Skip to main content
Testing. You are viewing the public testing version of GCN. For the production version, go to
New Announcement Feature, Code of Conduct, Circular Revisions. See news and announcements

GCN Circular 23185

GRB 180828A: Swift-XRT refined Analysis
2018-08-29T07:33:58Z (6 years ago)
Phil Evans at U of Leicester <>
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

We have analysed 6.6 ks of XRT data for GRB 180828A (Beardmore et al.
GCN Circ. 23182), 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

This circular is an official product of the Swift-XRT team.
Looking for U.S. government information and services? Visit