TITLE: GCN CIRCULAR NUMBER: 40996 SUBJECT: GRB250706B: Swift-XRT counterpart detection DATE: 25/07/06 21:33:59 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/40996. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...