GCN Circular 37334

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia
(SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB),
S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU) and P.A. Evans
(U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
SVOM/ECLAIRs-detected burst GRB 240828B, collecting 2.9 ks of Photon
Counting (PC) mode data between T0+13.5 ks and T0+25.0 ks. 

Two uncatalogued X-ray sources are detected, of which one ("Source 1")
is above the RASS 3-sigma upper limit at this position, and is
therefore likely the GRB afterglow. Using 2142 s of PC mode data and 3
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 = 63.45995, +16.71561 which is equivalent to:

RA (J2000): 04h 13m 50.39s
Dec(J2000): +16d 42' 56.2"

with an uncertainty of 2.6 arcsec (radius, 90% confidence). This
position is 5.5 arcmin from the SVOM/ECLAIRs position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.8 (+/-0.9).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+0.5, -0.4). The
best-fitting absorption column is  6.2 (+3.7, -2.7) x 10^21 cm^-2, in
excess of the Galactic value of 2.9 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 4.7 x 10^-11 (7.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.2 (+3.7, -2.7) x 10^21 cm^-2
Galactic foreground: 2.9 x 10^21 cm^-2
Excess significance: 1.9 sigma
Photon index:	     1.8 (+0.5, -0.4)

If the light curve continues to decay with a power-law decay index of
1.8, the count rate at T+24 hours will be 3.6 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (2.6 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/00021706.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021706.

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