SN 2024vjm: Faint Type Iax Supernova

• The paper presents a detailed multi-wavelength analysis of SN 2024vjm, highlighting its extremely low peak luminosity and slow photometric decline compared to typical SNe Iax.
• Using deep pre-explosion imaging and spectroscopic data, the study constrains the progenitor system to low-mass helium donors or double-degenerate scenarios, excluding massive stars.
• The findings challenge current explosion models by evidencing weak deflagration signatures and an 'inverse Phillips relation,' prompting the need for revised theoretical frameworks.

SN 2024vjm is an exceptionally faint member of the Type Iax subclass of thermonuclear supernovae, located in NGC 6744. Distinguished by its unusually low luminosity, slow photometric evolution, and spectroscopic signatures of a weak deflagration, SN 2024vjm presents an empirical and theoretical boundary for the diversity of explosion mechanisms and progenitor systems of Type Iax supernovae (Zimmerman et al., 9 Feb 2026, Kwok et al., 5 May 2025).

1. Discovery and Pre-Explosion Constraints

SN 2024vjm was discovered at a precise location in NGC 6744: α=19h09m25.7876s, δ=–63°50′01.7718″ (J2000), with astrometric precision of 10 mas using Gaia DR3 alignment. Leveraging the early visible-band (VIS) observations with the Euclid space mission, pre-explosion images reached a 5σ point-source depth of $m_{\rm VIS} \approx 26.6$ mag (AB), corresponding to an absolute magnitude limit of $M_{\rm VIS} \gtrsim -3.8$ mag, after accounting for a distance modulus $\mu = 29.86 \pm 0.10$ mag (D=9.39 $\pm$ 0.43 Mpc) and extinction correction $A_{\rm VIS} \approx 0.8$ mag. Forced PSF photometry at the SN site was used to conservatively set these limits, calibrated with a zeropoint offset $<0.1$ mag against local standards (Zimmerman et al., 9 Feb 2026).

Pre-explosion photometric limits tightly constrain any luminous progenitor or companion. Unlike SN Iax 2012Z, for which a luminous blue helium-giant companion was detected pre-explosion, no counterpart appears for SN 2024vjm down to these deep Euclid limits. This rules out massive stars (M > 8 M$_\odot$), luminous supergiants, and He giants (Zimmerman et al., 9 Feb 2026).

2. Progenitor System Constraints

Placement of the absolute magnitude limit on the Hertzsprung–Russell diagram, compared with evolutionary tracks and observed populations, demonstrates that all canonical single-star or He-stripped binary routes capable of producing $M_{\rm VIS} \lesssim -5$ mag are excluded. Only low-luminosity companion scenarios—specifically hot subdwarf B (sdB) donors ($L \lesssim 10^{2.5} L_\odot$, $T_{\rm eff} \gtrsim 20,000$ K) or double-degenerate (WD–WD) systems—remain compatible with the data. Survivors in Galactic populations and previous faint SN Iax events (e.g., SN 2008ha) all display companions/remnants brighter than the Euclid limit for SN 2024vjm, further reinforcing this constraint (Zimmerman et al., 9 Feb 2026).

A massive star origin is excluded by the combination of faintness in the VIS, absence of a bright pre-explosion source, and evolutionary population considerations. The data therefore strongly support progenitor scenarios involving a near-Chandrasekhar-mass white dwarf accreting from a low-mass helium donor at low rates (%%%%1$m_{\rm VIS} \approx 26.6$1%%%%1), eventually leading to a thin He shell deflagration (Zimmerman et al., 9 Feb 2026).

3. Photometric and Spectroscopic Properties

SN 2024vjm is among the faintest thermonuclear supernovae discovered, with $M_{\rm VIS} \gtrsim -3.8$2 to $M_{\rm VIS} \gtrsim -3.8$3 mag at peak—a full 3 magnitudes fainter than typical SNe Iax, and even below the luminosity of SN 2008ha (Zimmerman et al., 9 Feb 2026, Kwok et al., 5 May 2025). The follow-up campaign included multi-band photometry and panchromatic spectroscopy (optical through MIR) with JWST, Southern African Large Telescope, and ground-based facilities.

Key observed properties:

• Decline rates: $M_{\rm VIS} \gtrsim -3.8$4 mag (B), $M_{\rm VIS} \gtrsim -3.8$5 mag (r) (Zimmerman et al., 9 Feb 2026)
• Photospheric velocities: $M_{\rm VIS} \gtrsim -3.8$6 km s$M_{\rm VIS} \gtrsim -3.8$7, FWHM$M_{\rm VIS} \gtrsim -3.8$8 km s$M_{\rm VIS} \gtrsim -3.8$9 (NIR Co II) (Kwok et al., 5 May 2025)
• Early emergence of nebular (forbidden) MIR lines alongside permitted photospheric lines, indicating rapid transition to optically thin, mixed ejecta.

Forbidden lines ([Mg II], [Ni II], [Ar II], [Ne II], [Co II], [Co III]) all display centrally peaked, symmetric profiles across low-mass, IME, and IGE elements, indicating well-mixed ejecta, a hallmark of pure deflagration. Pure emission from O I 2.76 μm reveals low-velocity, unburned oxygen—direct evidence for incomplete core burning.

