A team of astronomers present a new method to detect supernovae hours after the explosion

Supernovae are enormous explosions that mark the final stages of a star’s life. As they are sudden and unpredictable, they have long been difficult to study, but today, thanks to high-cadence sky surveys, astronomers can discover new ones almost daily. A new study led by the Institute of Space Sciences (ICE-CSIC), with the participation of the Institute of Space Studies of Catalonia (IEEC), and published in the Journal of Cosmology and Astroparticles Physics (JCAP), presents a new method to detect supernovae hours after their eruption.

The pilot study focuses on a sample of ten supernovae using observations from the Gran Telescopio de Canarias (GTC). It shows how targeted protocols and fast telescope follow-up can capture the earliest spectra of these stellar explosions, ideally within 48 hours, or even 24 hours of their first light. This breakthrough offers an unprecedented opportunity to study the moments immediately following a star’s death and makes rapid detection essential for understanding their origins and evolution.

Supernovae fall into two broad categories, determined by the mass of the progenitor star. Thermonuclear supernovae involve stars whose initial mass did not exceed eight solar masses. “The most advanced evolutionary stage of these stars before the supernova is the white dwarf—very old objects that no longer have an active core producing heat. White dwarfs can remain in equilibrium for a long time, supported by a quantum effect called electron-degeneracy pressure”, explains Lluís Galbany, an astrophysicist at the Institute of Space Sciences (ICE-CSIC) and Institut d’Estudis Espacials de Catalunya (IEEC) and first author of the study.

If such a star is located in a binary system, it can siphon matter from its companion. The extra mass raises the internal pressure until the white dwarf explodes as a supernova.

The second supernovae major category involves very massive stars, above eight solar masses, “They shine thanks to nuclear fusion in their cores, but once the star has burned through progressively heavier atoms—right up to the point where further fusion no longer yields energy—the core collapses. At that point, the star collapses because gravity is no longer counterbalanced; the rapid contraction raises the internal pressure dramatically and triggers the explosion”, Galbany explains.

“The supernova’s spectrum tells us, for instance, whether the star contained hydrogen—meaning we are looking at a core-collapse supernova,” Galbany explains. “Knowing about the supernova in its very earliest moments also lets us seek other kinds of data on the same object, such as photometry from the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS) that we used in the study. Those light-curves show how brightness rises in the initial phase; if we see small bumps, it may mean another star in a binary system was swallowed by the explosion”, he adds. Additional checks cross-match data on the same patch of sky from other observatories.

Because this first study managed to gather data within 48 hours, the authors conclude that even faster observations are within reach. “We now know that a rapid-response spectroscopic program, well coordinated with deep photometric surveys, can realistically collect spectra within a day of the explosion, paving the way for systematic studies of the very earliest phases in forthcoming large surveys such as the La Silla Southern Supernova Survey (LS4) and the Legacy Survey of Space and Time (LSST), both in Chile”, Galbany concludes.