GCN Circular 23185

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

We have analysed 6.6 ks of XRT data for GRB 180828A (Beardmore et al.
GCN Circ. 23182
Not found
 
 
), from 121 s to 28.1 ks after the  BAT trigger. The
data comprise 1.8 ks in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. Using 2040 s of PC mode data and 4 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 =
268.71817, -25.79815 which is equivalent to:

RA (J2000): 17h 54m 52.36s
Dec(J2000): -25d 47' 53.3"

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=1.22 (+/-0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.1 (+1.3, -1.1). The
best-fitting absorption column is  2.5 (+1.3, -1.0) x 10^23 cm^-2, in
excess of the Galactic value of 1.4 x 10^22 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 1.2 x 10^-10 (7.0 x 10^-10) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     2.5 (+1.3, -1.0) x 10^23 cm^-2
Galactic foreground: 1.4 x 10^22 cm^-2
Excess significance: 4.0 sigma
Photon index:	     2.1 (+1.3, -1.1)

If the light curve continues to decay with a power-law decay index of
1.22, the count rate at T+24 hours will be 2.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x
10^-13 (1.8 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/00856977.

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