GCN Circular 39029
Subject
EP250125a: Swift/XRT detection of a fading counterpart
Event
Date
2025-01-25T14:04:22Z (6 months ago)
From
Jamie Kennea at Penn State <jak51@psu.edu>
Via
Web form
J. A. Kennea (PSU), K. L. Page, P. A. Evans (Leicester) and B. Sbarufatti (INAF) report on behalf of the Swift XRT Team.
At 03:26UT Swift performed a series of Target-of-Opportunity observations of the Einstein Probe transient EP250125a (GCN #39028), approximately 49 minutes after the EP trigger. In the XRT data we detect a bright X-ray source at the following coordinates: RA/Dec(J2000) = 175.36337, -21.71347
RA (J2000): 11h 41m 27.21s,
Dec (J2000): -21d 42' 48.5",
with an estimated error radius of 3.8 arc-seconds (90% confidence). This lies 2.1 arc-seconds from the reported EP FXT position of the transient, and 3.9 arc-seconds from the proposed optical afterglow of the source (GCN #39029).
We note that the source is fading with a 3-sigma significance. The peak flux seen by XRT is 4.3 (±1.0) ×10^-12 erg cm^-1 s^-1 (0.3-10 keV), dropping to a flux of 4.8 (±1.1) x 10^-13 erg cm^-1 s^-1 (0.3-10 keV) at a second observation taken at T0+8.9 hours.