GCN Circular 41249

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.