A Faint Progenitor System for the Faint Supernova 2024vjm

TL;DR

SN 2024vjm is an exceptionally faint Type Iax supernova in NGC 6744. Deep pre-explosion Euclid imaging constrains the progenitor to be fainter than the luminous Iax companion candidates, favoring WD progenitor channels with faint companions or double-degenerate WD systems. The bolometric and spectroscopic analyses yield a very low ejecta mass of ~$0.1$ M_sun and a nickel mass of ~${3.7e-3}$ M_sun, with a gamma-ray escape time of ~${83}$ days, producing a slow decline that runs opposite to the Phillips relation. No clear late-time excess from a bound remnant is detected, though infrared data imply dust formation and a shift of the SED to longer wavelengths. Collectively, the study shows that the faint end of SNe Iax arises from intrinsically faint WD progenitors and demonstrates the power of space-based pre-explosion imaging to constrain progenitor channels for thermonuclear transients.

Abstract

Type Ia Supernovae (SNe Ia) are well known for their role as standardizable cosmological candles. Their uniformity is credited to their single origin as thermonuclear explosions of White dwarf (WD) stars. Nevertheless, some SNe Ia break this regularity. Prominently, the Iax subclass are less energetic and remarkably diverse, raising questions about their progenitor systems. While no progenitor system of a normal SN Ia has ever been detected, a luminous blue star was identified in pre-explosion images of the site of the bright SN Iax SN 2012Z, suggested to be a helium giant companion star acting as a mass donor to a WD SN progenitor. This is in line with models of weak mass accretion of a WD from a binary companion, producing an explosion that does not fully disrupt the star. However, these models fail to explain the properties of the faintest Type Iax explosions, suggesting either they originate from other WD binary systems, or even from massive progenitor stars. Here, we present the faint SN Iax SN 2024vjm - possibly the faintest supernova observed to date. Using a deep pre-explosion image taken by the recently launched Euclid space mission, we show that its progenitor system must be fainter than the helium giant SN Iax progenitor candidate of SN 2012Z, as well as that of the luminous red companion or remnant of the faint SN 2008ha, and may require a subdwarf helium star as a mass donor. The deep image also provides strong arguments against a massive star origin for this faint supernova. Our observations argue that SN 2024vjm is a WD explosion, but we find that remarkably faint SNe Iax fade more slowly than bright ones, i.e., they evolve in an opposite manner from the famous Phillips relation that makes regular SNe Ia cosmological candles.

Figures (13)

• Figure 1: No bright stars at the location of SN 2024vjm. a post explosion q-band BG image of SN 2024vjm. SN 2024vjm is clearly seen at a spiral arm in the north-west of NGC 6744 (marked by a crosshair) b, the pre-explosion Euclid VIS-band image of NGC 6744 with the nearest $10"$ to the explosion site marked with a red rectangle. c, A zoom in on the 7.5$"$ by 15$"$ around the explosion site. The tip of the large star-forming region to the north of the explosion site is visible in the cutout. The astrometric location of SN 2024vjm is marked in a 0.1$"$ diameter circle (purple) corresponding to a single Euclid pixel. Sources identified by the Euclid pipeline are marked with 0.2$"$ diameter circles (red). S1, the nearest extended source to the location of SN 2024vjm, is marked in a cyan ellipse marking its extent. SN 2024vjm lies just north of S1's centre and is unlikely to be physically associated with the source.
• Figure 1: SN 2024vjm is surrounded by star-formation. The host redshifted $\rm H\alpha$ slice of the MUSE datacube is shown. SN 2024vjm is clearly visible at the centre and is marked by its name. The edges of two large star-forming regions are seen to the north ($\sim 80$pc) and south-west ($\sim140$ pc) of the SN. However, the SN lies away from large star-forming regions. The emission lines produced by the northern star-forming region are consistent with a solar abundence of metals.
• Figure 1: GP interpolation of the SN lightcurve. We find the interpolation (solid lines) to closely follow the data. Calculated $1\sigma$ confidence intervals are presented as transparent bands around the interpolation.
• Figure 2: The progenitor system of SN 2024vjm is fainter than previous Iax progenitor candidates. A Hertzsprung–Russell diagram showing the Euclid-VIS band brightness vs temperature. Two sets of BPASS stellar tracks are plotted; single-star H-rich (yellow-red shades) and stripped He tracks (green-blue). The terminal position of massive stripped BPASS tracks is also plotted with blue stars (for primary stars) and orange diamonds (secondary stars). The region occupied by the brightness of S1 is marked in translucent red, and the region occupied by the brightness at the SN location itself is marked in translucent grey. We note that the latter is likely contaminated by S1, and is therefore a strict upper limit of a possible progenitor brightness. We mark the region brighter than both with a darker shade of red. The progenitor system of SN 2024vjm is fainter than the inferred companion of SN 2012Z (blue circle), as well as the possible remnant of SN 2008ha (red circle), and also from that of galactic He Nova V445 Pup (green circle). Known Galactic WR stars Sander2019 (pink squares) are mostly excluded, though some are slightly fainter than the limit. Low-mass hot subdwarf-WD binary systems (black; Methods \ref{['sec:progenitor']}) lie well within the permitted brightness range. Such faint systems are possible progenitor systems for faint SNe Iax. Error bars represent $1\sigma$ uncertainties.
• Figure 2: Binning seeing-limited BG images improves the astrometric accuracy. Median astrometric position of identified Gaia sources within BG $i$-band images in single images (blue), four binned images (orange) and sixteen binned images (green). By combining more images the astrometric noise decreases by roughly the square root of the number of images. This demonstrates that the mean astrometric points are largely independent, and therefore, averaging multiple data points can achieve better accuracy. A single Euclid-VIS pixel is marked at $0.1"$ with a striped line. The brightness of SN 2024vjm within the $i$-band images is marked with a shaded grey region.