Double "acct": a distinct double-peaked supernova matching pulsational pair-instability models

The Big Picture

Imagine watching a firework shoot into the sky, burst brilliantly, then go nearly dark, only to explode again two months later, even more spectacularly. That’s what astronomers witnessed in late 2020 when they trained their telescopes on NGC 2981, a galaxy about 500 million light-years away. A star had apparently died twice.

The event, catalogued as SN 2020acct, didn’t match any known template for stellar explosions. Its light curve showed two distinct peaks separated by 58 days, with the star fading to just one-twentieth of its peak brightness in between. Something genuinely strange was going on.

An international team gathered observations from ultraviolet through infrared and ran detailed models to make their case: SN 2020acct may be the strongest example yet of pulsational pair instability, a rare mechanism that causes certain massive stars to tear themselves apart in violent pulses before finally dying.

Key Insight: SN 2020acct’s double-peaked light curve, unlike any previously known supernova, matches theoretical models of pulsational pair instability, where a star with a roughly 72 solar-mass helium core ejects shells of material in violent pulses before its terminal explosion.

How It Works

Very massive stars can end their lives with helium cores weighing more than about 45 times the mass of the Sun. At that point, temperatures inside climb so high that energetic photons spontaneously convert into electron-positron pairs, matched particles of matter and antimatter. The process bleeds energy from the star’s interior, robbing it of the pressure holding it up and triggering a catastrophic collapse.

For stars in a certain mass range, that collapse doesn’t kill the star immediately. Explosive oxygen burning reverses the implosion and blasts the outer layers into space. The star survives. For now. This is the pulsational pair instability (PPI) mechanism.

Ejected shells hurtle outward at thousands of kilometers per second. When the terminal explosion follows, fresh ejecta slams into those earlier shells, converting kinetic energy into light. That collision produced the bright initial flash seen as SN 2020acct’s first peak.

The observations fit. The first peak was luminous and markedly blue, with a g − r color of 0.27 ± 0.03 magnitudes. Its spectrum showed clear signatures of interaction with hydrogen-free circumstellar material (CSM), gas and debris shed during the pre-explosion pulses. The second peak arrived 58 days later and looked like a normal stripped-envelope supernova, the kind where the dying star had already lost its outer hydrogen layers. It showed characteristic calcium and oxygen emission lines.

The team ruled out every alternative explanation:

• Two unrelated supernovae coinciding within 58 days in one galaxy? The probability is vanishingly small.
• Binary star system explosions? The required orbital configuration and timing would be implausibly fine-tuned.
• A magnetar or accretion engine powering a rebrightening? The spectral evolution and timing match neither scenario. (A magnetar is an ultra-dense spinning neutron star; an accretion engine draws its power from material falling onto a black hole.)

What did match was a PPI model built around a star with a 72 solar-mass helium core. Hydrodynamic simulations by co-author S. E. Woosley reproduced both the timing and luminosity ratio of the two peaks. The initial pulse ejects a substantial mass of material, and when the terminal supernova follows, that collision lights up as the first, bluer, brighter peak.

One wrinkle remains. The host environment has a metallicity (enrichment in elements heavier than helium) of about 0.4 times solar. Standard models predict that stellar winds in such environments would strip too much mass before pair instability could kick in. The team proposes two workarounds: a late-stage binary merger combining two massive stars shortly before death, or a locally metal-poor pocket in the star-forming region that standard measurements missed.

Why It Matters

Pulsational pair instability has been a theoretical prediction for decades, but confirmed examples are rare and truly unambiguous cases nonexistent. SN 2020acct is now the strongest candidate. PPI probes the very top of the stellar mass function, the most extreme stars the universe produces.

Stars this massive were far more common in the early universe, when galaxies were metal-poor and stellar winds weaker. How they die shapes the chemical enrichment of early galaxies and the mass distribution of black holes that gravitational wave observatories like LIGO can detect.

The measured rate of SN 2020acct-like events (fewer than 3.3 × 10⁻⁸ per cubic megaparsec per year, or less than 0.1% of all core-collapse supernovae) is consistent with theoretical predictions. It also sets a benchmark for the Vera Rubin Observatory’s Legacy Survey of Space and Time (LSST), which will sweep the entire southern sky every few nights and catch thousands of unusual transients per year. Machine learning classifiers trained on events like this one will be critical for picking them out in real time.

Bottom Line: SN 2020acct is the most compelling observational match to pulsational pair instability models found to date. It shows that even stars in modestly metal-rich environments can die in this exotic, multi-stage fashion, and that surveys like LSST will soon test how common or rare the process truly is.