TITLE:   GCN CIRCULAR
NUMBER:  41680
SUBJECT: EP250903a: LCO candidate counterpart discovery
DATE:    25/09/03 18:47:51 GMT
FROM:    Andrew Levan at Radboud University <a.levan@astro.ru.nl>

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:

UPDATE: this candidate counterpart has been retracted (and the correct afterglow likely identified). See Levan et al. (GCN 41683).

We observed the location of EP250903a (Zhao et al.  GCN 41674) with the Sinistro instrument mounted on the 1-m telescopes at the LCO Siding Springs Observatory. 3 x 300 s exposures were obtained in the SDSS r filter at a start time of 14:44:15 UT on 2025-09-03 (~41 minutes post trigger).

Within the WXT and FXT error circles we locate a new source  in our stacked r-band image with r~21 mag (AB) calibrated against nearby SkyMapper stars and not corrected by Galactic extinction, at a location of

RA(J2000): 22:36:27.75
DEC(J2000): -49:51:25.6

The error is about 0.5” in each coordinate. We note the transient is offset ~1” from a Legacy survey object with magnitude r= 21.49 mag which has a photometric redshift of 0.24 +/- 0.11. In the legacy images the source appears to extend underneath the new source. Thus, our photometry likely includes some contribution from the host galaxy.