TITLE: GCN CIRCULAR NUMBER: 35600 SUBJECT: GRB 240122A: Swift-XRT afterglow detection DATE: 24/01/23 11:25:05 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), D.N. Burrows (PSU), J. D. Gropp (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (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 MAXI-detected burst GRB 240122A in a series of observations tiled on the sky. The total exposure time is 3.6 ks, distributed over 7 tiles; the maximum exposure at a single sky location was 1.3 ks. The data were collected between T0+29.5 ks and T0+35.6 ks, and are entirely in Photon Counting (PC) mode. Four uncatalogued X-ray sources are detected, of which one ("Source 3") is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. Using 930 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 = 93.05391, -19.14418 which is equivalent to: RA (J2000): 06h 12m 12.94s Dec(J2000): -19d 08' 39.0" with an uncertainty of 2.9 arcsec (radius, 90% confidence). This position is 14.5 arcmin from the MAXI position, and it is consistent with the GOTO optical detection (GCN 35596, Kumar et al. 2024). The light curve can be modelled with a power-law decay with a decay index of alpha=2.5 (+0.7, -2.1). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.82 (+0.33, -0.25). The best-fitting absorption column is consistent with the Galactic value of 1.0 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.0 (+/-0.8) x 10^21 cm^-2 Galactic foreground: 1.0 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.82 (+0.33, -0.25) If the light curve continues to decay with a power-law decay index of 2.5, the count rate at T+24 hours will be 0.012 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x 10^-13 (4.2 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021645. The results of the full analysis of the tiled XRT observations are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00119. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/35600. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...