Cosmic Ray Electrons Traced Back to Nearby Sources in Latest Study

Incorporating over 7 million data points encompassing particles recorded by CALET’s detector since 2015, this study benefits from the instrument’s unparalleled capability to detect electrons at the highest energies. This uniqueness contributes to a dataset containing more high-energy electrons than previous research, fortifying the statistical analysis and reinforcing the conclusion that one or more local sources of cosmic ray electrons exist.

Nicholas Cannady, an assistant research scientist with UMBC’s Center for Space Sciences and Technology and a study leader, emphasizes that the detection of nearby sources of cosmic ray electrons is one of the primary objectives of CALET. He highlights the significance of the study, noting that it allowed the research team to delve into a domain with fewer events and explore phenomena at the highest energies, marking an exciting advancement in their investigations.

Gaining deeper insights into the cosmos

According to the prevailing theory, the aftermath of supernovae, referred to as supernova remnants, is thought to generate high-energy electrons—a distinct category of cosmic rays. Since electrons rapidly lose energy once they depart from their source, the infrequent detection of high-energy electrons at CALET suggests an origin in relatively close supernova remnants, at least on a cosmic scale, as clarified by Cannady.

The outcomes of the study serve as a robust indication supporting the existing paradigm regarding high-energy electrons. According to Cannady, the prevailing understanding that these electrons originate from supernova remnants and undergo acceleration as currently hypothesized appears to be validated by the results. The insights derived from these findings contribute to a deeper comprehension of the processes within supernova remnants, thereby enhancing our understanding of the galaxy and its various sources.

CALET stands as a collaborative initiative orchestrated by teams hailing from Japan, Italy, and the United States, under the leadership of Shoji Torii. Noteworthy contributors to this endeavor from Japan include Torii, Yosui Akaike, and Holger Motz affiliated with Waseda University in Tokyo. Meanwhile, Louisiana State University takes the helm as the principal institution guiding the efforts in the United States.

Emerging data uncover novel origins of cosmic rays

Earlier investigations revealed a consistent decline in the influx of electrons at CALET as their energy levels ascended, reaching a discernible decrease around 1 teravolt (TeV) or 1 trillion electron volts. Notably, the occurrence of electrons with higher energy levels was exceptionally rare. However, contrary to expectations, the current study observed an unconventional pattern. Rather than witnessing the anticipated decline, the outcomes propose a plateauing effect, with indications of a subsequent increase in particle count at the highest energy levels—extending up to 10 TeV in some instances.

Earlier experiments were constrained to measuring particles only up to approximately 4 TeV. Consequently, the significance of the current study lies in the identification of potential nearby sources of cosmic ray electrons through the examination of event candidates exceeding this threshold. Cannady spearheaded the initiative to meticulously scrutinize each of these high-energy events, verifying their authenticity as genuine signals. Further exploration and in-depth analysis of these events are on the horizon, promising to unveil additional insights in the near future.

Confronting hurdles

Discerning between electrons and protons at elevated energy levels presents a formidable challenge, particularly given the abundance of protons compared to electrons. This abundance disparity complicates the precision of the analysis. To overcome this hurdle, the research team devised a specialized program. This software dissects the particles’ behavior upon striking the detector, offering a distinctive method to differentiate between electrons and protons in the data analysis process.

Distinguishing between protons and electrons hinges on their distinct breakdown patterns, allowing for the filtration of protons by analyzing the cascade of particles generated in the process. Yet, at the upper echelons of energy, the distinctions between protons and electrons diminish, presenting a heightened challenge in precisely isolating protons from the dataset.

In response to this challenge, Cannady took charge of the CALET team’s initiative to simulate the breakdown patterns of protons and electrons, each originating precisely from the direction of the high-energy events. This strategic approach significantly enhanced the team’s capacity to discern whether the events involved electrons or protons with the utmost accuracy.

The team, under Cannady’s guidance, has confidence in their assessment of the probability that certain events involve protons, asserting that their evaluation is conducted in a realistic manner. Following this meticulous analysis, a sufficient number of presumed electrons persist in the dataset, leading to the conclusion that a genuine signal exists.

Expanding horizons

T. Gregory Guzik, a physics professor at LSU and the lead collaborator for the U.S. CALET team, expresses enthusiasm over the additional examination of the data. This in-depth analysis indicates that electrons stemming from the three most promising candidates for nearby supernova remnants offer a plausible explanation for the observed high-energy arrivals.

Guzik suggests that the CALET observations present an intriguing prospect: the potential measurement of matter originating from a specific nearby supernova remnant right here on Earth. He highlights that ongoing CALET measurements over the International Space Station’s operational duration will contribute valuable insights into the origin and transit of relativistic matter within our galaxy.

According to Cannady, the most thrilling aspect of the research lies in the observation of phenomena at the highest energy levels. He expresses excitement over the identification of candidates surpassing 10 TeV, emphasizing that if these events are confirmed as genuine electron occurrences, it would serve as compelling evidence for the existence of a nearby source. Cannady underscores that this aligns with the primary mission of CALET, making the research both exciting and rewarding as it pushes the boundaries of previous observations.

Resources

1. ^NEWSPAPER Hansen, S. & University of Maryland Baltimore County. (2023, November 14). Study finds strongest evidence yet for local sources of cosmic ray electrons. Phys.org. [Phys.org]
2. ^JOURNAL Adriani, O., Akaike, Y., Asano, K., Asaoka, Y., Berti, E., Bigongiari, G., Binns, W. R., Bongi, M., Brogi, P., Bruno, A., Buckley, J. H., Cannady, N., Castellini, G., Checchia, C., Cherry, M. L., Collazuol, G., De Nolfo, G. A., Ebisawa, K., Ficklin, A. W., . . . Zober, W. V. (2023). Direct Measurement of the Spectral Structure of Cosmic-Ray Electrons+Positrons in the TeV Region with CALET on the International Space Station. Physical Review Letters, 131(19). [Physical Review Letters]