TITLE: GCN CIRCULAR NUMBER: 42891 SUBJECT: GRB 251129A: Swift-XRT afterglow detection DATE: 25/11/29 12:29:18 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> K.L. Page (U. Leicester), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU), A.P. Beardmore (U. Leicester), 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 251129A. We searched for X-ray sources in 732 s of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 2.3 ks, obtained between T0+16.5 ks and T0+28.5 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 2336 s of PC mode data and 4 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 = 224.33074, +79.29312 which is equivalent to: RA (J2000): 14h 57m 19.38s Dec(J2000): +79d 17' 35.2" with an uncertainty of 2.1 arcsec (radius, 90% confidence). This position is 26 arcsec from the SVOM/ECLAIRs position. The position is consistent with the optical afterglow (GCNs 42880, 42882, 42883, 42885, 42887) and the EP X-ray transient (GCN 42884). The light curve can be modelled can be modelled with a power-law decay with a decay index of alpha=6.23 (+0.32, -0.29). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.71 (+0.22, -0.20). The best-fitting absorption column is 7.3 (+3.7, -3.3) x 10^20 cm^-2, in excess of the Galactic value of 2.9 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 2.6 x 10^-11 (3.7 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 7.3 (+3.7, -3.3) x 10^20 cm^-2 Galactic foreground: 2.9 x 10^20 cm^-2 Excess significance: 2.2 sigma Photon index: 2.71 (+0.22, -0.20) If the light curve continues to decay with a power-law decay index of 6.2, the count rate at T+24 hours will be 1.1 x 10^-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x 10^-15 (4.2 x 10^-15) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/00021892. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021892. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/42891. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...