TITLE: GCN CIRCULAR NUMBER: 41108 SUBJECT: GRB 250716A: Swift-XRT afterglow detection DATE: 25/07/17 15:12:43 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

M.A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), A. Melandri (INAF-OAR) 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 250716A. We searched for X-ray sources in 2.6 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 2.6 ks, obtained between T0+79.5 ks and T0+91.7 ks.

An uncatalogued X-ray source is detected within the GOTO error region and is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. Using 2790 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 = 66.77171, -46.56649 which is equivalent to:

RA (J2000): 04h 27m 05.21s Dec(J2000): -46d 33' 59.4"

with an uncertainty of 2.7 arcsec (radius, 90% confidence). This position is 1.9 arcsec from the GOTO position.

The light curve can be modelled with a power-law decay with a decay index of alpha=2.755 (+0.015, -0.755).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.22 (+0.30, -0.27). The best-fitting absorption column is 6.9 (+6.4, -5.2) x 10^20 cm^-2, in excess of the Galactic value of 1.2 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 3.0 x 10^-11 (3.7 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 6.9 (+6.4, -5.2) x 10^20 cm^-2 Galactic foreground: 1.2 x 10^20 cm^-2 Excess significance: 1.8 sigma Photon index: 2.22 (+0.30, -0.27)

The results of the XRT-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/ToO_GRBs/00021845/Source1.php. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021845.

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

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