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

We present JWST observations of the Type Ia supernova (SN Ia) 2025rbs (D = 14.5 Mpc) at +1, +23, and +84 days after B-band maximum, spanning peak light through a wavelength-dependent transition toward the nebular phase. Combined with ground-based optical and near-infrared (NIR) data, our panchromatic spectra (0.4–14 µm) include the first maximum-light mid-infrared (MIR) spectrum and the earliest MIR spectroscopic sequence of an SN Ia to date. At peak light, the MIR spectrum exhibits 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, whereas the optical/NIR spectra remain transitional. The nebular spectrum reveals strongly stratified ejecta, with stable Ni concentrated at the lowest velocities, radioactive Co at intermediate velocities but absent within ∼ 2000 km s−1, and Ar occupying an outer shell. We detect small-scale substructure in [Ca IV] 3.21 µm with fractional amplitudes of a few percent and a characteristic velocity scale of ∼ 800 km s−1, which may reflect compositional structure, ionization variations, or both. Radiative-transfer calculations substantially underpredict these MIR Mg II features despite approximately reproducing the NIR Mg II 1.0927 µm line, suggesting that the relative strengths of these transitions are sensitive to the treatment of Mg ionization and excitation. These observations demonstrate that MIR spectroscopy beginning near maximum light simultaneously probes the emerging inner ejecta and rapidly fading outer burning products, providing new constraints for explosion and radiative-transfer models.

Improvements to AI Systems:

1. Multiwavelength Spectral Synthesis for Supernova Modeling

• Improve radiative-transfer AI models (e.g., neural emulators for TARDIS or ARTIS) to jointly fit optical, NIR, and MIR spectra across multiple epochs.

• The improved system can self-consistently predict line strengths (e.g., Mg II 1.0927 µm vs. MIR Mg II features) by learning ionization/excitation corrections from observed mismatches, reducing systematic errors in abundance and density inversions.

2. Time-Dependent Ejecta Stratification Inference

• Develop an AI that uses sequential spectra (peak → +23d → +84d) to reconstruct 3D elemental stratification (e.g., stable Ni at low velocity, Co at intermediate, Ar outer) without assuming spherical symmetry.

• The improved system can output velocity-resolved abundance maps and detect mixing or clumping from small-scale line substructure (e.g., [Ca IV] 3.21 µm with 800 km/s features), enabling automated identification of turbulent or compositional inhomogeneities.

3. Early-Phase MIR Line Identification and Classification

• Train a transformer-based classifier on synthetic and observed MIR spectra to distinguish permitted vs. forbidden lines, and to flag early-emerging species (e.g., [Ni III–IV], [Ar II–III]) that indicate burning products.

• The improved system can autonomously annotate new SN Ia MIR spectra, providing real-time alerts for unusual ejecta composition or explosion mechanisms.

4. Nebular-Phase Transition Prediction

• Build a predictive model that, given peak-light MIR spectra, forecasts the timing and spectral evolution of the transition to full nebular phase (as seen at +84d) across wavelengths.

• The improved system can estimate explosion parameters (e.g., Ni mass, ejecta kinetic energy) earlier in the supernova’s life, aiding rapid classification and cosmological distance calibration.

5. Substructure Detection and Physical Interpretation

• Enhance AI vision models for spectral line profile analysis to detect fractional amplitude variations (few percent) and characteristic velocity scales (e.g., 800 km/s) in emission lines.

• The improved system can automatically separate Doppler-broadened bulk motion from small-scale clumping or ionization fronts, linking these to explosion hydrodynamics or magnetic field effects.

6. Cross-Instrument Data Fusion

• Create a multimodal AI that fuses ground-based optical/NIR and JWST MIR data (0.4–14 µm) with different resolutions and epochs, handling missing or sparse sampling.

• The improved system can generate a complete, time-resolved panchromatic spectral sequence, enabling robust comparisons with explosion models and reducing observational biases in distance measurements.

What the Improved AI System Can Do:

• Automatically produce physically consistent 3D ejecta models from multi-epoch, multiwavelength spectra of SNe Ia.

• Predict MIR line strengths and ionization states from NIR/optical inputs, reducing the need for expensive MIR observations.

• Detect subtle compositional or dynamical substructure in unresolved lines, offering new probes of explosion asymmetries.

• Provide early, accurate estimates of Ni mass and ejecta geometry, improving SN Ia cosmology and progenitor channel discrimination.

We present JWST observations of the Type Ia supernova (SN Ia) 2025rbs (D= 14.5 Mpc) at +1, +23, and +84 days after B-band maximum, spanning peak light through a wavelength-dependent transition toward the nebular phase. Combined with ground-based optical and near-infrared (NIR) data, our panchromatic spectra (0.4-14 mu m) include the first maximum-light mid-infrared (MIR) spectrum and the earliest MIR spectroscopic sequence of an SN Ia to date. At peak light, the MIR spectrum exhibits 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, whereas the optical/NIR spectra remain transitional. The nebular spectrum reveals strongly stratified ejecta, with stable Ni concentrated at the lowest velocities, radioactive Co at intermediate velocities but absent within 2000 km s-1, and Ar occupying an outer shell. We detect small-scale substructure in [Ca IV] 3.21 mu m with fractional amplitudes of a few percent and a characteristic velocity scale of 800 km s-1, which may reflect compositional structure, ionization variations, or both. Radiative-transfer calculations substantially underpredict these MIR Mg II features despite approximately reproducing the NIR Mg II 1.0927 mu m line, suggesting that the relative strengths of these transitions are sensitive to the treatment of Mg ionization and excitation. These observations demonstrate that MIR spectroscopy beginning near maximum light simultaneously probes the emerging inner ejecta and rapidly fading outer burning products, providing new constraints for explosion and radiative-transfer models.