AstroNote 2026-51

SN 2025ajmz (ZTF25acjbjpa) was discovered on 2025-12-29 (2025TNSTR5215....1S) in the nearby spiral galaxy NGC 3982, located at a distance of 15.6 Mpc (Wright 2006), at a projected separation of ~0.4 arcmin (~1.8 kpc at the distance of NGC 3982) from the host's nucleus. We obtained spectroscopic observations using the Next Generation Palomar Spectrograph (NGPS) on the Palomar 200-in Hale Telescope beginning at 2025-12-29 09:18:16 UTC. The spectrum shows Balmer series emission lines superimposed on a blue continuum, leading us to classify this as a Type IIn SN (2026TNSCR..14....1S). Additional spectroscopic follow-up observations showed no significant change in the electron-scattering broadened emission line profiles. However, higher-resolution spectra show a narrow P-Cygni profile with an absorption velocity of ~100 km/s superimposed on the H-alpha IIn-like profile. 

The transient exhibits an unusual evolution compared to typical Type IIn events: it has brightened by approximately 1 mag over 100 days while maintaining a relatively red colour (g − r ≈ 0.5). ZTF forced photometry reveals intermittent detections spanning the past ~6 years with absolute magnitudes between −9 and −11 in the r band. The overall spectral appearance, luminosity, and light-curve evolution are consistent with both LBV-like eruptions (Smith et al. 2011) and pre-SN eruptions seen before Type IIn SNe (Pastorello et al. 2013, N. L. Strotjohann et al. 2021). 

Continued monitoring is encouraged.