Known as superluminous supernovae, these events are typically 10 to 100 times brighter than a normal supernova, but much rarer. We’ve seen around 100 so far, but many aspects of these events remain elusive.
Why are they so much brighter than normal supernovae, for example, and what stars cause them ? Astronomers hope to answer these and more questions in the coming years, with several studies underway to understand these events like never before.
Training
Dr Ragnhild Lunnan, from Stockholm University, Sweden, is one of the co-investigators of the SUPERS project, which is trying to determine which stars lead to the formation of superluminous supernovae. With dozens already found, the team is building the largest collection of these events in an effort to learn more about them.
“By following the evolution of these supernovae at a very late stage, you can decode their (structure),” he said. “This tells you things about the star that exploded, and possibly how it exploded.”
To find these explosions, Lunnan and his team are using a camera called the Zwicky Transient Facility (ZTF) , part of the Palomar Observatory in California, USA, to examine the sky. Only one supernova is expected per galaxy per century, and only one in 1,000 or even one in 10,000 of those that are superluminous. But by observing many galaxies simultaneously with the ZTF, it is possible to detect these events.
Superluminous supernovae are found more often in star-forming galaxies than in older galaxies, which means they are likely explosions of young stars, Lunnan notes.
“Also, very often you find them in galaxies that are chemically primitive, called low metallicity, and we think this is a clue as well,” he said. ‘We think they are associated with very massive and metal-poor stars. But beyond that, we really don’t know. “
In 2018, Lunnan and his team discovered a super-luminous supernova with a giant shell of material around it, which it must have expelled in the last years of its short life. “That discovery (of the shell) is another clue that the stars must be very massive,” Lunnan clarifies.
The exact process that causes a superluminous supernova is another matter . Stars can generally become supernovae by collapsing independently or by sharing material with a small dense star known as a white dwarf before an explosion occurs, known as a Type 1a supernova. But what happens in a super-bright event?
Dr. Avishay Gal-Yam from the Weizmann Institute of Sciences in Israel, coordinator of the Fireworks project, has been trying to answer this question. The project has been using observations of the night sky from cameras like the ZTF that have a fast cadence, meaning they show an event shortly after it occurred, to study cosmic explosions.
We previously only saw supernovae about two weeks after they occurred, but ZTF’s constant observations of the sky allow us to see them in about a day or two. And that’s particularly useful for superluminous supernovae. A regular supernova can glow and fade over a period of weeks, but a super-luminous supernova can last several times longer, while also reaching its maximum brightness slower.
“They are evolving relatively slowly,” he said. “The time it takes for the explosion to peak can be a couple of months, sometimes even longer. Therefore, the studies of these objects do not focus on quick observations, but on a continuous monitoring campaign that takes months and sometimes years . “
So far, Gal-Yam and her team have published several studies, examining some of the theories about how these events occur. One idea is that a regular supernova leaves behind a rapidly spinning and highly magnetized neutron star, called a magnetar, which acts like a giant magnet and pumps energy into the supernova explosion.
But Gal-Yam’s most favored theory is the one Lunnan advocates: that massive collapsing stars are the cause. “What can generate so much energy that it can generate such luminous emissions, both in terms of the amount of energy and the amount of time the emission continues to occur?” “The most intriguing (theory) is an explosion from a very massive star 100 times more massive than the sun.”
Distance
While many questions about superluminous supernovae remain unanswered, they are already proving useful as distance markers in the universe. Called “standard candles,” bright events like supernovae can tell us how far away a particular galaxy is, since we know how bright they should be.
“The idea here is a standard candle, an object of known luminosity,” said Mark Sullivan, coordinator of the SPCND project who analyzed how explosive events like this could be useful for cosmological studies. “If you can find it in the sky and measure how bright it appears to be to us on Earth, you can tell how far away it is.”
Original article
This article was originally published in Horizon, the EU Research and Innovation Magazine
