TITLE: GCN CIRCULAR NUMBER: 37746 SUBJECT: GRB 241002B: Swift-XRT afterglow detection DATE: 24/10/09 09:51:52 GMT FROM: Phil Evans at U of Leicester V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), C. Salvaggio (INAF-OAB) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has conducted further observations of the field of the Fermi/GBM-detected burst GRB 241002B (GCN Circs. 37668 and 37711), also detected by Swift-BAT GUANO (GCN Circ. 37704). The observations now extend from T0+134.5 ks to T0+600.2 ks. The source previously reported, "Source 4", is fading with 2.8 sigma significance and thus is believed to be the GRB afterglow. Using 2711 s of PC mode data and 3 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 = 328.32006, -58.94706 which is equivalent to: RA (J2000): 21h 53m 16.81s Dec(J2000): -58d 56' 49.4" with an uncertainty of 3.2 arcsec (radius, 90% confidence). This position is 3.2 arcsec from the position of candidate optical counterpart GOTO24gpc/AT 2024xbg (GCN Circs. 37676 and 37694). The light curve can be modelled with a power-law decay with a decay index of alpha=0.9 (+0.6, -0.4). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.7 (+0.9, -0.7). The best-fitting absorption column is 1.3 (+2.8, -0.9) x 10^21 cm^-2, consistent with the Galactic value of 3.0 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 4.6 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.3 (+2.8, -0.9) x 10^21 cm^-2 Galactic foreground: 3.0 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.7 (+0.9, -0.7) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021721. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021721. This circular is an official product of the Swift-XRT team.