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GCN Circular 30915

Subject
GRB 210928A: Swift-XRT analysis (replaces GCN 30914)
Date
2021-10-01T17:48:53Z (3 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
P. A. Evans and K.L. Page (U. Leicester) report on behalf of the Swift-XRT team:

GCN Circ (30914) was sent in error, prematurely. The corrected information is below,
and we apologise for the confusion.

We have analysed 5.3 ks of XRT data for the Fermi-detected burst GRB 210928A
(GCN Circs. 30912, 30913). The data were originally collected as a ToO to
observe U Sco, the possible counterpart to the Fermi-LAT source Fermi J1623-1752
(ATEL #14939). While U Sco was not detected, a new transient, Swift
J162315.0-175233 was discovered (ATEL #14945). Further Swift observations were
requested and are ongoing. The discovery of an unannounced Fermi-GBM trigger has
identified this object as GRB 210928A (ATEL #14948), and the manual insertion 
of the already-observed data into the GRB analysis system resulted in the premature
dispatch of this circular.

The Swift-XRT observations at present extend from 67.4 ks to 262.9 ks after the
GBM trigger time. The data are entirely in Photon Counting (PC) mode. Using 1636
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 = 245.51109, -17.94788 which is equivalent to:

RA (J2000): 16h 22m 02.66s
Dec(J2000): -17d 56' 52.4"

with an uncertainty of 4.4 arcsec (radius, 90% confidence). This
position is 29 arcsec from the Fermi-LAT  position. 

The light curve can be modelled with a power-law decay with a decay index of
alpha=1.3 (+1.8, -1.3); however comprising only 3 data points and an upper
limit, this should be viewed with the caution the large error-bars imply.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.4 (+0.6, -0.3). The
best-fitting absorption column is  2.342 (+3.358, -0.021) x 10^21
cm^-2, consistent with the Galactic value of 2.3 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 4.8 x 10^-11
(5.5 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.342 (+3.358, -0.021) x 10^21 cm^-2
Galactic foreground: 2.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.4 (+0.6, -0.3)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00014850.

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