AstroNote 2025-309

The transient AT 2025aapf/ZTF25abxcnsf was first detected in the ZTF public stream on 2025 Oct 11.5 UT with an apparent magnitude of g=20.1+/-0.4 AB mag and first reported to TNS on 2025 Oct 16 (Sollerman et al.; Transient Report No. 273550). The light curve has slowly risen to an apparent peak near g~19.7 AB mag with persistent blue colors (g-r ~ -0.3 mag). The transient was flagged by the ML classifier tdescore (Stein et al., 2024, ApJL, 965, L14) due to its blue colors and lack of evidence for past variability or AGN activity in the host galaxy. 

We obtained a spectrum of AT 2025aapf on 2025 Oct 23.6 UT using the Low Resolution Imaging Spectrometer on the Keck-I telescope covering the range 3100-10300 Angs. The spectrum reveals a very blue continuum, with some host galaxy emission superposed that is consistent with star light from an early-type galaxy at redshift z=0.180 (e.g., Ca II H+K). No narrow nebular emission lines or broad spectral features from the transient are detected. We therefore classify this transient as a likely TDE-featureless.

Swift follow-up observations have been requested.

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 Grants No. AST-1440341, AST-2034437, and currently Award #2407588. ZTF receives additional funding from the ZTF partnership. Current members include 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.