AstroNote 2026-31

We obtained a spectrum of AT2026cha with the Goodman High Throughput Spectrograph on SOAR (Program ID SOAR2026A-018, PI Andreoni) of AT2026cha/ZTF26aaffxis (AstroNote #2026-29; Jayaraman et al., 2026) on MJD 61082.18 (2026-02-11 04:32), ~6 d after initial detection by the Zwicky Transient Facility (ZTF). We used a grating of 400 lines/mm and a 1.0'' wide slit mask in the M1 spectroscopic setup (hereafter 400M1) with 2x2 binning using the GHTS Red Camera. The 400M1 spectra cover a wavelength range of 3800 - 7040 Å. The spectra were reduced using Pypeit (Prochaska 2020), using arcs taken immediately after target observation and calibration images from the same night. Flux calibration was performed using observations of HR 1996 taken on the same night as the observations with an identical 400M1 setup and 2x2 binning.

Compared to the earlier Gemini-S spectrum, the SOAR spectrum shows broad features developing at ~4500Å, 5250Å, and 5950Å. Fits to the SOAR spectrum with SuperFit/NGSF (AstroNote #2022-191; Howell et al., 2005, ApJ, 634, 1190) identify tentative matches to Type Ic-BL SN (2003jd). The light-curve evolution of 2026cha is similar to that of the Ic-BL supernovae SN2018gep (Ho et al., 2019) and iPTF16asu (Whitesides et al., 2016), which also showed a rapid rise and decay with blue peak colors and post-peak reddening. Further observations are planned.