In 2023, a supernova near our planet provided scientists with a great chance to investigate how these explosions accelerate cosmic rays to almost the speed of light. However, the surprising discovery was that NASA’s Fermi Gamma-ray Space Telescope did not detect any of the expected high-energy gamma-ray light from these particles.
On May 18, 2023, a supernova called SN 2023ixf was discovered in the Pinwheel galaxy, located about 22 million light-years away in the constellation Ursa Major. This supernova is the most luminous one found nearby since 2008.
A researcher at the University of Trieste in Italy, Guillem Martí-Devesa, stated, “Astrophysicists previously estimated that supernovae convert about 10% of their total energy into cosmic ray acceleration. But we have never observed this process directly. With the new observations of SN 2023ixf, our calculations should show the energy these particles should produce.”The energy conversion is as low as 1% within a few days after the explosion. This does not eliminate supernovae as cosmic ray factories, but it does indicate that we still have more to learn about their production. The paper, led by Martí-Devesa while at the University of Innsbruck in Austria, will be published in a future edition of Astronomy and Astrophysics. Trillions of trillions of cosmic rays collide with Earth’s atmosphere every day, with roughly 90% of them being hydrogen nuclei, or protons, and the rest being electrons or the nuclei of heavier elements. Scientists have been studying the origins of cosmic rays since the early 1900s.The origin of cosmic rays is difficult to trace as the particles cannot be traced back to their sources. This is because cosmic rays are electrically charged and change direction as they travel to Earth due to encountering magnetic fields. According to Elizabeth Hays, the Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, gamma rays travel directly to Earth. Hays explains that cosmic rays produce gamma rays when they interact with the matter in their environment. The Fermi telescope is the most sensitive gamma-ray telescope in orbit, and when it does not detect an expected signal, scientists are tasked with explaining the absence. Solving this mystery will help in constructing a more accurate understanding of the origins of cosmic rays.
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Astrophysicists have long suspected supernovae as major contributors to cosmic rays.
These explosions occur when a star at least eight times the Sun’s mass runs out of fuel. The core collapses and then rebounds, sending a shock wave outward through the star. This shock wave accelerates particles, creating cosmic rays. When these cosmic rays collide with other matter and light surrounding the star, they produce gamma rays.
Supernovae have a significant impact on a galaxy’s interstellar environment. Their blast waves and expanding cloud of debris can last for more than 50,000 years. In 2013, Fermi discoveredThe measurements indicated that supernova remnants within the Milky Way galaxy were speeding up cosmic rays, resulting in the production of gamma-ray light upon impact with interstellar matter. However, astronomers argue that the remnants are not generating enough high-energy particles to align with the measurements conducted on Earth.
One hypothesis suggests that supernovae may accelerate the most powerful cosmic rays in our galaxy during the initial explosion’s first days and weeks. However, supernovae are infrequent, only occurring a few times per century in a galaxy like the Milky Way. On average, a supernova occurs within distances of approximately 32 million light-years.Once a year.
After one month of observations, during which visible light telescopes first spotted SN 2023ixf, Fermi had not detected any gamma rays.
“Not detecting gamma rays unfortunately does not necessarily mean that there are no cosmic rays,” explained co-author Matthieu Renaud, an astrophysicist at the Montpellier Universe and Particles Laboratory, part of the National Center for Scientific Research in France. “We need to investigate all the underlying assumptions about acceleration mechanisms and environmental conditions in order to interpret the absence of gamma rays as an upper limit for cosmic ray production.”
The researchers put forward several potential scenarios.ios that might have impacted Fermi’s ability to detect gamma rays from the event, such as how the explosion scattered debris and the density of material surrounding the star. Fermi’s observations offer a unique chance to examine the conditions immediately following the supernova explosion. Further observations of SN 2023ixf at different wavelengths, new simulations and models based on this event, and future research on other young supernovae will help astronomers identify the mysterious sources of the universe’s cosmic rays.
Journal Reference: G. Martí-Devesa, C. Cheung, N. Di LEarly-time gamma-ray constraints on cosmic-ray acceleration in the core-collapse SN 2023ixf with the Fermi Large Area Telescope by M. Renaud, G. Principe, N. Omodei, F. Acero was published in the journal Astronomy & Astrophysics in 2024. The article can be accessed using the DOI: 10.1051/0004-6361/202349061.