TITLE: GCN CIRCULAR NUMBER: 44084 SUBJECT: EP260321a: ZTF and Rubin detections of the candidate optical counterpart to EP260321a DATE: 26/03/22 23:06:20 GMT FROM: Tomas Ahumada <tomas.ahumada@noirlab.edu> T. Ahumada (NOIRLab), X. J. Hall (CMU), D. A. Perley (LJMU) on behalf of the Zwicky Transient Facility We report the Zwicky Transient Facility detection of ZTF26aaonmha (AT 2026gzf), the candidate optical counterpart to EP260321a (Huang et al., GCNs 44068, 44075) announced by Lee et al (GCN 44070, see also Aryan et al., GCN 44081, Tanvir et al. GCN 44082). The ZTF detection was observed at 2026-03-22 07:35:25 (~19 hr after the EP-trigger) in the ZTF-g band at a magnitude of 19.7 +/- 0.2 mag. The region was also observed in ZTF-r band at 2026-03-22 07:12:13, but resulted in a non-detection limit of 19.6 mag. The source has no previous activity in the ZTF survey, with the most recent upper limit of g = 20.2 mag on 2026-03-21 08:06:02 (4.5 hr before the X-ray detection). The ZTF non-detection prior to the EP detection, together with the post-EP ZTF trigger, strongly supports the optical association. We additionally note that the source is detected in the Rubin alert stream, although we caution that the alerts show a dipole structure and could be spurious detections. The variability in the Rubin data fluctuates in around r = 23.8 mag, with the most prominent pre-EP detection at r = 21.1 mag, ~25 days before the X-ray flare. The combined Rubin and ZTF lightcurve can be found at https://babamul.caltech.edu/objects/LSST/314003014107006318 We are pursuing and encouraging follow-up observations to classify the source and validate the X-ray and optical association. Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC. View this GCN Circular online at https://gcn.nasa.gov/circulars/44084. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...