GCN Circular 38713

D.N. Burrows (PSU), M. A. Williams (PSU), S. Dichiara (PSU), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester), M. Ferro (INAF-OAB),
R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/GBM-detected burst GRB 241228B. We searched for X-ray sources in 
1.8 ks of Photon Counting (PC) mode data. The total exposure at the
position of the afterglow (see below) is 1.8 ks, obtained between
T0+40.0 ks and T0+41.8 ks.

An uncatalogued X-ray source is detected within the estimated 3-sigma
Fermi/GBM error region. Using 1809 s of PC mode data and 1 UVOT image,
we find an enhanced XRT position (using the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
127.77288, +6.84823 which is equivalent to:

RA (J2000): 08h 31m 05.49s
Dec(J2000): +06d 50' 53.6"

with an uncertainty of 3.2 arcsec (radius, 90% confidence). This
position is 0.6 arcsec from the optical source GOTO24jmz and is
believed to be the afterglow. The light curve is consistent with a
constant source of mean count rate 5.7e-02 ct/sec. A power-law fit
gives an index of -1.281 (+4.492, -0.030).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.4 (+0.5, -0.3). The
best-fitting absorption column is  3 (+15, -0) x 10^20 cm^-2,
consistent with the Galactic value of 2.9 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 4.8 x 10^-11 (5.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3 (+15, -0) x 10^20 cm^-2
Galactic foreground: 2.9 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.4 (+0.5, -0.3)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021750.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021750.

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