GCN Circular 41379

C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), S. Dichiara (PSU), J.A.
Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), R. Brivio (INAF-OAB) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 6.7 ks of XRT data for GRB 250814A, from 163 s to 40.1
ks after the   trigger. The data comprise 184 s in Windowed Timing (WT)
mode (the first 10 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The best available XRT position
 (using the promptly downlinked event data, the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec =
306.30792, 48.39903 which is equivalent to:

RA (J2000): 20h 25m 13.90s
Dec(J2000): +48d 23' 56.5"

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=1.49 (+0.27, -0.28), followed by a break at T+338 s to
an alpha of 3.15 (+0.26, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.33 (+0.24, -0.22). The
best-fitting absorption column is  1.2 (+0.4, -0.3) x 10^22 cm^-2, in
excess of the Galactic value of 6.5 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.9 (+/-0.4) and a
best-fitting absorption column of 1.0 (+0.4, -0.3) x 10^22 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 5.5 x 10^-11 (9.3 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.0 (+0.4, -0.3) x 10^22 cm^-2
Galactic foreground: 6.5 x 10^21 cm^-2
Excess significance: 1.8 sigma
Photon index:	     1.9 (+/-0.4)

If the light curve continues to decay with a power-law decay index of
3.15, the count rate at T+24 hours will be 1.3 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.0 x
10^-18 (1.2 x 10^-17) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01342439.

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