GCN Circular 20917

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

We have analysed 7.0 ks of XRT data for GRB 170318A, from 98 s to 41.1
ks after the  BAT trigger. The data comprise 94 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 power-law decay with an
index of alpha=1.09 (+0.51, -0.29), followed by a break at T+208 s to
an alpha of 2.74 (+0.76, -0.17).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 0.84 (+0.20, -0.19). The
best-fitting absorption column is  6.3 (+2.7, -1.9) x 10^21 cm^-2,
consistent with the Galactic value of 4.4 x 10^21 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 0.92 (+0.35,
-0.30) and a best-fitting absorption column of 6.4 (+4.6, -1.9) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 8.3 x 10^-11 (9.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.4 (+4.6, -1.9) x 10^21 cm^-2
Galactic foreground: 4.4 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     0.92 (+0.35, -0.30)

If the light curve continues to decay with a power-law decay index of
2.74, the count rate at T+24 hours will be 4.0 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.4 x
10^-17 (3.8 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/00743065.

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