SN 2025qe: A case study of diversity in low-luminosity thermonuclear explosions

Abstract

We present optical photometric and spectroscopic observations of SN 2025qe, a Type Iax supernova in IC 0529, monitored from $-10$ to $+150$ rest-frame days relative to $g'$-band maximum using the GROWTH-India and the Himalayan Chandra Telescopes. SN 2025qe reaches a peak $g'$-band absolute magnitude of $M_{g'} \approx -16.16 \pm 0.15$ mag, placing it in the sparsely sampled luminosity gap between the brightest and faintest members of the SNe Iax class. The multiband light curves show pronounced wavelength-dependent broadening at late times, with the redder bands remaining brighter than in other SNe Iax. Semi-analytical (Arnett) modeling of the pseudobolometric light curve yields a synthesized $^{56}$Ni mass of $0.02\,M_\odot$, an ejecta mass of $0.45\,M_\odot$, and a kinetic energy of $7.0\times 10^{49}$ erg. Three-dimensional pure-deflagration models of Chandrasekhar-mass carbon-oxygen white dwarfs reproduce the peak luminosity and $^{56}$Ni yield but underpredict the slow, red post-maximum decline; models including energy input from a bound remnant better match the late-time evolution, especially in the red bands. We model early-time spectra using TARDIS with a modified deflagration model and identify late-time spectral features using syn++. We qualitatively try to explain the persistent red-band flux with a two-component ejecta structure comprising a dense inner region, possibly including a bound remnant, and a density-enhanced outer shell that sustains redder emission. Together, the photometric and spectroscopic properties of SN 2025qe highlight the diversity of ejecta structure and evolution among SNe Iax with similar peak luminosities, emphasizing the need for detailed observations and modeling of their ejecta.