GCN Circular 22572

P.A. Evans (U. Leicester) and C. B. Markwardt (GSFC) report on behalf
of the Swift-XRT team:

We have analysed 4.2 ks of XRT data for GRB 180331A (Markwardt et al.
GCN Circ. 22568
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 ), from 115 s to 23.0 ks after the  BAT trigger. The
data comprise 70 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. 22569
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.64 (+0.21, -0.19), followed by a break at T+368 s to
an alpha of 3.47 (+0.30, -0.25); it then breaks again at T+1.2 ks to a 
decay of alpha=1.2 (+0.5, -2.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.35 (+0.21, -0.20). The
best-fitting absorption column is  4.9 (+1.7, -1.5) x 10^21 cm^-2, in
excess of the Galactic value of 2.9 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 6.0 x 10^-11 (7.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.9 (+1.7, -1.5) x 10^21 cm^-2
Galactic foreground: 2.9 x 10^21 cm^-2
Excess significance: 2.3 sigma
Photon index:	     1.35 (+0.21, -0.20)

If the light curve continues to decay with a power-law decay index of
1.2, the count rate at T+24 hours will be 3.3e-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
2.0^-14 (2.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00820347.

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