We propose a reclassification of SN 2026cha (ZTF26aaffxis) as a Type IIb supernova. In TNS Classification Report No. #22971, the spectrum of SN 2026cha taken with ESO-NTT was blue and featureless. In AstroNote #2026-31 (Hinds et al. 2026), ZTF26aaffxis was later classified as a Type Ic-BL supernova at a redshift of z=0.0393, based on spectral fits using SuperFit/NGSF (AstroNote #2022-191; Howell et al. 2005, ApJ, 634, 1190).
We obtained a follow-up spectrum with the Nordic Optical Telescope (NOT) using ALFOSC with Grism 4. The data were reduced using PypeIt (Prochaska et al. 2020), with arc and standard-star observations from the same night. Our spectrum reveals a clear P-Cygni profile of Halpha, with a blue-shifted absorption trough corresponding to at least v ~ 18,500 km/s. This feature is inconsistent with a Type Ic-BL supernova.
We also performed spectral classification using the Python package SNID-SAGE (https://github.com/FiorenSt/SNID-SAGE, F. Stoppa in prep. 2026), which yields matches to well-studied Type IIb Supernova 2011dh.
This spectroscopic and photometric evolution is unusual, further follow-up is encouraged.
| Catalog | Name | Reported RA | Reported DEC | Reported Obj-Type | Reported Redshift | Host Name | Host Redshift | Remarks | TNS RA | TNS DEC | TNS Obj-Type | TNS Redshift |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| TNS | 2026cha [ZTF26aaffxis] | 08:32:33.106 | -26:28:37.90 | SN Ic-BL | 0.0393 | 08:32:33.106 | -26:28:37.90 | SN IIb | 0.0393 |
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