TITLE: GCN CIRCULAR NUMBER: 39584 SUBJECT: EP250304a: Swift-XRT detection of a fading X-ray source DATE: 25/03/04 09:18:19 GMT FROM: K.L. Page at U Leicester klp5@leicester.ac.uk

K.L. Page, P.A. Evans (U.Leicester) and J. DeLaunay (PSU) report on behalf of the Swift XRT Team:

On 2025 March 04 at 02:59 UT, Swift started observing EP250304a, 5.2 ks after the Einstein Probe trigger (GCN Circ. 39580). A fading X-ray source was identified, at a position of RA, Dec = 208.39408, -42.8046, which is equivalent to

RA (J2000): 13h 53m 34.58s Dec (J2000): -42d 48′ 16.7″

with an uncertainty of 4.1 arcsec (radius, 90% confidence). This is consistent with the FXT position (GCN Circ. 39580), as well as the TRT optical counterpart given in GCN Circ. 39583.

The mean observed X-ray flux in the first snapshot was (3.5 +/- 0.4) x 10^-12 erg cm^-2 s^-1 (0.3 - 10 keV). By the time of the second snapshot, at 10.3 ks after the trigger, the source was no longer detected, having faded to a 3-sigma upper limit of 2.1 x 10^-12 erg cm^-2 s^-1.

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