TITLE: GCN CIRCULAR NUMBER: 36558 SUBJECT: GRB 240527B: Swift-XRT afterglow detection DATE: 24/05/29 05:53:44 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> J. D. Gropp (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Fermi/LAT-detected burst GRB 240527B, collecting 5.2 ks of Photon Counting (PC) mode data between T0+27.3 ks and T0+135.5 ks. Eight uncatalogued X-ray sources are detected, of which one ("Source 7") is above the RASS 3-sigma upper limit at this position and fading with >3-sigma significance, and is therefore likely the GRB afterglow. Using 1624 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 = 128.15387, -14.43738 which is equivalent to: RA (J2000): 08h 32m 36.93s Dec(J2000): -14d 26' 14.6" with an uncertainty of 2.0 arcsec (radius, 90% confidence). This position is 4.0 arcmin from the Fermi/LAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.23 (+0.16, -0.14). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.70 (+0.08, -0.06). The best-fitting absorption column is consistent with the Galactic value of 5.7 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 3.8 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 5.7 (+/-1.9) x 10^20 cm^-2 Galactic foreground: 5.7 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.70 (+0.08, -0.06) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021694. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021694. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/36558. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...