GCN Circular 20963

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), D.N. Burrows
(PSU), A. Cholden-Brown (PSU), S. J. LaPorte (PSU), B. Mingo (U.
Leicester) and	report on behalf of the Swift-XRT team: report on
behalf of the Swift-XRT team:

We have analysed 7.3 ks of XRT data for GRB 170330A, from 89 s to 53.1
ks after the  BAT trigger. The data comprise 404 s in Windowed Timing
(WT) mode (the first 7 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=0.71 (+0.07, -0.06), followed by a break at T+439 s to
an alpha of 1.061 (+0.029, -0.025).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.00 (+/-0.10). The
best-fitting absorption column is  4.0 (+0.6, -0.5) x 10^21 cm^-2, in
excess of the Galactic value of 3.0 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.88 (+0.12, -0.11)
and a best-fitting absorption column of 4.0 (+/-0.6) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.2 x 10^-11 (6.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.0 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 3.0 x 10^21 cm^-2
Excess significance: 2.6 sigma
Photon index:	     1.88 (+0.12, -0.11)

If the light curve continues to decay with a power-law decay index of
1.061, the count rate at T+24 hours will be 0.021 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.7 x
10^-13 (1.3 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/00744773.

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