MADRID, 19 (EUROPA PRESS)
The first flashes of a supernova could be analyzed within hours of its occurrence to determine its origin, thanks to a new method of astronomical observation.
Supernovae appear to our eyes—and to astronomical instruments—as bright flashes that appear in the sky without warning, in places where nothing was visible moments before. The flash is caused by the colossal explosion of a star.
Because of their sudden and unpredictable nature, supernovae have long been difficult to study, but today, thanks to extensive, continuous, and frequent surveys of the sky, astronomers can discover new ones almost daily.
However, it is crucial to develop protocols and methods to detect them promptly; only then will we be able to understand the events and celestial bodies that triggered them.
In a pilot study, Lluís Galbany of the Institute of Space Sciences (ICE-CSIC) in Barcelona and his colleagues present a methodology that allows them to obtain the earliest possible supernova spectra, ideally within 48 or even 24 hours of first light. The results have been published in the Journal of Cosmology and Astroparticle Physics.
Supernovae are enormous explosions that mark the final stages of a star's life. They are divided into two broad categories, determined by the mass of the progenitor star. "Thermonuclear supernovae are stars whose initial mass did not exceed eight solar masses," explains Galbany, first author of the study.
"The most advanced evolutionary stage of these stars, before the supernova, is the white dwarf: very old objects that no longer have an active, heat-producing core. White dwarfs can remain in equilibrium for a long time, thanks to a quantum effect called electron degeneracy pressure."
If such a star is in a binary system, he continues, it can absorb matter from its companion. The additional mass increases the internal pressure until the white dwarf explodes as a supernova.
"The second main category is very massive stars, those greater than eight solar masses," says Galbany. They shine thanks to nuclear fusion in their cores, but once the star has burned progressively heavier atoms, to the point where fusion no longer produces energy, the core collapses. At that point, the star collapses because gravity is no longer counteracted; the rapid contraction dramatically increases the internal pressure and triggers the explosion.
The first hours and days after the explosion preserve direct clues about the progenitor system: information that helps distinguish competing explosion patterns, estimate critical parameters, and study the local environment. "The sooner we detect them, the better," Galbany notes.
DAYS OR WEEKS
Historically, obtaining such early data was difficult because most supernovae were discovered days or weeks after the explosion. Modern wide-field, high-speed surveys, which cover large swaths of the sky and revisit them frequently, are changing that perspective and enabling discoveries in just hours or days. Protocols and criteria are still needed to fully exploit these surveys, and Galbany's team tested these rules using observations from the Gran Telescopio de Canarias (GTC). Their study reports on 10 supernovae: half thermonuclear and half core-collapse. Most were observed within six days of the estimated explosion, and in two cases, within 48 hours.
The protocol begins with a rapid candidate search based on two criteria: the bright signal must have been absent in the previous night's images, and the new source must be within a galaxy. When both conditions are met, the team activates the GTC's OSIRIS instrument to obtain a spectrum.
"The supernova spectrum tells us, for example, whether the star contained hydrogen, which means we're dealing with a core-collapse supernova," Galbany explains.
Understanding the supernova in its earliest moments also allows for the exploration of other types of data on the same object, such as photometry from the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS), which is used in the study.
These light curves show how the brightness increases in the initial phase; if small bulges are observed, it could mean that another star in a binary system was absorbed by the explosion. Additional verification is based on data from the same area of the sky from other observatories.
Since this first study managed to collect data in 48 hours, the authors conclude that even faster observations are possible. "What we just published is a pilot study," says Galbany.
