Surprising lack of cosmic rays in nearby supernovae questions theory

Supernova SN 2023ixf provided astronomers with an unusual opportunity when it exploded last year. Unfortunately, it deepens rather than solves the problem of interpreting cosmic rays. It also casts doubt on supernova models, but hey, if we had all the answers, we wouldn’t need scientists.

We now detect thousands of supernovae each year, but most of them are billions of light-years away, limiting how much we know about them individually. By comparison, SN 2023ixf is 21 million light-years away, just around the sky block.

This gives amateurs the opportunity to collect some stunning images of supernovae and their host galaxy’s windmills. For professional astronomers, this is the best opportunity to study the production of gamma rays from supernovae since the launch of the Fermi Space Telescope in 2008.

Astrophysicists previously estimated that supernovae convert about 10% of their total energy into cosmic ray acceleration, Dr. Guillem Matt-Devesa of the University of Trieste said in a statement. But we’ve never observed this process directly.

Cosmic rays are charged particles (mainly protons) that are accelerated to nearly the speed of light as a result of some astronomical event. They are constantly bombarding the Earth and have important effects on the atmosphere, such as producing the carbon-14 we use to measure the age of artifacts. However, when writers say that they were produced by astronomical events, they are making assumptions based on no obvious alternative explanation. We do not know what events produced them and in what quantities.

It looked like this should be easy to test until we found something dramatic happening and subsequently looked for light. However, there are two problems with this. First, delays can be quite long. Even if an object travels at 99.9% the speed of light, if the distance is large enough, it may lag simultaneously emitted photons by thousands of years. Second, magnetic fields, including those of the Earth, bend the paths of incoming charged particles so we can’t accurately identify the direction they came from.

Fortunately, cosmic rays produce gamma rays when they pass through objects other than a perfect vacuum. Once formed, gamma rays cannot be deflected by magnetic fields (or other factors other than the gravity of very large masses). Cosmic ray bursts encountering gas clouds surrounding supernovae should produce gamma rays whose origins can be traced. SN 2023ixf therefore provides an excellent opportunity to see how many relevant gamma rays we can find.

The answer is no, at least not anything significantly above the background. Fermi is the most sensitive gamma-ray telescope in orbit, so when it doesn’t detect the expected signal, scientists have to explain the phenomenon. Dr. Elizabeth Hayes of NASA’s Goffard Space Flight Center said solving this mystery will lead to a more accurate understanding of the origin of cosmic rays.

Based on the new observations of SN 2023ixf, we calculate that the energy conversion was as low as 1% in the days after the explosion. That doesn’t rule out supernovae serving as cosmic ray factories, Matt-DeVisa added, but it does mean we need to know more about their production.

There are different types of supernovae, so just because a supernova doesn’t seem to produce a lot of cosmic rays, doesn’t mean a lot of cosmic rays aren’t produced. To test this, we need more nearby supernovae, so it would be nice to have one discovered last week at almost the same distance. The authors also considered the possibility that 2023ixf did produce a large number of cosmic rays, but that they, and the resulting gamma rays, were far away from us. In this case, more study subjects may also be convenient.

However, the findings remain confusing. Supernovae (except Type Ia) involve a giant star collapsing and then rebounding, creating a shock wave that is supposed to drive its previous particles to produce cosmic rays. If 2023ixf doesn’t create a lot, that needs some explanation.

Additionally, Fermi observations demonstrate that supernova remnants from thousands of years ago are still emitting cosmic rays at us, which then produce gamma rays when they pass through nearby material thrown up before the explosion.

However, the rate at which these remnants are produced does not account for all the cosmic rays we see; either most are released in the explosion itself, or there is some other source. 10% of a supernova’s energy is converted into cosmic rays, which could well explain the quantities and spectra we see after accounting for the energy produced by the remnants. If that number is 1%, then most are unaccounted for. That’s why Fermis failed to detect any issues in 2023ixf, raising more questions than it answered.

The research will be published in Astronomy & Astrophysics.