AstroNote 2025-347

We report Keck/LRIS spectroscopy of SN 2025anhj, a Type Ibn supernova discovered by the Zwicky Transient Facility (ZTF; ZTF25acipmxl). The classification is based on weak He I P-Cygni features, C II and C III emission lines. The source is blue and still rising, and we encourage further follow-up observations.

SN 2025anhj was discovered at the J2000 coordinates
RA = 00:23:38.496, Dec = −01:27:32.77
on 2025 December 17 at 06:02:01.996 UT (JD = 2461026.7514) by ZTF (2025TNSTR5072....1S). The discovery magnitude was r = 20.18 AB mag. The lightcurve is blue and rising, brightening by ≈ 1.8 mag over 4 days, with a g-r color of -0.31 mag on MJD 60130.2.

SN 2025anhj was flagged by a ZTF fast transient filter (e.g., Ho et al. 2023) based on its rapid early evolution, brightening at a rate of > 1 mag/day during the first few days after discovery. The transient is visually associated with a faint, distant galaxy at a projected separation of 0.013 arc minute, with a photometric redshift of z = 0.164 ± 0.0452, making it unlikely to be a nearby stellar contaminant and a strong candidate for photometric follow-up. Subsequent LT ugriz photometry (PI Perley) confirmed its very blue colors, further motivating prompt spectroscopic observations.

We obtained a 300 s Keck/LRIS long-slit spectrum of the transient at its position on 2025 December 21 (PI Kasliwal), using a 1.0’’ wide slit oriented at the parallactic angle in the low-resolution configuration. The 600/4000 grism and 400/8500 grating were used for the blue and red arms, respectively, providing wavelength coverage of ≈3165–5643 Å on the blue side and ≈5359–10256 Å on the red side, with spectral resolutions of ~5 Å and ~9 Å (R ~ 900). The spectrum exhibits multiple emission features at a common redshift of z ≈ 0.067. We identify weak P-Cygni He I 3889, 4471, 5876, 6678, 7065 emission features with expansion velocities of approximately 835 km s⁻¹, along with prominent carbon emission features, including strong [C II] 6580, 7236 and [C III] 5696, consistent with a Type Ibn supernova.

Given its rising lightcurve, further follow-up observations are strongly encouraged.

Based on observations at the W. M. Keck Observatory, operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community.  We are most fortunate to have the opportunity to conduct observations from this mountain.

Based on observations obtained with the Samuel Oschin Telescope 48-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.