TITLE: GCN CIRCULAR NUMBER: 41325 SUBJECT: GRB 250812A: Swift-XRT afterglow detection DATE: 25/08/12 13:34:45 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

T. Sbarrato (INAF-OAB), D.N. Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of GRB 250812A. We searched for X-ray sources in 977 s of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 977 s, obtained between T0+6.7 ks and T0+12.4 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. Using 1351 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 = 32.80701, -43.16360 which is equivalent to:

RA (J2000): 02h 11m 13.68s Dec(J2000): -43d 09' 49.0"

with an uncertainty of 3.0 arcsec (radius, 90% confidence). The light curve is consistent with a constant source of mean count rate 1.5e-01 ct/sec. A power-law fit gives an index of 0.16 (+0.64, -0.16).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.97 (+0.40, -0.23). The best-fitting absorption column is consistent with the Galactic value of 1.5 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 3.1 x 10^-11 (3.2 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.5 (+/-6.9) x 10^20 cm^-2 Galactic foreground: 1.5 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.97 (+0.40, -0.23)

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00019994. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00028.

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

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