AstroNote 2025-250

ZTF22aakyeps was first reported to the TNS by Rehemtulla et al. (TNS Astronomical Transient Report No. 263777), with a first detection on 2025 July 23.278 (UT) at m_g=19.84 AB mag, which followed a non-detection at m_r=20.14 AB mag on 2025 July 22.436. The transient is located about 0.4” from the nucleus of the host galaxy WISE J174859.45+252945.3. The host has a public DESI DR1 spectrum (target ID: 2843124669874177) that exhibits an early-type galaxy spectrum of an old stellar population and weak nebular emission lines at redshift z=0.0949. The public ZTF light curve of SN 2025sei/ZTF22aakyeps in g band rapidly rose to a peak on 2025 July 30.276 at m_g= 17.17 +/- 0.02 mag, which corresponds to an absolute magnitude of M_g=-20.5 AB mag.

Optical spectra (range 3500-10500 Angs) of SN 2025sei/ZTF22aakyeps were obtained on 2025 Aug 02.204 with the Kast spectrograph on the Shane 3 m telescope at Lick Observatory. After subtraction of the archival DESI spectrum, the Kast spectra reveal a blue continuum with a series of absorption minima between 3800 and 4600 Angs in the rest frame. We identify these with the characteristic strong O II features found in Type I SLSNe at early times (Quimby et al. 2011, Nature, 474, 487), consistent with the high peak luminosity of the transient.

The rapid rise of this supernova is very short compared to known samples of SLSNe (e.g., Lunnan et al. 2018, ApJ, 852, 81; Chen et al. 2023, ApJ, 943, 41; Gomez et al. 2024, MNRAS, 535, 471) and the massive, quiescent, early-type host galaxy is also unusual for SLSNe. 

Further multiwavelength observations have been triggered to better characterize this unusual supernova.

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), Caltech/IPAC, and the University of Washington at Seattle, USA.