GCN Circular 22462

P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J.
LaPorte (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester),
A.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA)
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 180305A (Axelsson et al. GCN Circ. 22457
Not found
 
 
),
collecting 4.7 ks of Photon Counting (PC) mode data between T0+30.6 ks
and T0+53.0 ks. 

Four uncatalogued X-ray sources are detected, of which one ("Source 2")
is above the RASS limit and fading with 3-sigma significance, and is
therefore likely the GRB afterglow. Using 3574 s of PC mode data and 6
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 = 49.61821, +32.10994 which is equivalent to:

RA (J2000): 03h 18m 28.37s
Dec(J2000): +32d 06' 35.8"

with an uncertainty of 1.7 arcsec (radius, 90% confidence). This
position is 2.7 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.7 (+0.5, -0.8).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.5 (+/-0.4). The
best-fitting absorption column is  4.7 (+3.2, -2.3) x 10^21 cm^-2,
consistent with the Galactic value of 2.4 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 5.4 x 10^-11 (6.9 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.7 (+3.2, -2.3) x 10^21 cm^-2
Galactic foreground: 2.4 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.5 (+/-0.4)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020789.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020789.

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