GCN Circular 40950

J.P. Osborne (U. Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio
(INAF-OAB), M. Ferro (INAF-OAB), M.A. Williams (PSU), S. Dichiara
(PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:

We have analysed 2.0 ks of XRT data for GRB 250704A, from 2.9 ks to 8.7
ks after the  SVOM/ECLAIRs trigger. The data are entirely in Photon
Counting (PC) mode. A bright, fading source is detected inside the
SVOM/ECLAIRs error region (Cao et al., GCN 40934
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 ), which we propose as
the GRB afterglow. Using 2029 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 =
17.19414, -17.32397 which is equivalent to:

RA (J2000): 01h 08m 46.59s
Dec(J2000): -17d 19' 26.3"

with an uncertainty of 2.8 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.3 (+0.6, -0.5).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.03 (+0.31, -0.28). The
best-fitting absorption column is  6.0 (+6.7, -4.5) x 10^20 cm^-2,
consistent with the Galactic value of 1.6 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 3.2 x 10^-11 (3.8 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.0 (+6.7, -4.5) x 10^20 cm^-2
Galactic foreground: 1.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.03 (+0.31, -0.28)

If the light curve continues to decay with a power-law decay index of
1.3, the count rate at T+24 hours will be 2.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.9 x
10^-14 (9.1 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00019907.

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