TITLE:   GCN CIRCULAR
NUMBER:  19408
SUBJECT: GRB 160509A: Swift-XRT afterglow detection
DATE:    16/05/09 15:04:24 GMT
FROM:    Jamie A. Kennea at PSU/Swift-XRT  <kennea@swift.psu.edu>

J. A. Kennea (PSU), T.G.R. Roegiers (PSU), 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), L.M. McCauley (PSU),
C. Pagani (U. Leicester) and P.A. Evans (U. Leicester) report on behalf
of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 160509A (Longo et al. GCN Circ. 19403) in
a series of observations tiled on the sky. The total exposure time is
1.7 ks, distributed over 7 tiles; the maximum exposure at a single sky
location was 560 s. The data were collected between T0+7.3 ks and
T0+7.5 ks, and are entirely in Photon Counting (PC) mode. 

Using 250 s of XRT Photon Counting mode data and 1 UVOT images, we find
an astrometrically corrected X-ray position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 311.75375, 76.10837 which is equivalent to:

RA (J2000):  20 47 00.90
Dec (J2000): +76 06 30.1

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This
position is 24.8 arcmin from the Fermi/LAT position, and 11 arcseconds
from the preliminary XRT position (GCN 19407).	The light curve is
consistent with a constant source of mean count rate 1.4 ct/sec. We
cannot determine fading at this time.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.7 (+/-0.3). The
best-fitting absorption column is  4.6 (+2.3, -1.8) x 10^21 cm^-2, in
excess of the Galactic value of 2.1 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 4.9 x 10^-11 (6.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.6 (+2.3, -1.8) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^21 cm^-2
Excess significance: 2.2 sigma
Photon index:	     1.7 (+/-0.3)

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/xrt_products/TILED_GRB00054/index_2.php.
The results of the full analysis of the tiled XRT observations are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00054.

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