TITLE: GCN CIRCULAR NUMBER: 38525 SUBJECT: GRB 241209B: Swift-XRT afterglow detection DATE: 24/12/10 14:36:15 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

M. A. Williams (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), S. Dichiara (PSU), J.A. Kennea (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected burst GRB 241209B. We searched for X-ray sources in 3.1 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 3.1 ks, obtained between T0+50.8 ks and T0+57.9 ks.

An uncatalogued X-ray source is detected within the estimated 3-sigma SVOM/ECLAIRs error region (459 arcsec) and is believed to be the afterglow, given its positional coincidence with the SVOM/VT optical transient (GCN 38516). Using 1725 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 = 194.63185, +76.17637 which is equivalent to:

RA (J2000): 12h 58m 31.64s Dec(J2000): +76d 10' 34.9"

with an uncertainty of 4.5 arcsec (radius, 90% confidence). This position is 5.7 arcmin from the SVOM/ECLAIRs position. We cannot determine at the present time whether the source is fading.

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021739. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021739.

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

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