GCN Circular 37531
Subject
GRB 240916A: Swift-XRT afterglow detection
Date
2024-09-17T06:38:56Z (a month ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M.G.
Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M. Capalbi
(INAF-IASFPA), J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams
(PSU) 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 240916A, centred on the
position of the candidate optical counterpart detected by GOTO:
GOTO24fzn/AT2024vlp (Gompertz et al., GCN Circ. 37522), collecting 1.6
ks of Photon Counting (PC) mode data between T0+61.0 ks and T0+70.9 ks.
An uncatalogued X-ray source is spatially consistent with the GOTO
position and is fading with 1.6 sigma significance and thus is believed
to be the GRB afterglow. Using 687 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 =
235.91357, -7.76503 which is equivalent to:
RA (J2000): 15h 43m 39.26s
Dec(J2000): -07d 45' 54.1"
with an uncertainty of 3.2 arcsec (radius, 90% confidence). This
position is 1.0 arcsec from the GOTO position.
The light curve can be modelled with a power-law decay with a decay
index of alpha=2.96 (+0.03, -2.88).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.9 (+0.8, -0.6). The
best-fitting absorption column is 3.3 (+2.2, -1.7) x 10^21 cm^-2, in
excess of the Galactic value of 1.4 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 2.6 x 10^-11 (7.1 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 3.3 (+2.2, -1.7) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^21 cm^-2
Excess significance: 1.8 sigma
Photon index: 2.9 (+0.8, -0.6)
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021717.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021717.
This circular is an official product of the Swift-XRT team.