JWST Spectroscopy of Type Ia Supernova 2025rbs from Maximum Light to the Nebular Phase

JWST observed the nearby Type Ia supernova 2025rbs (D = 14.5 Mpc) at +1, +23, and +84 days after B-band maximum, capturing peak light and the transition to the nebular phase. Combined with ground-based optical and near-infrared data, these panchromatic spectra (0.4–14 μm) represent the first maximum-light mid-infrared (MIR) spectrum of an SN Ia and the earliest MIR spectroscopic sequence ever obtained.

At peak light, the MIR spectrum shows a continuum with permitted and forbidden features including Si II, Ni II, and early-emerging [Ni III–IV] and [Ar II–III]. By +23 days, the MIR is dominated by forbidden lines with a weak continuum, and by +84 days it is fully nebular, while the optical/NIR spectra remain transitional.

The nebular spectrum reveals strongly stratified ejecta: stable Ni is concentrated at the lowest velocities, radioactive Co is present at intermediate velocities but absent within ~2000 km/s, and Ar occupies an outer shell. Small-scale substructure is detected in [Ca IV] 3.21 μm with fractional amplitudes of a few percent and a characteristic velocity scale of ~800 km/s, likely reflecting compositional structure or ionization variations.

Radiative-transfer calculations substantially underpredict the MIR Mg II features despite approximately reproducing the NIR Mg II 1.0927 μm line, indicating that the relative strengths of these transitions are sensitive to the treatment of Mg ionization and excitation. These observations demonstrate that MIR spectroscopy starting near maximum light simultaneously probes the emerging inner ejecta and rapidly fading outer burning products, providing new constraints for explosion and radiative-transfer models.