TITLE: GCN CIRCULAR NUMBER: 41658 SUBJECT: GRB 250830B: Swift-XRT afterglow detection DATE: 25/09/02 14:31:53 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU) 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 MAXI-detected burst GRB 250830B. The observations now extend from T0+11.1 ks to T0+223.0 ks and have a total exposure time of 6.0 ks. . The source previously reported, "Source 1", is fading with 2.5 sigma significance and thus is believed to be the GRB afterglow. Using 2349 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 = 105.74037, -64.85289 which is equivalent to: RA (J2000): 07h 02m 57.69s Dec(J2000): -64d 51' 10.4" with an uncertainty of 3.0 arcsec (radius, 90% confidence). This position is 2.3 arcmin from the EP position. The light curve can be modelled with a power-law decay with a decay index of alpha=0.43 (+0.15, -0.16). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.4 (+0.8, -0.5). The best-fitting absorption column is 1.5 (+6.0, -0.4) x 10^21 cm^-2, consistent with the Galactic value of 1.1 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 5.4 x 10^-11 (6.1 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.5 (+6.0, -0.4) x 10^21 cm^-2 Galactic foreground: 1.1 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.4 (+0.8, -0.5) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021858. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021858. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41658. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...