TITLE: GCN CIRCULAR NUMBER: 40566 SUBJECT: GRB 250521C (short): Zwicky Transient Facility identification of a possible fast optical transient counterpart DATE: 25/05/29 02:45:22 GMT FROM: Igor Andreoni at JSI/UMD/NASA <igor.andreoni@gmail.com> Igor Andreoni (UNC), Anna Ho (Cornell), Vishwajeet Swain (IITB), Michael Coughlin (UMN), on behalf of the ZTF Collaboration The optical transient ZTF25aarhkyn / AT2025mgj was first detected by the Zwicky Transient Facility (ZTF, Bellm et al. 2019, Graham et al. 2019) on 2025-05-23 03:42 UT at a magnitude r = 17.61 +- 0.07 (AB) during the regular survey. The previous non-detection at the transient location was on 2025-05-09 05:23. AT2025mgj was identified as a rapidly evolving transient by the ''ZTF Realtime Search and Triggering'' project (ZTFReST; Andreoni & Coughlin et al., 2021) and by a custom filter for fast transient discovery (Ho et al., 2022) within the ZTF Collaboration. A fading rate of ~0.4 mag/day was observed in r-band in the first two days after the initial detection. This was followed by a slower evolution. The latest photometric data point, r~18.7 mag, was taken on 2025-05-27 04:07 UT. Only r-band data points are available, so we do not have color information. The rapidly fading transient AT2025mgj is spatially and temporally consistent with the “likely short” gamma-ray burst GRB 250521C detected by the Fermi GBM instrument on 2025-05-21 13:31:01.20 UT (Fermi GBM team, GCN Circular 40515). The GRB location was reported with a statistical uncertainty of 16.05 deg, the angular separation between AT2025mgj and the center of the GBM localization is 11.05 deg. Archival ZTF and Legacy Survey images do not reveal high S/N detections. A faint (r=24.21 mag) source may be present near the transient location in Legacy Survey DR10, which is classified as a likely galaxy according to Tractor modeling, although classification is challenging at such faint magnitudes. We caution that, with the data in hand, we cannot exclude that AT2025mgj is a Galactic source. The transient was found at a relatively low Galactic latitude (l, b = 161.329994, 21.4 deg) and the light curve behavior may resemble a CV. Follow-up observations are strongly encouraged to determine the nature of this optical transient and its possible association with GRB 250521C. 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, and OKC, University of Stockholm, Sweden. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA. View this GCN Circular online at https://gcn.nasa.gov/circulars/40566. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...