GCN Circular 37337

A.P. Beardmore (U. Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio
(INAF-OAB), M. Ferro (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU),
M. A. Williams (PSU), J.P. Osborne (U. Leicester) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 240828A in a series of observations tiled
on the sky. The total exposure time is 3.3 ks, distributed over 3
tiles; the maximum exposure at a single sky location was 2.3 ks. The
data were collected between T0+42.1 ks and T0+60.2 ks, and are entirely
in Photon Counting (PC) mode. 

Six uncatalogued X-ray sources are detected, of which one ("Source 3")
is above the RASS 3-sigma upper limit at this position, and is
therefore likely the GRB afterglow.  The position of this source is RA,
Dec=225.5130, +36.5285 which is equivalent to:

RA (J2000): 15:02:3.12
Dec(J2000): +36:31:42.5

with an uncertainty of 3.9 arcsec (radius, 90% confidence).  This
position is 6.8 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=3.043 (+0.027, -0.580).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.47 (+0.33, -0.24). The
best-fitting absorption column is  consistent with the Galactic value
of 1.3 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 3.7 x 10^-11 (3.8 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.3 (+/-6.6) x 10^20 cm^-2
Galactic foreground: 1.3 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.47 (+0.33, -0.24)

If the light curve continues to decay with a power-law decay index of
3.043, the count rate at T+24 hours will be 0.014 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.3 x
10^-13 (5.3 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021707.
The results of the full analysis of the tiled XRT observations are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00127.

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