TITLE: GCN CIRCULAR NUMBER: 41407 SUBJECT: GRB 250818B: Swift-XRT afterglow detection DATE: 25/08/18 05:50:58 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

M. Ferro (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M.A. Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), 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 GRB 250818B. We searched for X-ray sources in 604 s of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 604 s, obtained between T0+1.7 ks and T0+2.3 ks.

An uncatalogued X-ray source is detected and 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=46.0552, -3.1251 which is equivalent to:

RA (J2000): 03h 04m 13.24s Dec(J2000): -03h 07m 30.24s

with an uncertainty of 3.6 arcsec (radius, 90% confidence). The light curve is consistent with a constant source of mean count rate 7.1e-01 ct/sec, though only covers a short interval of time. The position is also consistent with the fading optical counterpart candidate GOTO25fzq/AT 2025ukm reported by GOTO (GCN Circ. 41406).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.67 (+0.26, -0.20). The best-fitting absorption column is 8.6 (+8.1, -2.1) x 10^20 cm^-2, consistent with the Galactic value of 6.5 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 4.0 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 8.6 (+8.1, -2.1) x 10^20 cm^-2 Galactic foreground: 6.5 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.67 (+0.26, -0.20)

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/03000030. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00031.

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

View this GCN Circular online at https://gcn.nasa.gov/circulars/41407. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...