While examining a nearby supernova with NASA’s Fermi gamma-ray space telescope, an effort to discover how these stellar explosions ignite charged particles called cosmic rays, scientists have uncovered a bigger mystery.
The team found that the supernova, named SN 2023ixf, completely lacks the gamma-ray emissions that should be present when cosmic ray particles are accelerated to near-light speeds. This is a discovery that could challenge our understanding of supernovae. Scientists have long thought they were cosmic ray factories, emitting gamma rays in enormous quantities.
SN 2023ixf is a ‘new’ supernova (at least as we see it here on Earth) that was discovered on May 18, 2023. It is located in the Messier 101 (M101) galaxy, also known as the ‘pinwheel galaxy’, located around 21 million light-years from Earth. Caused by the death and collapse of a supergiant star with a mass estimated at 12 times that of the Sun, SN 2023ixf is the brightest relatively close-to-Earth supernova spotted by Fermi since the telescope in 2008 started looking for these events.
Yet this powerful event is missing an important ingredient. That is extremely strange.
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“Astrophysicists previously estimated that supernovae convert about 10% of their total energy into cosmic ray acceleration,” team member and researcher Guillem Martí-Devesa of the University of Trieste said in a statement. “But we have never directly observed this process. With the new observations of SN 2023ixf, our calculations result in an energy conversion of only 1% within a few days after the explosion.
“This doesn’t rule out supernovas as cosmic ray factories, but it does mean we still have more to learn about their production.”
Mysterious cosmic ray factories
Trillions of cosmic rays collide in Earth’s atmosphere every day, and about 90% of these charged particles are hydrogen atoms; the rest are free electrons or the nuclei of heavier elements.
However, the source of cosmic rays is difficult to investigate. That’s because these charged particles, as they travel millions of light years to reach Earth, encounter a host of magnetic fields that redirect them. This endless bouncing back and forth means that the trajectory of cosmic rays is virtually impossible to reconstruct. High-energy photons or gamma rays do not experience such deflections and can therefore be used as tracers of cosmic ray production.
“However, gamma rays travel directly to us,” Elizabeth Hays, the Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in the statement. ‘Cosmic rays produce gamma rays when they interact with matter in their environment. Fermi is the most sensitive gamma-ray telescope in orbit, so if it doesn’t detect an expected signal, scientists must explain its absence.
“Solving that mystery will provide a more accurate picture of the origins of cosmic rays.”
Supernovae occur when stars about eight times as massive as the Sun run out of fuel needed for nuclear fusion in their cores. This also ends the outflow of energy that created radiation pressure to support a star against its own gravity.
As this cosmic tug-of-war that has lasted millions of years comes to an end, with gravity the clear victor, the star’s core collapses. The outer layers are then blown out in a supernova explosion.
This shed material creates a shock wave that shoots out of the dying star and crashes into the surrounding gas and dust, accelerating particles and creating cosmic rays. These shock waves can last for up to 50,000 years, affecting interstellar matter during that time. And when cosmic rays in particular interact with interstellar gas and dust, gamma photons are created.
In 2013, Fermi discovered that this phenomenon occurs around supernova remnants in our own galaxy, the Milky Way. This discovery revealed that these supernova remnants do not create enough high-energy particles to match the measurements made by scientists on Earth. One reason for this may be that supernovas only accelerate particles to create the most energetic cosmic rays during the first days after the star that launches them collapses.
This picture became even more complicated when Fermi watched SN 2023ixf for months after the supernova was first detected in visible light by other telescopes. Despite the Fermi observations just after the supernova explosion, the NASA space telescope still saw no gamma rays from SN 2023ixf.
The team has a few possible explanations for why this supernova produces cosmic rays, but not gamma rays, at least none that Fermi can detect. One theory is that the supernova debris is unevenly distributed and aligned in such a way that the gamma rays don’t flow toward Earth, so Fermi can’t find them. Another possibility is that the debris around this supernova absorbs the gamma rays produced.
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Astronomers will now continue to study SN 2023ixf in other wavelengths of light and create computer models to figure out what could be causing its strange appearance.
“Unfortunately, seeing no gamma rays does not mean there are no cosmic rays,” Matthieu Renaud, team member and astrophysicist at the Montpellier Universe and Particles Laboratory, said in the statement. “We need to work through all the underlying hypotheses regarding acceleration mechanisms and environmental conditions to convert the absence of gamma rays into an upper limit for cosmic ray production.”
The team’s research has been accepted for publication in the journal Astronomy and Astrophysics.