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CALET Discovers Nearby, Young Cosmic Ray Electron Sources

A new study employing data from the International Space Station’s CALorimetric Electron Telescope (CALET) instrument has discovered evidence for nearby, young sources of cosmic ray electrons, which will lead to a better understanding of how the galaxy functions as a whole.

CALET Discovers Nearby, Young Cosmic Ray Electron Sources

CALET has been collecting cosmic ray data from the International Space Station (ISS) since 2015. Here it is visible in the inset, attached to the ISS. Image Credit: NASA

More than seven million data points from particles arriving at CALET’s detector since 2015 were analyzed for this study, and CALET is the only facility that can detect electrons at the highest energies. Consequently, the data contains higher numbers of electrons at high energies than any other investigation.

This strengthens the statistical analysis of the data and provides evidence in favor of the hypothesis that there are one or more local sources of electrons from cosmic rays.

This is one of the primary things that CALET is made to look for. We were really able to push into the realm where we have few events and start to look for things at the highest energies, which is exciting.

Nicholas Cannady, Assistant Research Scientist, Center for Space Sciences and Technology, University of Maryland, Baltimore County

A Better Understanding of the Galaxy

The current idea states that these high-energy electrons, a particular kind of cosmic ray, are produced by supernova remnants—the leftovers of exploding stars. According to Cannady, supernova remnants that are quite close (on a cosmic scale) are thought to be the source of the rare electrons that arrive at CALET with high energy because electrons lose energy quickly after leaving their source.

Cannady added, “The study’s results are a strong indicator that the paradigm that we have for understanding these high-energy electrons—that they come from supernova remnants and that they are accelerated the way that we think they are—is correct. It gives insight into what’s going on in these supernova remnants, and offer a way to understand the galaxy and these sources in the galaxy better.

CALET is a collaborative project led by Shoji Torii that developed and is administered by groups in Japan, Italy, and the United States. Torii, Yosui Akaike, and Holger Motz of Waseda University in Tokyo are the primary contributors to this study in Japan, and Louisiana State University is the primary contributor in the United States. Physical Review Letters published the findings.

New Data Lead to New Cosmic Ray Sources

Previous research discovered that when energy increased up to around 1 teravolt (TeV), or 1 trillion electron volts, the number of electrons arriving at CALET fell consistently. The number of electrons with even higher energies entering was incredibly minimal. However, CALET did not see the expected dropoff in this investigation. Instead, the results imply that at the highest energies, the number of particles plateaus and then increases—all the way up to 10 TeV in a few cases.

Previous tests could only measure particles up to roughly 4 TeV, so the highest energy event candidates in this study represent an important new source of information regarding probable close sources of cosmic ray electrons. Cannady spearheaded the effort to independently evaluate each of those occurrences to validate they constitute a true signal and a deeper dive into those events is on the way.

Addressing Challenges

At high energies, it is difficult to discriminate between electrons and protons, and there are many more protons coming than electrons, making proper analysis impossible. An algorithm built by the researchers analyzes how the particles break down when they reach the detector to distinguish them apart.

Since protons and electrons disintegrate differently, comparing the cascade of particles they produce in that process can help to filter out the protons. However, at high energies, the differences between protons and electrons become smaller, making it more difficult to accurately eliminate only the protons from the data.

Cannady led the CALET team’s work to replicate the breakdown patterns of both protons and electrons coming from the exact direction each of the high-energy events arrived from to address this. This improved the team’s capacity to distinguish whether the events were caused by electrons or protons as precisely as feasible.

We believe we are evaluating the likelihood of events being protons in a realistic fashion,” Cannady stated.

After that careful review, there are still enough assumed electrons in the dataset to determine that there is an actual signal.

Pushing Boundaries

The lead of the US CALET project, T. Gregory Guzik, is thrilled that additional data analysis revealed that the high-energy arrivals may be explained by electrons originating from the top three candidates for nearby supernova remnants. Guzik is an LSU professor of physics.

These CALET observations open the tantalizing possibility that matter from a particular nearby supernova remnant can be measured at Earth. Continued CALET measurement through the life of the International Space Station will help shed new light on the origin and transport of relativistic matter in our galaxy.

T. Gregory Guzik, Professor, Department of Physics & Astronomy, Louisiana State University

The most exciting part is seeing things at the highest energies. We have some candidates above 10 TeV—and if it is borne out that these are real electron events, it is really a smoking gun for clear evidence of a nearby source. This is essentially what CALET was put up to do, so it is exciting to be working on this and to finally be getting results that are pushing the bounds of what we have seen before,” Cannady concluded.

Journal Reference

Adriani, O., et al. (2023) Direct Measurement of the Spectral Structure of Cosmic-Ray Electrons + Positrons in the TeV Region with CALET on the International Space Station Aggregation. Physical Review Letters. doi:10.1103/PhysRevLett.131.191001

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