TITLE:   GCN CIRCULAR
NUMBER:  16767
SUBJECT: GRB 140903A: Swift-XRT refined Analysis
DATE:    14/09/03 20:57:43 GMT
FROM:    Massimiliano de Pasquale at IASF-Palermo  <ucasmd0@ucl.ac.uk>

M. De Pasquale, A. Maselli (IASF-Palermo), J. R. Cummings (NASA/UMBC)
reports on behalf of the Swift-XRT team:

We have analysed 2.9 ks of XRT data for GRB 140903A (Cummings  et al.
GCN Circ. 16763),  from 74 s to 7.3 ks after the  BAT trigger. The data
are entirely in Photon Counting (PC) mode. Using 3407 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 = 238.01361, +27.60259 which is equivalent to:

RA (J2000): 15h 52m 03.27s
Dec(J2000): +27d 36' 09.3"

with an uncertainty of 1.7 arcsec (radius, 90% confidence).

The light curve can be modelled with  a power-law decay with a decay
index of alpha=0.21 (+/-0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.71 (+0.21, -0.20). The
best-fitting absorption column is  1.2 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 3.3 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 4.0 x 10^-11 (4.7 x 10^-11) erg
cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column:         1.2 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^20 cm^-2
Excess significance: 2.5 sigma
Photon index:         1.71 (+0.21, -0.20)

If the light curve continues to decay with a power-law decay index of
0.21, the count rate at T+24 hours will be 0.11 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.5 x
10^-12 (5.3 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/00611599.

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