TITLE:   GCN CIRCULAR
NUMBER:  23045
SUBJECT: GRB 180727A: Swift-XRT refined Analysis
DATE:    18/07/28 14:58:03 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

V. D'Elia (ASDC), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J.
LaPorte (PSU), Z. Liu (NAOC / U. Leicester), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA), A.
Melandri (INAF-OAB) and A.P. Beardmore report on behalf of the
Swift-XRT team:

We have analysed 5.9 ks of XRT data for GRB 180727A (Beardmore et al.
GCN Circ. 23043), from 58 s to 80.1 ks after the  BAT trigger. The data
comprise 24 s in Windowed Timing (WT) mode (the first 7 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. Using 503 s of PC mode data and 2 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 = 346.66632, -63.05183
which is equivalent to:

RA (J2000): 23h 06m 39.92s
Dec(J2000): -63d 03' 06.6"

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=2.4 (+0.3, -0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+/-0.4). The
best-fitting absorption column is  2.1 (+1.8, -1.4) x 10^21 cm^-2, in
excess of the Galactic value of 2.1 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 3.9 x 10^-11 (5.0 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.1 (+1.8, -1.4) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^20 cm^-2
Excess significance: 2.2 sigma
Photon index:	     1.9 (+/-0.4)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00850231.

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