GCN Circular 22511

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

We have analysed 6.1 ks of XRT data for GRB 180316A (Melandri et al.
GCN Circ. 22500
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 ), from 101 s to 28.6 ks after the  BAT trigger. The
data comprise 534 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. 22501
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 ).

The late-time light curve (from T0+4.4 ks) can be modelled with an
initial power-law decay with an index of alpha=-0.0 (+/-0.6), followed
by a break at T+8729 s to an alpha of 2.1 (+/-0.3).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.94 (+0.07, -0.04). The
best-fitting absorption column is  2.038 (+0.286, -0.012) x 10^21
cm^-2, consistent with the Galactic value of 2.0 x 10^21 cm^-2
(Willingale et al. 2013). The PC mode spectrum has a photon index of
2.05 (+/-0.16) and a best-fitting absorption column of 3.3 (+/-0.7) x
10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.7 x 10^-11 (5.7 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.3 (+/-0.7) x 10^21 cm^-2
Galactic foreground: 2.0 x 10^21 cm^-2
Excess significance: 3.2 sigma
Photon index:	     2.05 (+/-0.16)

If the light curve continues to decay with a power-law decay index of
2.1, the count rate at T+24 hours will be 2.9 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (1.7 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/00814677.

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