GCN Circular 20828

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

We have analysed 6.7 ks of XRT data for GRB 170306A, from 119 s to 28.8
ks after the  BAT trigger. The data comprise 29 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 refined XRT position is RA,
Dec = 263.0690, -44.7482 which is equivalent to:

RA (J2000): 17 32 16.56
Dec(J2000): -44 44 53.5

with an uncertainty of 3.6 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.25 (+0.28, -0.21), followed by a break at T+559 s to
an alpha of 0.36 (+0.10, -0.26).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.05 (+0.30, -0.19). The
best-fitting absorption column is  3.98 (+1.64, -0.16) x 10^21 cm^-2,
consistent with the Galactic value of 3.8 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 3.8 x 10^-11 (6.1 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.98 (+1.64, -0.16) x 10^21 cm^-2
Galactic foreground: 3.8 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.05 (+0.30, -0.19)

If the light curve continues to decay with a power-law decay index of
0.36, the count rate at T+24 hours will be 0.011 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.3 x
10^-13 (6.9 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/00741220.

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