GCN Circular 40996
Subject
GRB250706B: Swift-XRT counterpart detection
Event
Date
2025-07-06T21:33:59Z (a day ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), J.A. Kennea (PSU), D.N.
Burrows (PSU), M.A. Williams (PSU), A.P. Beardmore (U. Leicester), J.P.
Osborne (U. Leicester), A. D'Ai (INAF-IASFPA) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:
Swift has performed follow-up observations of the SVOM detected burst
GRB250706B (GCN 40989), collecting 1.6 ks of XRT data from 1.4 ks to
3.0 ks after the SVOM/ECLAIRs trigger. The data are entirely in Photon
Counting (PC) mode. A candidate X-ray counterpart has been found with a
position RA, Dec = 41.2296, -50.0604 which is equivalent to:
RA (J2000): 02 44 55.11
Dec(J2000): -50 03 37.5
with an uncertainty of 3.5 arcsec (radius, 90% confidence).
We consider the X-ray source the afterglow of GRB 250706B, even if this
position appears only marginally consistent with the optical
counterpart detected by TRT (Zhu et al., GCN 40991) and from SVOM/VT
(GCN 40992).
The light curve can be modelled with an initial power-law decay with an
index of alpha=0.93 (+0.17, -0.21), followed by a break at T+2679 s to
an alpha of 8.0 (+0.0, -1.4).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.50 (+/-0.07). The
best-fitting absorption column is 1.11 (+0.24, -0.23) x 10^21 cm^-2,
in excess of the Galactic value of 2.8 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 4.3 x 10^-11 (4.8 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.11 (+0.24, -0.23) x 10^21 cm^-2
Galactic foreground: 2.8 x 10^20 cm^-2
Excess significance: 5.9 sigma
Photon index: 1.50 (+/-0.07)
If the light curve continues to decay with a power-law decay index of
8.0, the count rate at T+24 hours will be 1.3 x 10^-11 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.6 x
10^-22 (6.2 x 10^-22) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00019912.
This circular is an official product of the Swift-XRT team.