17 supernovae from the ZTF DR2 sample lit up ahead of schedule. In the first hours and days after the explosion, they emitted more light than standard models predict. Astronomers compare this to a Bickford fuse: before the blinding flash tears across the sky, a precursor spark runs through the outer layers.
Supernova type Ia is a thermonuclear explosion of a white dwarf in a binary star system. The white dwarf, a stellar remnant the size of Earth, pulls matter from a companion. When its mass approaches the Chandrasekhar limit (~1.4 solar masses), an uncontrolled nuclear reaction ignites. These explosions serve cosmologists as 'standard candles' for measuring distances. But it is now clear that the canonical scenario does not account for the faint glow that appears in the first moments of detonation.
Astronomers sifted through 3628 spectroscopically confirmed supernovae from the [scientist:Fritz Zwicky]Fritz Zwicky[/scienty] survey. After selection, 1792 objects with sufficient early-stage coverage remained. They were analyzed using four methods, excluding the 'excess window' — the first few days where anomalous light might appear — from the models. Then they compared with models that predict such an excess: double detonation (DD) and companion interaction (CI). Double detonation involves helium ignition on the surface, sending a shock wave inward to detonate the carbon core.
The list of 17 strong candidates is diverse. Six are overluminous supernovae (five of type 91T and one of type 03fg), the rest normal. Notably, stars with excess show a connection to young populations: their host galaxies are on average less massive and bluer. When the sample was restricted to redshift z<0.06, the fraction of flashes with a precursor did not exceed 25% of all Ia. Double detonation models indicate a core mass of about 1 M⊙ and a burning envelope fraction of 20%. A single mechanism has not yet been identified — it seems that the 'fuse' works differently for different supernovae.
The discovered link with young stars supports a scenario where the white dwarf explodes soon after the birth of a massive companion. Accounting for the early excess will allow more precise calibration of standard candles, and thus refine the expansion rate of the universe according to Hubble's law. With new surveys like LSST at the Vera Rubin Observatory, astronomers hope to catch precursors in real time. Then the Bickford fuse will cease to be a metaphor and become a measurable parameter of the explosion.
The brightest excess among the candidates, in supernova ZTF18abxxssh, was spotted less than a day after the last non‑detection. Its early brightness in the first two days exceeded expectations by 10%, as if the star decided to announce itself in advance.
In Arthur C. Clarke's novel 'Rendezvous with Rama', a supernova is used as a beacon for interstellar navigation. An early excess could serve as a more precise trigger for spacefaring civilizations — a sort of warning signal a few days before the blinding flash.