TITLE: GCN CIRCULAR NUMBER: 35859 SUBJECT: GRB 240225B: Swift-XRT afterglow detection DATE: 24/03/04 13:18:03 GMT FROM: Chiara Salvaggio at INAF OABrera <chiara.salvaggio@inaf.it> C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), A. A. Breeveld (UCL-MSSL), P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed further follow-up observations of the MAXI-detected burst GRB 240225B (Nakajima et al., GCN Circ. 35796; Joshi et al., GCN Circ. 35798; Kawakubo et al., GCN Circ. 35811; Frederiks et al., GCN Circ. 35835; Cheung et al., GCN Circ. 35848). The data were collected between T0+461.7 ks and T0+508.9 ks, and are entirely in Photon Counting (PC) mode for a total exposure time of 1.4 ks. The uncatalogued X-ray source reported by D'Ai et al. ("Source 1"; GCN Circ. 35810), is still detected at an average count rate of ~ 1.3e-2 ct/s and shows signs of fading with >3-sigma significance, and is therefore likely the GRB afterglow. Using 1113 s of XRT Photon Counting mode data and 1 UVOT image, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 128.36159, 27.07601 which is equivalent to: RA (J2000): 08 33 26.78 Dec (J2000): +27 04 33.6 with an uncertainty of 3.9 arcsec (radius, 90% confidence). This position is consistent with the reported optical afterglow position (Gompertz et el., GCN Circ. 35805; Liu et al., GCN Circ. 35812; Malesani et al., GCN Circ. 35819; Wise et al., GCN Circ. 35820; Gompertz et el., GCN Circ. 35824; Pankov et al., GCN Circ. 35826; Moskvitin et al., GCN Circ. 35828; Amit et al., GCN Circ. 35030; Sasada et al., GCN Circ. 35831; Moskvitin et al., GCN Circ. 35839). This position may be improved as more data are received. The latest position can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). The X-ray afterglow light curve can be modelled with a power-law with no breaks and decay index alpha = 1.09 (+0.16, -0.13). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.1 (+0.8, -0.6). The best-fitting absorption column is 1.0 (+2.0, -1.0) × 10^21 cm^-2, at a redshift of 0.946 (Schneider et al., GCN Circ. 35832), in addition to the Galactic value of 4.0 × 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 2.72 x 10^-11 (3.7 x 10^-11) erg cm^-2 count^-1. The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/00021675. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/35859. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...