TITLE: GCN CIRCULAR NUMBER: 30915 SUBJECT: GRB 210928A: Swift-XRT analysis (replaces GCN 30914) DATE: 21/10/01 17:48:53 GMT 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.