TITLE: GCN CIRCULAR NUMBER: 44120 SUBJECT: EP260324a: ZTF discovery of the optical counterpart ZTF26aapviim/AT2026hir DATE: 26/03/26 21:32:04 GMT FROM: akashjanaki98@gmail.com Akash Anumarlapudi (UNC CH), Robert Stein (UMd), Tomas Ahumada (NOIRLab), Jillian Rastinejad (UMd), Mansi Kasliwal, Lin Yan (Caltech), D. Y. Li, C.C.Jin, W. Yuan (NAO, CAS) report, on behalf of the ZTF collaboration. Einstein Probe detected a long-lived X-ray transient (EP260324a; Wu et al., ATel #17728) beginning on 2026-03-24T07:04:20 (hereafter 't0') that triggered the Wide-field X-ray Telescope (WXT) multiple times over the following day. Subsequent observations with the Follow-up X-ray Telescope (FXT) proposed a likely association of this transient with the galaxy SDSS J084412.34+820110.9. Here, we report the Zwicky Transient Facility (ZTF; Bellm et al. 2019, Graham et al. 2019) discovery of the optical counterpart, ZTF26aapviim / AT2026hir. AT2026hir was detected by the ZTF survey on 2026-03-26T04:51:22, but forced photometry revealed an earlier detection at 2026-03-24T04:17:28. This first detection is ~3 hours before the first EP detection. The source thus exhibited a moderately fast rise (0.7 mag per day). ZTF light curve of AT2026hir is provided below: | Time (days since t0) | Band | Magnitude | Error | Limiting magnitude | | -------- | -------- | -------- | -------- | ------- | -0.12 | r | 20.14 | 0.15 | 20.52 | 0.07 | g | -- | -- | 17.93 | 1.91 | r | 18.71 | 0.11 | 19.42 | 2.07 | g | 18.38 | 0.08 | 19.49 | We note that AT2026hir was also reported to TNS by ATLAS (Tonry et al. 2018), with detections beginning at 2026-03-26T07:32:13.920 (~2 days after the EP discovery). The transient is 0.3" away from the nucleus (PS1 position) of the host galaxy identified by ATel #17728 (Wu et al.), so it may be nuclear. The host galaxy itself does not display any obvious evidence of being an AGN (W1-W2=0.2, no detection in milliquas (Flesch et al. 2023), and no previous significant ZTF detections). 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/44120. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...