SN 2024bfu, SN 2025qe, and the early light curves of type Iax supernovae

TL;DR

This study offers some of the earliest optical spectroscopic and photometric data for two Type Iax SNe, SN 2024bfu and SN 2025qe, using multi-survey coverage to tightly constrain the time of first light and the shape of the early light curve. By applying a unified early-rise model across bands, the authors derive $t_0$, rise times, and rise indices, finding systematically shorter rise times and shallower rises in SNe Iax compared to normal SNe Ia, particularly in blue bands. Spectroscopy of SN 2025qe reveals persistent carbon features (C II/C III) from the earliest epochs through maximum light, implying unburned material throughout the ejecta and supporting a highly mixed structure compatible with pure deflagration scenarios. Across a sample of 14 SNe Iax and 87 SNe Ia, the results favor significant mixing and a broader diversity in early-time behavior for SNe Iax, while highlighting the need for larger, uniformly observed samples to quantify the statistical significance of the identified trends and to explore possible alternative explosion channels such as mergers.

Abstract

Type Iax supernovae (SNe Iax) are one of the most common subclasses of thermonuclear supernova and yet their sample size, particularly those observed shortly after explosion, remains relatively small. In this paper we present photometric and spectroscopic observations of two SNe Iax discovered shortly after explosion, SN 2024bfu and SN 2025qe. Both SNe were observed by multiple all-sky surveys, enabling tight constraints on the moment of first light and the shape of the early light curve. Our observations of SN 2025qe begin <2d after the estimated time of first light and represent some of the earliest observations of any SN Iax. Spectra show features consistent with carbon absorption throughout the evolution of SN 2025qe, potentially indicating the presence of unburned material throughout the ejecta. We gather a sample of SNe Iax observed by ATLAS, GOTO, and ZTF shortly after explosion and measure their rise times and early light curve power-law rise indices. We compare our results to a sample of normal SNe Ia and find indications that SNe Iax show systematically shorter rise times, consistent with previous work. We also find some indication that SNe Iax show systematically lower rise indices than normal SNe Ia. The low rise indices observed among SNe Iax are qualitatively consistent with extended $^{56}$Ni distributions and more thoroughly-mixed ejecta compared to normal SNe Ia, similar to predictions from pure deflagration explosions.

Figures (20)

• Figure 1: Stacked $giz$-band Pan-STARRS images of SN 2024bfu (Panel a; $z = 0.036$) and SN 2025qe (Panel b; $z = 0.007$). The locations of both SNe are marked by crosshairs.
• Figure 2: Light curves of SN 2024bfu (Panel a) and SN 2025qe (Panel b). Fits to the observations using Eqn. \ref{['eq:rise']} are shown as coloured lines based on randomly sampling the posterior distributions. Observations included in the fits are shown as filled points, while those not included are shown unfilled. Offsets are applied to each filter for clarity and are given by horizontal lines. Observations from different facilities are denoted by different symbols. The epochs of discovery (dashed) and first light (dotted) are marked as vertical lines. Epochs of spectroscopy are denoted by 'S'. Phases are given relative to the GOTO $L$-band maximum.
• Figure 3: Spectra of SN 2024bfu (Panel a) and SN 2025qe (Panel b) are shown in black. Comparison spectra of SN 2005hk and SN 2019muj are shown in red. Phases of SN 2024bfu and SN 2025qe are given relative to GOTO $L$-band maximum, while SN 2005hk and SN 2019muj are given relative to $V$-band. Spectra have been corrected for Milky Way and host galaxy extinction, where appropriate, and are vertically offset for clarity. Features discussed in the text are marked by vertical lines.
• Figure 4: Zoom-in of the region surrounding Ciii $\lambda$4 647 (left) and Cii $\lambda$6 580 & $\lambda$7234 (right) for SN 2025qe (black) and SN 2019muj (red). Shaded regions indicate velocity offsets in steps of 2 000 km s$^{-1}$.
• Figure 5: Peak absolute magnitudes and rise times for normal SNe Ia (squares) and SNe Iax (circles). Predictions from pure deflagration models fink-2014kromer-15lach--22--def are shown as triangles, while predictions from pulsationally assisted gravitationally confined detonations are shown as pentagons lach--22--pdd. Each band is given by a different colour. Cumulative-density histograms for peak absolute magnitude and rise time are shown in the top and right-hand side panels, respectively. Within these panels, normal SNe Ia are given by a dashed line, while SNe Iax are given by a solid line. Distributions are shown based on randomly sampling the SNe within each band and the posteriors of our UltraNest fits 10 000 times, with prominent lines showing the median and shaded regions showing the 1$\sigma$ deviation.