TITLE:   GCN CIRCULAR
NUMBER:  17597
SUBJECT: GRB 150317A: Swift-XRT refined Analysis
DATE:    15/03/17 21:12:25 GMT
FROM:    Boris Sbarufatti at INAF-OAB/IASFPA  <boris.sbarufatti@brera.inaf.it>

A.P. Beardmore (U. Leicester), A. Amaral-Rogers (U. Leicester), M. de
Pasquale (INAF-IASFPA), A. Maselli  (INAF-IASFPA), A. Melandri
(INAF-OAB), D.N. Burrows (PSU), J.A. Kennea (PSU), B. Sbarufatti
(INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift-XRT
team:

We have analysed 15 ks of XRT data for GRB 150317A (Siegel et al. GCN
Circ. 17592), from 68 s to 42.4 ks after the  BAT trigger. The data
comprise 19 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. Using 5905 s of PC mode data and 7 UVOT images, we
find an enhanced XRT position (using the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
138.98467, +55.46573 which is equivalent to:

RA (J2000): 09h 15m 56.32s
Dec(J2000): +55d 27' 56.6"

with an uncertainty of 1.8 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.90 (+0.06, -0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.3 (+/-0.3). The
best-fitting absorption column is  2.9 (+0.6, -0.5) x 10^22 cm^-2, in
excess of the Galactic value of 3.0 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 5.9 x 10^-11 (2.0 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.9 (+0.6, -0.5) x 10^22 cm^-2
Galactic foreground: 3.0 x 10^20 cm^-2
Excess significance: 9.2 sigma
Photon index:	     2.3 (+/-0.3)

If the light curve continues to decay with a power-law decay index of
0.90, the count rate at T+24 hours will be 2.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.6 x
10^-13 (5.3 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/00635148.

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