AstroNote 2026-96

This transient exhibits a long-lived and peculiar lightcurve. It was first detected by ZTF at MJD 60935 with an r-band magnitude of ~20.2. It rapidly rose to ~18.5 mag within 2 days and has remained at approximately this brightness for over 200 days, resembling a Type II SN lightcurve at early phases. The lightcurve shows a couple of low-amplitude fluctuations, varying between r ~18.6 and 18.1 mag.

A spectrum was obtained using SEDM on the Palomar 60-inch telescope (2025-09-25), but no classification was reported at the time due to low SNR. Upon careful re-analysis, the spectrum is consistent with a Type Ia supernova at z = 0.054, showing features broadly consistent with normal SN Ia and some Ia subtypes.

The transient drew renewed attention due to its persistent brightness well beyond typical SN timescales. We obtained a follow-up spectrum with NGPS on the Palomar 200-inch telescope on 2026-04-01. The spectrum reveals strong H-alpha emission with a velocity of ~7500 km s⁻¹, along with prominent Ca II NIR emission. The H-alpha emission is consistent with z = 0.054 and indicates ongoing interaction with H-rich circumstellar material (CSM). The spectral properties resemble Ia-CSM events such as SN 2013dn, but with a significantly longer-lived and flat light curve.

Given its continued brightness and unusual evolution, further spectroscopic and photometric follow-up is strongly encouraged.

NGPS is a new medium-resolution, single-object, long-slit optical spectrograph on the Palomar 200-in Hale Telescope at Palomar Observatory. NGPS replaces the Double Beam Spectrograph (DBSP; Oke & Gunn 1982), the workhorse Palomar 200-in optical spectrograph for four decades. NGPS is designed to be over a factor of three higher throughput than DBSP.  NGPS is a partnership between Caltech, Peking University, the National Astronomical Observatories of the Chinese Academy of Sciences, and Jet Propulsion Laboratory. The NGPS team acknowledges generous support from Caltech, the National Science Foundation (2018866), the Heising Simons Foundation (2020-1865), Strategic Priority Research Program of the Chinese Academy of Sciences (XDB0550100), and the National Natural Science Foundation of China (11927804).