TITLE: GCN CIRCULAR NUMBER: 39312 SUBJECT: EP250207B: Chandra localization of the X-ray counterpart DATE: 25/02/13 20:29:54 GMT FROM: Eleonora Troja <nora.gsfc@gmail.com> E. Troja (U Rome) reports on behalf of the ERC BHianca team: We imaged the field of the fast X-ray transient EP250207B (Zhou et al. GCN 39266) with the Chandra X-ray Observatory. Observations started on 2025 Feb 12.8 UT (~4.9 days after the trigger), using the ACIS-S instrument, for a total exposure time of 24.5 ks. Within the FXT localization (Zhou et al. GCN 39266) a single X-ray source is detected with significance >99.99% (Kraft, Burrows & Nousek, 1990) at a position consistent with the NOT optical candidate (Liu et al. GCN 39300). The inferred X-ray flux is consistent with the rapid afterglow decay reported by the EP/FXT observations (Zhou et al. GCN 39266). This supports the association between the Chandra source and EP250207B and confirms that the candidate reported by Liu et al. (GCN 39300) is its optical counterpart. No X-ray source is visible at the position of any other optical candidate (Eyles-Ferris et al. GCN 39281, Fraser et al. GCN 39287). Although the Chandra localization implies a 10 arcsec offset from the center of the nearby galaxy WISEA J111002.65-075211.9, the probability of a chance alignment remains relatively small (<1%, Bloom et al. 2002). Further observations to establish the nature of EP250207B and its distance scale are encouraged. We thank the Chandra director, Pat Slane, for awarding discretionary time and the observatory staff for rapidly scheduling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/39312. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...