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GCN Circular 35587

GRB 240118A: Swift-XRT afterglow detection
2024-01-20T22:41:31Z (5 months ago)
Phil Evans at U of Leicester <>
A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi 
(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), A. Tohuvavohu (U. Toronto),
D.N. Burrows (PSU), J. D. Gropp (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 240118A, collecting 4.7 ks of Photon
Counting (PC) mode data between T0+96.2 ks and T0+109.0 ks. 

Two uncatalogued X-ray sources are detected consistent with being
within 296 arcsec of the Fermi/LAT position, of which one ("Source 3")
is believed to be the afterglow. Using 4697 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 = 18.53587, +29.79798 which is equivalent to:

RA (J2000): 01h 14m 08.61s
Dec(J2000): +29d 47' 52.7"

with an uncertainty of 2.7 arcsec (radius, 90% confidence). This
position is 2.1 arcmin from the Fermi/LAT position.  The light curve is
consistent with a constant source of mean count rate 3.6e-02 ct/sec. A
power-law fit gives an index of 1.9 (+1.0, -2.8).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+/-0.6). The
best-fitting absorption column is  1.1 (+0.8, -0.6) x 10^22 cm^-2, in
excess of the Galactic value of 7.1 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 4.5 x 10^-11 (9.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.1 (+0.8, -0.6) x 10^22 cm^-2
Galactic foreground: 7.1 x 10^20 cm^-2
Excess significance: 3.1 sigma
Photon index:	     2.2 (+/-0.6)

The results of the XRT-team automatic analysis are available at
The results of the full analysis of the XRT observations are available

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

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