Cosmic ray protons reveal new spectral structures at high energies

Cosmic rays constitute high-energy protons and atomic nuclei that originate from stars (both within our galaxy and from other galaxies) and are accelerated by supernovae and other high-energy astrophysical objects. Our current understanding of the Galactic cosmic ray energy spectrum suggests that it follows a power-law dependence, in that the spectral index of protons detected within a certain energy range goes down by power law as energy increases. But recent observations made using magnetic spectrometers for low energy levels and calorimeters for high energy levels has hinted at a deviation from this power-law variation, with the spectral index of protons becoming larger around an energy of few hundred GeV at energies up to 10 TeV. Following this "spectral hardening," characterized by a smaller absolute value of the spectral index, a "spectral softening" has been detected above 10 TeV using the CALorimetric Electron Telescope (CALET), a space telescope installed at the International Space Station. However, better measurements with high statistics and low uncertainty need to be performed over a broad energy spectrum for the confirmation of these spectral structures.

This is exactly what a team of international researchers led by Associate Professor Kazuyoshi Kobayashi from Waseda University in Japan set out to do. "With the data collected by CALET over roughly 6.2 years, we have put forth a detailed spectral structure of the cosmic ray protons. The novelty of our data lies in the high-statistics measurement over a broader energy range of 50 GeV to 60 TeV," elaborates Kobayashi. The findings of their study, which included contributions from Professor Emeritus Shoji Torii from Waseda University (PI, or Principal Investigator, of CALET project) and Professor Pier Simone Marrocchesi from University of Siena in Italy, was published in the journal Physical Review Letters on 1 September 2022.

The new observations confirmed the presence of spectral hardening and softening below and above 10 TeV, suggesting that the proton energy spectrum is not consistent with a single power law variation for the entire range. Moreover, the spectral softening starting at around 10 TeV is consistent with a previous measurement reported by the Dark Matter Particle Explorer (DAMPE) space telescope. Interestingly enough, the transition by spectral softening was found to be sharper than that by spectral hardening.

The variations and the uncertainty in the new CALET data were controlled using Monte Carlo simulations. The statistics was improved by a factor of around 2.2 and the spectral hardening feature was confirmed with a higher significance of more than 20 sigmas.

Talking about the significance of this research, Kobayashi remarks, "This result will significantly contribute to our understanding of cosmic ray acceleration by supernovae and the propagation mechanism of cosmic rays. The next step would be to extend our measurement of the proton spectra to even higher energies with reduced systematic uncertainties. This should be accompanied by a shift in the theoretical understanding to accommodate the new observations."

Eventually, it's not just about cosmic rays, though. Rather, the study goes on to show just how much we still do not understand about our Universe, and that it's worthwhile to ponder over it.

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Reference

DOI: https://doi.org/10.1103/PhysRevLett.129.101102

Authors: O. Adriani^1,2 ,Y. Akaike,^3,4 K. Asano,⁵ Y. Asaoka,⁵ E. Berti,^1,2 G. Bigongiari,^6,7 W. R. Binns,⁸ M. Bongi,^1,2 P. Brogi,^6,7 A. Bruno,⁹ J. H. Buckley,⁸ N. Cannady,^10,11,12 G. Castellini,¹³ C. Checchia,^6,7 M. L. Cherry,¹⁴ G. Collazuol,^15,16 K. Ebisawa,¹⁷ A. W. Ficklin,¹⁴ H. Fuke,¹⁷ S. Gonzi,^1,2 T. G. Guzik,¹⁴ T. Hams,¹⁰ K. Hibino,¹⁸ M. Ichimura,¹⁹ K. Ioka,²⁰ W. Ishizaki,⁵ M. H. Israel,⁸ K. Kasahara,²¹ J. Kataoka,²² R. Kataoka,²³ Y. Katayose,²⁴ C. Kato,²⁵ N. Kawanaka,²⁰ Y. Kawakubo,¹⁴ K. Kobayashi,^3,4,* K. Kohri,²⁶ H. S. Krawczynski,⁸ J. F. Krizmanic,¹¹ P. Maestro,^6,7 P. S. Marrocchesi,^6,7,† A. M. Messineo,^27,7 J. W. Mitchell,¹¹ S. Miyake,²⁸ A. A. Moiseev,^29,11,12 M. Mori,³⁰ N. Mori,² H. M. Motz,³¹ K. Munakata,²⁵ S. Nakahira,¹⁷ J. Nishimura,¹⁷ G. A. de Nolfo,⁹ S. Okuno,¹⁸ J. F. Ormes,³² S. Ozawa,³³ L. Pacini,^1,13,2 P. Papini,² B. F. Rauch,⁸ S. B. Ricciarini,^13,2 K. Sakai,^10,11,12 T. Sakamoto,³⁴ M. Sasaki,^29,11,12 Y. Shimizu,¹⁸ A. Shiomi,³⁵ P. Spillantini,¹ F. Stolzi,^6,7 S. Sugita,³⁴ A. Sulaj,^6,7 M. Takita,⁵ T. Tamura,¹⁸ T. Terasawa,⁵ S. Torii,³,‡ Y. Tsunesada,^36,37 Y. Uchihori,³⁸ E. Vannuccini,² J. P. Wefel,¹⁴ K. Yamaoka,³⁹ S. Yanagita,⁴⁰ A. Yoshida,³⁴ K. Yoshida,²¹ and W. V. Zober⁸

