TITLE: GCN CIRCULAR NUMBER: 41885 SUBJECT: GRB 250919A: Swift-XRT afterglow detection DATE: 25/09/19 14:03:29 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Lanava (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of GRB 250919A/EP250919a. We searched for X-ray sources in 1.7 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 1.7 ks, obtained between T0+21.4 ks and T0+23.1 ks, taking T0 as the Fermi trigger time. An uncatalogued X-ray source is detected within the estimated 3-sigma Einstein Probe/WXT error region (35 arcsec), also consistent with the Fermi-LAT error region, and is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. Using 1732 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 298.55415, -48.83775 which is equivalent to: RA (J2000): 19h 54m 13.00s Dec(J2000): -48d 50' 15.9" with an uncertainty of 2.5 arcsec (radius, 90% confidence). The light curve is consistent with a constant source of mean count rate 2.3e+00 ct/sec. A power-law fit gives an index of 1.0 (+1.9, -1.0). However, given the position of this X-ray saource is consistent with the optical counterpart with a measured redshift of 1.145 (GCN 41883), as well as being above the RASS 3-sigma upper limit, we believe this to be the X-ray afterglow. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.52 (+0.13, -0.11). The best-fitting absorption column is 6.7 (+4.1, -1.6) x 10^20 cm^-2, consistent with the Galactic value of 5.1 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 4.5 x 10^-11 (4.8 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6.7 (+4.1, -1.6) x 10^20 cm^-2 Galactic foreground: 5.1 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.52 (+0.13, -0.11) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/03000101. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/EP/EP_FIELD00075. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41885. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...