TITLE: GCN CIRCULAR NUMBER: 38899 SUBJECT: GRB 250109A / EP250109a: Swift detection of the X-ray afterglow DATE: 25/01/10 20:57:21 GMT FROM: Jamie Kennea at Penn State jak51@psu.edu

J. A. Kennea (PSU), P. A. Evans and K. L. Page (Leicester) report on behalf of the Swift/XRT team:

Swift/XRT performed a further 3.1ks of exposure on GRB 250109A (GCN #38887) AKA EP250109a (GCN #38864), following on from a rapid 1ks observation which did not detect the counterpart (GCN #38867). Combining the two datasets, we detect one point source inside the EP WXT error circle, at the following position: RA/Dec(J2000) = 88.80698, -12.4958, which is equivalent to:

RA(J2000) = 05h 55m 13.67s, Dec(J2000) = -12° 29′ 44.8″,

with an estimated uncertainty of 5.8 arc-seconds radius (90% containment). This position lies 6 arc-seconds from the EP FXT position, and 1.1 arc-seconds from the SVOM-VT reported optical position (GCN #38872). Given the localization coincidence, this source is likely to be the afterglow of GRB 250109A.

The combined XRT flux from this afterglow is 9(+5,-4) x 10^-14 erg/s/cm^2 (0.3-10 keV).

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