 1.4 x 10^22 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.3 (+/-0.6) and a
best-fitting absorption column of 6.2 (+2.1, -1.7) x 10^22 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 8.0 x 10^-11 (3.5 x 10^-10) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.2 (+2.1, -1.7) x 10^22 cm^-2
Galactic foreground: 1.4 x 10^22 cm^-2
Excess significance: 4.6 sigma
Photon index:	     2.3 (+/-0.6)

If the light curve continues to decay with a power-law decay index of
2.96, the count rate at T+24 hours will be 4.5 x 10^-8 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x
10^-18 (1.6 x 10^-17) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00743086.

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