TITLE: GCN CIRCULAR NUMBER: 34601 SUBJECT: GRB 230827B: Swift-XRT afterglow detection DATE: 23/08/30 13:26:29 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

A. Tohuvavohu (U. Toronto), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), D.N. Burrows (PSU), J. D. Gropp (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the Fermi/GBM-detected burst GRB 230827B (GCN Circ. 34584), collecting 3.6 ks of Photon Counting (PC) mode data between T0+231.3 ks and T0+249.7 ks.

An uncatalogued X-ray source is detected consistent with being within 9.9 arcsec of the position of ZTF23abaanxz/AT2023qxj (GCN Circ. 34574) and is believed to be the afterglow. Using 3821 s of PC mode data and 4 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 = 299.63944, +54.46318 which is equivalent to:

RA (J2000): 19h 58m 33.47s Dec(J2000): +54d 27' 47.5"

with an uncertainty of 2.6 arcsec (radius, 90% confidence). This position is 0.8 arcsec from the ZTF position. The light curve is consistent with a constant source of mean count rate 7.0e-02 ct/sec. A power-law fit gives an index of 2.490 (+0.015, -2.990).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.59 (+0.33, -0.30). The best-fitting absorption column is 4.0 (+2.1, -1.0) x 10^21 cm^-2, consistent with the Galactic value of 3.0 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 5.0 x 10^-11 (6.4 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 4.0 (+2.1, -1.0) x 10^21 cm^-2 Galactic foreground: 3.0 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.59 (+0.33, -0.30)

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021620. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021620.

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

--- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...