Affiliations:

1. Department of Physics, University of Florence, Via Sansone, 1—50019 Sesto Fiorentino, Italy
2. INFN Sezione di Florence, Via Sansone, 1—50019 Sesto Fiorentino, Italy
3. Waseda Research Institute for Science and Engineering, Waseda University, 17 Kikuicho, Shinjuku, Tokyo 162-0044, Japan
4. JEM Utilization Center, Human Spaceflight Technology Directorate, Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan
5. Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa, Chiba 277-8582, Japan
6. Department of Physical Sciences, Earth and Environment, University of Siena, via Roma 56, 53100 Siena, Italy
7. INFN Sezione di Pisa, Polo Fibonacci, Largo B. Pontecorvo, 3—56127 Pisa, Italy
8. Department of Physics and McDonnell Center for the Space Sciences, Washington University, One Brookings Drive, St. Louis, Missouri 63130-4899, USA
9. Heliospheric Physics Laboratory, NASA/GSFC, Greenbelt, Maryland 20771, USA
10. Center for Space Sciences and Technology, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA
11. Astroparticle Physics Laboratory, NASA/GSFC, Greenbelt, Maryland 20771, USA
12. Center for Research and Exploration in Space Sciences and Technology, NASA/GSFC, Greenbelt, Maryland 20771, USA
13. Institute of Applied Physics (IFAC), National Research Council (CNR), Via Madonna del Piano, 10, 50019 Sesto Fiorentino, Italy
14. Department of Physics and Astronomy, Louisiana State University, 202 Nicholson Hall, Baton Rouge, Louisiana 70803, USA
15. Department of Physics and Astronomy, University of Padova, Via Marzolo, 8, 35131 Padova, Italy
16. INFN Sezione di Padova, Via Marzolo, 8, 35131 Padova, Italy
17. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan
18. Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa, Yokohama, Kanagawa 221-8686, Japan
19. Faculty of Science and Technology, Graduate School of Science and Technology, Hirosaki University, 3, Bunkyo, Hirosaki, Aomori 036-8561, Japan
20. Yukawa Institute for Theoretical Physics, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
21. Department of Electronic Information Systems, Shibaura Institute of Technology, 307 Fukasaku, Minuma, Saitama 337-8570, Japan
22. School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
23. National Institute of Polar Research, 10-3, Midori-cho, Tachikawa, Tokyo 190-8518, Japan
24. Faculty of Engineering, Division of Intelligent Systems Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
25. Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan
26. Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
27. University of Pisa, Polo Fibonacci, Largo B. Pontecorvo, 3—56127 Pisa, Italy
28. Department of Electrical and Electronic Systems Engineering, National Institute of Technology (KOSEN), Ibaraki College, 866 Nakane, Hitachinaka, Ibaraki 312-8508, Japan
29. Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
30. Department of Physical Sciences, College of Science and Engineering, Ritsumeikan University, Shiga 525-8577, Japan
31. Faculty of Science and Engineering, Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
32. Department of Physics and Astronomy, University of Denver, Physics Building, Room 211, 2112 East Wesley Avenue, Denver, Colorado 80208-6900, USA
33. Quantum ICT Advanced Development Center, National Institute of Information and Communications Technology, 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan
34. College of Science and Engineering, Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo, Sagamihara, Kanagawa 252-5258, Japan
35. College of Industrial Technology, Nihon University, 1-2-1 Izumi, Narashino, Chiba 275-8575, Japan
36. Graduate School of Science, Osaka Metropolitan University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan
37. Nambu Yoichiro Institute for Theoretical and Experimental Physics, Osaka Metropolitan University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan
38. National Institutes for Quantum and Radiation Science and Technology, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan
39. Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan
40. College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan

About Waseda University

Located in the heart of Tokyo, Waseda University is a leading private research university that has long been dedicated to academic excellence, innovative research, and civic engagement at both the local and global levels since 1882. The University ranks number one in Japan in international activities, including the number of international students, with the broadest range of degree programs fully taught in English. To learn more about Waseda University, visit https://www.waseda.jp/top/en