TITLE: GCN CIRCULAR NUMBER: 41249 SUBJECT: GRB 250806A: Swift-XRT counterpart detection DATE: 25/08/06 15:03:16 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

S. Campana (INAF-OAB), A. Melandri (INAF-OAR), 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) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

We have analysed 972 s of XRT data for GRB 250806A, from 343 s to 1.4 ks after the SVOM/ECLAIRs trigger. The data are entirely in Photon Counting (PC) mode. We find an uncatalogued X-ray source at RA, Dec = 348.4298, +1.3550 which is equivalent to:

RA (J2000): 23 13 43.16 Dec(J2000): +01 21 18.1

with an uncertainty of 3.7 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay index of alpha=1.9 (+/-0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.5 (+0.4, -0.3). The best-fitting absorption column is 1.7 (+1.6, -1.2) x 10^21 cm^-2, consistent with the Galactic value of 5.4 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 4.6 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.7 (+1.6, -1.2) x 10^21 cm^-2 Galactic foreground: 5.4 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.5 (+0.4, -0.3)

If the light curve continues to decay with a power-law decay index of 1.9, the count rate at T+24 hours will be 2.4 x 10^-5 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x 10^-15 (1.3 x 10^-15) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00019983.

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

View this GCN Circular online at https://gcn.nasa.gov/circulars/41249. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...