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Researchers Discover the Origin of Milky Way’s Cosmic Rays

For the first time, astronomers have successfully determined the electron and proton components of cosmic rays in a supernova remnant. Novel imaging analysis of radio, X-ray and gamma-ray radiation showed that at least 70% of the very-high-energy gamma rays discharged from cosmic rays are caused by relativistic protons.

Researchers Discover the Origin of Milky Way’s Cosmic Rays.
Schematic images of gamma-ray production from cosmic-ray protons and electrons. Cosmic-ray protons interact with interstellar protons such as molecular and atomic hydrogen gas. The interaction creates a neutral pion that quickly decays into two gamma-ray photons (hadronic process). Cosmic-ray electrons energize interstellar photons (mainly Cosmic Microwave Background; CMB) into gamma-ray energy via inverse Compton scattering (leptonic process). Image Credit: Astrophysics Laboratory, Nagoya University.

In modern astrophysics, the acceleration site of protons, which are the key constituents of cosmic rays, has been a mystery for over a century. This is the first time where researchers have quantified the number of cosmic rays being generated in a supernova remnant. This is a milestone in the explanation of the origin of cosmic rays.

Cosmic rays are particles exhibiting the highest energy in the universe. Their origin has been a mystery since their breakthrough in 1912. As cosmic rays boost the chemical evolution of interstellar matter, learning their origin is crucial to comprehend the evolution of the Milky Way Galaxy.

The cosmic rays were considered to be expedited by supernova remnants (the after-effects of supernova explosions) in the Milky Way Galaxy and moved to the Earth with speed nearly equal to that of light.

Recent developments achieved in the gamma-ray analysis have shown that several supernova remnants emit gamma rays at teraelectronvolts (TeV) energies. In case the gamma rays are generated by protons, the key component of cosmic rays, then the supernova remnant origin of cosmic rays can be confirmed.

Yet, gamma rays are also generated by electrons and, therefore, it is vital to identify if the electron or proton origin is dominant and to calculate the proportion of both the contributions.

The research outcomes offer strong evidence of gamma rays originating from the proton component, which is considered to be the key constituent of cosmic rays. This also supports the theory that Galactic cosmic rays are generated by supernova remnants.

The originality of this study is that gamma-ray radiation is characterized by a linear combination of electron and proton components. Astronomers were aware of a relation that the intensity of gamma-ray from protons is proportional to the interstellar gas density achieved by radio-line imaging observations.

However, the gamma rays produced from electrons are also anticipated to be in the same ratio to X-ray intensity from electrons. Thus, they exhibit the total gamma-ray intensity as the total of two gamma-ray components, both the electron and proton origin. This observation leads to combined learning of three independent observables. This approach was initially proposed in this study.

Consequently, it was revealed that gamma rays from electrons and protons make up 30% and 70% of the total gamma rays, respectively. This is also the first time that both the origins have been determined.

The results also illustrate that the gamma rays emitted from protons dominate the interstellar gas-rich regions, while gamma rays emitted from electrons are more in the gas-poor region. This shows that both the mechanisms operate together and support the assumptions of previous theoretical studies.

This novel method could not have been accomplished without international collaborations.

Yasuo Fukui, Emeritus Professor, Nagoya University

Yasuo Fukui is the project lead and has precisely determined the interstellar gas density distribution using the NANTEN radio telescope and Australia Telescope Compact Array since 2003.

The gamma-ray resolution was inadequate to execute a complete analysis at that time, but Professor Gavin Rowell and Dr. Sabrina Einecke of the University of Adelaide and the H.E.S.S. team significantly enhanced the spatial resolution and sensitivity of gamma rays over the years, enabling a comparison with interstellar gas.

Dr. Hidetoshi Sano from the National Astronomical Observatory of Japan headed the X-ray imaging analysis of archival datasets obtained from the European X-ray satellite XMM-Newton. Dr. Einecke and Professor Rowell collaborated closely with Professor Fukui and Dr. Sano on making detailed studies that examined the correlations across the X-ray, gamma-ray and radio emission.

This novel method will be applied to more supernova remnants using the next-generation gamma-ray telescope CTA (Cherenkov Telescope Array) in addition to the existing observatories, which will greatly advance the study of the origin of cosmic rays,” added the researchers.

Journal Reference:

Fukui, Y., et al. (2021) Pursuing the Origin of the Gamma Rays in RX J1713.7-3946 Quantifying the Hadronic and Leptonic Components. The Astrophysical Journal.


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