GCN Circular 41696

Subject

EP250903a: retraction of the LCO candidate, and discovery of the likely counterpart

Event

EP250903a

Date

2025-09-04T13:25:08Z (a month ago)

Edited On

2025-09-04T13:37:16Z (a month ago)

From

Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>

Edited By

Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>

Via

Web form

A. J. Levan (Radboud and Warwick), J. A. Quirola-Vasquez (Radboud), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), A. Martin-Carrillo (UCD), R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radboud), F. E. Bauer (SSI and UTA), and J. Chacón (PUC) report on behalf of a larger collaboration:

We observed again the field of the fast X-ray transient EP250903a (Zhao et al., GCN 41674

). Our first observation was carried out with LCO/Sinistro (3x300 s exposure, mean epoch 0.68 hr after the trigger) and was reported by Levan et al. (GCN ). We obtained a second, deeper observation with LCO/Sinistro (9x300 s exposure, mean epoch 3.79 hr after the trigger), and a follow-up observation with Gemini/GMOS-S (4x60 s exposure, mean epoch 19.38 hr after the trigger).

Besides, a more accurate X-ray localization of EP250903a is now available, thanks to both Swift/XRT (https://www.swift.ac.uk/EP/) and EP/FXT (Zhang et al., GCN 41693

The candidate afterglow proposed by Levan et al. (GCN 41680

) is outside both the XRT and FXT error circles. Furthermore, image subtraction with respect to the Legacy Survey leaves no detectable residual. The magnitudes of this object reported by Xin et al. (GCN ) using SVOM/VT are also consistent with the Legacy Survey archival values (r = 21.49, g = 21.95). Given the low S/N of the original detection, the most likely explanation is that there was no flux excess, and we hereby retract this afterglow candidate.

Inspection of the second (deeper) LCO epoch and Gemini images reveals however an object not visible in the Legacy survey, at a position consistent with both the Swift/XRT and EP/FXT X-ray localization, at coordinates:

RA(J2000) = 22:36:27.02 Dec(J2000) = -49:51:14.1

The estimated error is 0.5". This object is seen to fade between the two observations, and is not detected in (deeper) images from the Legacy Survey. We conclude that this is the actual optical afterglow of EP250903a. We report for the transient the following r-band magnitudes, calibrated against the SkyMapper catalog and not corrected for Galactic extinction:

Time since trigger (hr)	Telescope	Magnitude (AB)
0.68	LCO	> 21.7
3.79	LCO	23.2 +- 0.3
19.38	Gemini	24.3 +- 0.2
N/A	Legacy	> 24.8

We apologize for any inconvenience that our previous announcement may have caused.

We acknowledge excellent support from the Gemini staff, in particular Roque Ruiz.