Supernovae: How to spot them at record speed

(Press-News.org) Supernovae appear to our eyes—and to astronomical instruments—as brilliant flashes that flare up in the sky without warning, in places where nothing was visible just moments before. The flash is caused by the colossal explosion of a star. Because supernovae are sudden and unpredictable, they have long been difficult to study, but today, thanks to extensive, continuous, high-cadence sky surveys, astronomers can discover new ones almost daily.

It is crucial, however, to develop protocols and methods that detect them promptly; only in that way can we 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 can obtain the earliest possible spectra of supernovae—ideally within 48 hours, or even 24 hours, of the “first light.” The results have just been published in the Journal of Cosmology and Astroparticle Physics (JCAP).

Supernovae are enormous explosions that mark the final stages of a star’s life. They 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,” 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 core producing heat. White dwarfs can remain in equilibrium for a long time, supported by a quantum effect called electron-degeneracy pressure.”

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

“The second major category involves very massive stars, above eight solar masses,” Galbany says. “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.”

The first hours and days after the blast preserve direct clues to the progenitor system—information that helps distinguish competing explosion models, estimate critical parameters, and study the local environment. “The sooner we see them, the better,” Galbany notes. Historically, obtaining such early data was difficult because most supernovae were discovered days or weeks after the explosion. Modern wide-field, high-cadence surveys—covering large swaths of sky and revisiting them frequently—are changing that picture and allowing discoveries within mere hours or days.

Protocols and criteria are still needed to exploit these surveys fully, and Galbany’s team tested such rules using observations from the Gran Telescopio de Canarias (GTC). Their study reports on ten supernovae: half thermonuclear, 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 search for candidates based on two criteria: the light signal must have been absent in the previous night’s images, and the new source must lie within a galaxy. When both conditions are met, the team triggers the OSIRIS instrument on the GTC to obtain a spectrum.

“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.” 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. “What we have just published is a pilot study,” Galbany says. “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.”


 

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