While examining a nearby supernova with NASA's Fermi gamma-ray space telescope, an effort meant to discover how these stellar explosions ignite charged particles called cosmic rays, scientists have uncovered a bigger mystery.
The team found the supernova, designated SN 2023ixf, to be completely lacking 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 supernovas. Scientists have long believed them to be cosmic ray factories, pumping out gamma-rays in immense quantities.
SN 2023ixf is a "new" supernova (at least as we see it here on Earth) that was discovered on May 18, 2023. It's situated in the galaxy Messier 101(M101), 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 to be around 12 times that of the sun, SN 2023ixf is the brightest supernova relatively close to Earth that's been spotted by Fermi since the telescope started searching for these events in 2008.
Yet, this powerful event lacks a key ingredient. That's extremely odd.
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"Astrophysicists previously estimated that supernovas convert about 10% of their total energy into cosmic ray acceleration," team member and University of Trieste researcher Guillem Martí-Devesa said in a statement. "But we have never observed this process directly. With the new observations of SN 2023ixf, our calculations result in energy conversion as low as 1% within a few days after the explosion.
"This doesn’t rule out supernovas as cosmic ray factories, but it does mean we have more to learn about their production.”
Mysterious cosmic ray factories
Trillions of cosmic rays slam into the atmosphere of Earth every day, and around 90% of these charged particles are atomic nuclei of hydrogen; the rest are free electrons or the nuclei of heavier elements.
However, the source of cosmic rays has been difficult to investigate. That is because, as these charged particles travel millions of light-years to reach Earth, they encounter a multitude of magnetic fields that divert them. This endless bouncing around means the trajectory of cosmic rays is nearly impossible to reconstruct. High-energy photons or gamma rays don't experience such deflections and can, therefore, be used as tracers of cosmic ray production.
"Gamma rays, however, 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 when it doesn’t detect an expected signal, scientists must explain the absence.
"Solving that mystery will build a more accurate picture of cosmic ray origins."
Supernovas happen when stars around eight times as massive as the sun run out of fuel needed for nuclear fusion at their cores. This also ends the outflow of energy that has been providing radiation pressure to support a star against its own gravity.
As this cosmic tug of war that has raged for millions of years ends, with gravity as the clear victor, the core of the star collapses. The outer layers are then blasted outward in a supernova explosion.
This punched-off material causes a shockwave that races out from the dying star, slamming into surrounding gas and dust and accelerating particles as well as creating cosmic rays. These shockwaves can last for as long as 50,000 years, influencing interstellar matter during that time. And when the cosmic rays in particular interact with interstellar gas and dust, they create gamma-ray photons.
In 2013, Fermi discovered this phenomenon happening around supernova remnants in our own galaxy, the Milky Way. This discovery revealed that these supernova remnants aren't creating enough high-energy particles to match scientists' measurements 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 was complicated further when Fermi looked at SN 2023ixf for months after the supernova was first detected by other telescopes in visible light. Despite the Fermi observations coming right after the supernova explosion, the NASA space telescope still saw no gamma rays from SN 2023ixf.
The team has a few possible explanations as to why this supernova might be producing 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 aren't streaming toward Earth, so Fermi can't find them. Another possibility is the debris around this supernova could be absorbing any gamma rays being produced.
Astronomers will now continue to study SN 2023ixf in other wavelengths of light as well as create computer models to learn what may be causing its odd appearance,
"Unfortunately, seeing no gamma rays doesn’t mean there are no cosmic rays," Matthieu Renaud, team member and an astrophysicist at the Montpellier Universe and Particles Laboratory said in the statement. "We have to go through all the underlying hypotheses regarding acceleration mechanisms and environmental conditions in order 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.