An empirical hallmark is the rare photometric behavior: fainter SNe Iax such as 2024vjm decline more slowly, an "inverse Phillips relation" ($\mu = 29.86 \pm 0.10$0) contrary to the classical SNe Ia sequence ($\mu = 29.86 \pm 0.10$1) (Zimmerman et al., 9 Feb 2026).

Quantity	Value (SN 2024vjm)	Reference
$\mu = 29.86 \pm 0.10$2 (peak)	$\mu = 29.86 \pm 0.10$3 to $\mu = 29.86 \pm 0.10$4 mag	(Zimmerman et al., 9 Feb 2026)
$\mu = 29.86 \pm 0.10$5	$\mu = 29.86 \pm 0.10$6 mag	(Zimmerman et al., 9 Feb 2026)
$\mu = 29.86 \pm 0.10$7 (NIR, +12 d)	$\mu = 29.86 \pm 0.10$8 km/s	(Kwok et al., 5 May 2025)
Ejecta velocity (forb. lines)	$\mu = 29.86 \pm 0.10$9 km/s	(Kwok et al., 5 May 2025)

4. Explosion Physics and Modeling

Modeling efforts using 1D radiative transfer codes (CMFGEN) and 3D hydrodynamical deflagration sequences from Fink et al. (2014) and Kromer et al. (2015) have not yet reproduced the faint luminosity and slow decline simultaneously. The N1def model ($\pm$0, $\pm$1) is too luminous, fades too fast, and underpredicts O I and [Mg II] strengths. The N5def hybrid C/O/Ne model ($\pm$2, $\pm$3) matches the Ni yield for SN 2024vjm but fails to match the slow evolution and observed IR continuum (Kwok et al., 5 May 2025).

Analysis of the bolometric tail yields:

• $\pm$4 (Zimmerman et al., 9 Feb 2026), or $\pm$5 (Arnett’s rule) (Kwok et al., 5 May 2025)
• $\pm$6-ray escape time $\pm$7–$\pm$8 days
• Ejecta mass $\pm$9–$A_{\rm VIS} \approx 0.8$0, depending on the estimator and assumed velocities

A weak deflagration is favored, where the flame is insufficient to disrupt the entire WD, leaving a bound remnant that likely traps radioactive energy and produces a prolonged IR continuum. The presence of [Mg II], [Ne II], [O I], and centrally peaked kinematics are direct diagnostics of this mixing.

A plausible implication is that future modeling must include bound remnant contributions to yield the IR continuum and stretch the photometric timescales as required to match SN 2024vjm. The explosion likely involved a very small number of ignition spots ($A_{\rm VIS} \approx 0.8$110), producing extremely mixed, low-mass, and low-energy ejecta (Kwok et al., 5 May 2025).

5. Implications for the Type Iax Population and Cosmology

SN 2024vjm extends the observed diversity of SNe Iax and constrains the lower boundary of thermonuclear SN luminosities. Contrary to Chandrasekhar-mass SNe Ia, whose peak brightness and decline rate are tightly anti-correlated (Phillips relation), the faintest SNe Iax (including 2024vjm) exhibit slower fading when fainter. The "inverse Phillips relation" ($A_{\rm VIS} \approx 0.8$2 in $A_{\rm VIS} \approx 0.8$3 vs. $A_{\rm VIS} \approx 0.8$4) is statistically robust for this subclass ($A_{\rm VIS} \approx 0.8$5, $A_{\rm VIS} \approx 0.8$6) (Zimmerman et al., 9 Feb 2026).

From a progenitor perspective, luminous blue or red giants, He giants, and single massive stars are excluded. The accretion rates and companion luminosities require a near-Chandrasekhar WD with a low-mass sdB donor or a double-WD system. Observationally, SN 2024vjm and comparable events now define the most extreme end of weak deflagration explosions, challenging both explosion theory and population synthesis (Zimmerman et al., 9 Feb 2026, Kwok et al., 5 May 2025).

6. Future Prospects and Open Questions

Outstanding theoretical challenges include reproducing the faintness, slow fading, and emission line strengths in synthetic models. The need for models incorporating bound remnant heating is clear, and further detailed panchromatic studies of similar events are expected to provide more stringent constraints. The continued absence of observed massive or luminous companions for the faintest Iax events implies that direct detection of progenitors will remain rare except in the nearest galaxies or with future ultra-deep imaging campaigns.

A plausible implication is that SN 2024vjm and similar events will contribute to refining the classification, progenitor mapping, and explosion mechanisms of thermonuclear SNe, and may inform the lower-mass, low-accretion parameter space of binary stellar evolution.

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Key References:

• (Zimmerman et al., 9 Feb 2026) Zimmerman et al. "A Faint Progenitor System for the Faint Supernova 2024vjm"
• (Kwok et al., 5 May 2025) "JWST and Ground-based Observations of the Type Iax Supernovae SN 2024pxl and SN 2024vjm: Evidence for Weak Deflagration Explosions"