TITLE: GCN CIRCULAR NUMBER: 43213 SUBJECT: GRB 251222A: Swift-XRT afterglow detection DATE: 25/12/23 11:12:39 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU), J.P. Osborne (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected burst GRB 251222A. We searched for X-ray sources in 2.0 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 2.0 ks, obtained between T0+10.7 ks and T0+17.4 ks. An uncatalogued X-ray source is detected within the estimated 3-sigma SVOM/ECLAIRs error region (49 arcsec) and is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. Using 1130 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 = 77.19502, -7.21354 which is equivalent to: RA (J2000): 05h 08m 46.81s Dec(J2000): -07d 12' 48.7" with an uncertainty of 2.5 arcsec (radius, 90% confidence). This position is 36 arcsec from the SVOM/ECLAIRs position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.9 (+0.8, -0.7). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.92 (+0.31, -0.20). The best-fitting absorption column is 1.05 (+0.91, -0.12) x 10^21 cm^-2, consistent with the Galactic value of 9.3 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 3.6 x 10^-11 (4.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.05 (+0.91, -0.12) x 10^21 cm^-2 Galactic foreground: 9.3 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.92 (+0.31, -0.20) 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 4.7 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x 10^-13 (2.0 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/ToO_GRBs/00021895/Source1.php. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021895. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/43213. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...