AstroNote 2026-236

We report the transformation of supernova precursor candidate AT2026sfninto a type IIn supernova beginning on 07-28-2026. As presented in Astronote 2026-217 from 07-14-2026, AT2026sfn was identified as a low luminosity transient in the ZTF alert stream and spectroscopic follow-up with Gemini-S GMOS and P200 NGPS showed narrow Balmer series emission lines with electron-scattering redward “wings.” Given the similarity to other precursor events such as Event A of SN 2023ldh (Pastorello+ 2025), the previous “Precursor Watch” Astronote prediction was that “the terminal explosion could occur within the next ~60 days.”

On 07-28-2026, AT2026sfn showed a significant light curve rise and bluer g-r colors (see attached light curve plot), prompting additional spectroscopic follow-up with SEDM and NGPS at Palomar Observatory as well as NOT ALFOSC. As shown in the attached spectral time-series plot, the latest spectra of AT2026sfn show a substantially bluer continuum with prominent IIn-like Balmer series Lorentzian line profiles as well as He II + N III emission near 4686 angstroms – all these observables being consistent with the spectra of young type IIn supernovae (comparison plot attached). The current light curve shows that AT2026sfn has risen to M_g ~ -17.5 mag and the supernova will continue to be monitored spectroscopically. 

This AstroNote is 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.