TITLE:   GCN CIRCULAR
NUMBER:  46241
SUBJECT: EP261105a: Euclid DR1 Archival Observations Reveal Underlying Source
DATE:    26/11/12 23:00:40 GMT
FROM:    Gregory Corcoran  at University College Dublin  <gregory.corcoran@ucdconnect.ie>

G. Corcoran (UCD), S. J. Brennan (MPE), S. J. Smartt (Oxford/QUB), B. P. Gompertz (Birmingham), A. Martin-Carrillo (UCD), A. J. Levan (Radboud/Warwick), J. H. Gillanders (Oxford), on behalf of the ENGRAVE collaboration.

We examined the field of the fast X-ray transient EP261105a (Guo et al., GCN 46155) in the newly released Euclid DR1 foundations data release (Aussel et al. 2026). At this time, calibrated data for this field is only available in the VIS band.

At the position of the candidate optical counterpart (Ma et al., GCN 46160), there is an underlying source that appears point-like in nature with a cataloged m_vis = 24.93 +/- 0.06 (MAG_AUTO). There is some visual evidence of extended emission around the source and the SVOM position of EP261105A (Ma et al, GCN 46160) is marginally offset (~0.15 arcsec) from the Euclid source. 

The Euclid images alone cannot distinguish between a foreground Galactic source, an unresolved extended object associated with the z=0.076 galaxy (Waseem et al., GCN 46170; Gillanders et al., GCN 46231) or a higher redshift chance alignment. Assuming this faint source corresponds to a higher redshift galaxy and considering a half-light radius of 0.1 arcsec and an offset of 2x the half-light radius, we calculate the probability of a chance alignment with such a source to be ~0.12% (cf., Bloom et al., 2002).