GCN Circular 22578

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

We have analysed 6.6 ks of XRT data for GRB 180331B, from 671 s to 25.2
ks after the  BAT trigger. The data are entirely in Photon Counting
(PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.30 (+/-0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.80 (+0.13, -0.12). The
best-fitting absorption column is  8.7 (+3.5, -3.2) x 10^20 cm^-2, in
excess of the Galactic value of 1.6 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.7 x 10^-11 (4.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.7 (+3.5, -3.2) x 10^20 cm^-2
Galactic foreground: 1.6 x 10^20 cm^-2
Excess significance: 3.7 sigma
Photon index:	     1.80 (+0.13, -0.12)

If the light curve continues to decay with a power-law decay index of
0.30, the count rate at T+24 hours will be 0.082 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x
10^-12 (3.5 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00820513.

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