TITLE: GCN CIRCULAR NUMBER: 37337 SUBJECT: GRB 240828A: Swift-XRT afterglow detection DATE: 24/08/29 12:58:35 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

A.P. Beardmore (U. Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams (PSU), J.P. Osborne (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 Fermi/LAT-detected burst GRB 240828A in a series of observations tiled on the sky. The total exposure time is 3.3 ks, distributed over 3 tiles; the maximum exposure at a single sky location was 2.3 ks. The data were collected between T0+42.1 ks and T0+60.2 ks, and are entirely in Photon Counting (PC) mode.

Six 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. The position of this source is RA, Dec=225.5130, +36.5285 which is equivalent to:

RA (J2000): 15:02:3.12 Dec(J2000): +36:31:42.5

with an uncertainty of 3.9 arcsec (radius, 90% confidence). This position is 6.8 arcmin from the Fermi/LAT position.

The light curve can be modelled with a power-law decay with a decay index of alpha=3.043 (+0.027, -0.580).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.47 (+0.33, -0.24). The best-fitting absorption column is consistent with the Galactic value of 1.3 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 3.7 x 10^-11 (3.8 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.3 (+/-6.6) x 10^20 cm^-2 Galactic foreground: 1.3 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.47 (+0.33, -0.24)

If the light curve continues to decay with a power-law decay index of 3.043, the count rate at T+24 hours will be 0.014 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.3 x 10^-13 (5.3 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/00021707. The results of the full analysis of the tiled XRT observations are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00127.

This circular is an official product of the Swift-XRT team.

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