by Examining a nearby supernova with NASA’s Fermi gamma-ray space telescope, an effort to discover how these massive explosions ignite charged particles called cosmic rays, scientists have uncovered a bigger mystery.
The team found that the supernova, named SN 2023ixf, completely lacked 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 extremes. Scientists have long believed that they are cosmic ray factories, pumping out gamma rays in massive 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 galaxy Messier 101(M101), known as the “pinwheel galaxy,” which located around him. 21 million light years from Earth. SN 2023ixf is the brightest relatively close-to-Earth supernova seen by Fermi since the telescope began searching for these events in 2008 as the cause of the death and collapse of a massive star estimated to be about 12 times more massive more than the falling sun.
However, this powerful event lacks a key ingredient. That is very strange.
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“Astrophysicists previously estimated that supernovas change about 10% of their total energy in the acceleration of cosmic rays,” University of Trieste team member and 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 lead to an energy conversion as low as 1% within a few days of the explosion.
“This does not rule out supernovae as cosmic ray factories, but it does mean that we have more to learn about their production.”
A mysterious cosmic ray factory
Trillions of cosmic rays slam into the Earth’s atmosphere every day, and around 90% of these charged particles are hydrogen atomic nuclei; the rest are free electrons or the nuclei of the heavier elements.
However, investigating the source of cosmic rays has been difficult. That is because, as these charged particles travel millions of light years to reach Earth, they encounter many magnetic fields that redirect them. This endless bouncing means that it is almost impossible to reconstruct the trajectory of cosmic rays. High-energy photons or gamma rays do not experience such deflection and, therefore, can be used as tracers of cosmic ray production.
“Gamma, however, travels directly to us,” Elizabeth Hays, the Fermi project scientist at NASA 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 find an expected signal, scientists need to explain the absence .
“Solving that mystery will lead to a more accurate picture of the origin of cosmic rays.”
Supernovas occur when stars about eight times as massive as the sun run out of fuel needed for nuclear fusion at their cores. This also eliminates the outflow of energy that was providing radiation pressure to support a star against its own gravity.
As this cosmic tug of war that has been raging for millions of years comes to an end, with gravity as the clear winner, the core of the star collapses. The layers out there are blown away in a supernova explosion.
This punching material creates a shock wave that travels away from the dying star, slamming into the surrounding gas and dust and accelerating particles as well as creating cosmic rays. These shock waves can last as long as 50,000 years, and they affect interstellar matter during that time. And when the cosmic rays interact in particular with interstellar gas and dust, they create gamma-ray photons.
In 2013, Fermi discovered that this phenomenon was occurring around the remnants of a supernova in our own galaxy, the Milky Way. This discovery showed that these supernova remnants are not producing enough high-energy particles to match the measurements of scientists on Earth. One reason for this could be that supernovas only accelerate particles to create the most energetic cosmic rays in the first days after the star that sends them collapses.
This picture was further complicated when Fermi watched SN 2023ixf for months after the supernova was first observed by other telescopes in visible light. Despite Fermi’s observations coming immediately after the supernova explosion, NASA’s space telescope did not see any gamma rays from SN 2023ixf.
The team has several possible explanations for why this supernova might be producing cosmic rays but not gas 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 are not streaming towards Earth, so Fermi cannot detect them. Another possibility is that the debris around this supernova could be absorbing any jets being produced.
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Astronomers will continue to study SN 2023ixf in other wavelengths of light as well as create computer models to find out what causes its odd appearance,
“Unfortunately, not seeing any cosmic rays does not mean that there are no cosmic rays,” said Matthieu Renaud, team member and astrophysicist at the Montpellier Universe and Particles Laboratory in the statement. “We need to go through all the basic hypotheses regarding acceleration mechanisms and environmental conditions to convert the absence of gamma rays into an upper limit for the production of cosmic rays.”
The team’s research has been accepted for publication in the journal Astronomy and Astrophysics.
