Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Review Article
  • Published:

The hunt for PeVatrons as the origin of the most energetic photons observed in the Galaxy

Nature Astronomy volume 8, pages 425–431 (2024)Cite this article

Subjects

Abstract

Ultrarelativistic particles called cosmic rays permeate the Milky Way, propagating through the turbulent Galactic magnetic fields. The mechanisms under which these particles increase their energy can be reasonably described by current theories of acceleration and propagation of cosmic rays. There are, however, still many open questions as to how to reach petaelectronvolt (PeV) energies, the maximum energy believed to be attained in our Galaxy, and in which astrophysical sources (dubbed PeVatrons) this ultrahigh-energy acceleration happens. In this Review, we describe the theoretical conditions for plasma acceleration to these energies and the Galactic sources in which these conditions are possible. These theoretical predictions are then compared to the latest experimental results, summarizing the state of the art of our current knowledge of PeVatrons. We finally describe the prospects to keep advancing the understanding of these elusive objects, still unidentified more than 100 years after the discovery of cosmic rays.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: CR all-particle spectrum measured by different experiments in the region above 5 TeV.
Fig. 2: Magnetic field versus the size of potential Galactic PeVatrons.
Fig. 3: Sensitivity of different instruments with spectra of selected UHE sources.

Similar content being viewed by others

References

  1. Blasi, P. The origin of Galactic cosmic rays. Astron. Astrophys. Rev. 21, 70 (2013).

    ADS  Google Scholar 

  2. Amato, E. The origin of Galactic cosmic rays. Int. J. Mod. Phys. D 23, 1430013 (2014).

    ADS  MathSciNet  Google Scholar 

  3. Gabici, S. et al. The origin of Galactic cosmic rays: challenges to the standard paradigm. Int. J. Mod. Phys. D 28, 1930022–339 (2019).

    ADS  Google Scholar 

  4. Atoyan, A. M. & Aharonian, F. A. On the mechanisms of gamma radiation in the Crab Nebula. Mon. Not. R. Astron. Soc. 278, 525–541 (1996).

    ADS  Google Scholar 

  5. Meyer, M., Horns, D. & Zechlin, H. S. The Crab Nebula as a standard candle in very high-energy astrophysics. Astron. Astrophys. 523, A2 (2010).

    ADS  Google Scholar 

  6. LHAASO Collaboration et al. Peta-electron volt gamma-ray emission from the Crab Nebula. Science 373, 425–430 (2021).

    ADS  Google Scholar 

  7. Cao, Z. et al. The First LHAASO catalog of gamma-ray sources. Astrophys. J. Suppl. S. 271, 25 (2024).

  8. Amato, E. & Blasi, P. Cosmic ray transport in the Galaxy: a review. Adv. Space Res. 62, 2731–2749 (2018).

    ADS  Google Scholar 

  9. Amato, E. & Casanova, S. On particle acceleration and transport in plasmas in the Galaxy: theory and observations. J. Plasma Phys. 87, 845870101 (2021).

    Google Scholar 

  10. Spitzer, L. Physics of Fully Ionized Gases Vol. 31 (American Journal of Physics, 1962).

  11. Hillas, A. M. The origin of ultra-high-energy cosmic rays. Annu. Rev. Astron. Astrophys. 22, 425–444 (1984).

    ADS  Google Scholar 

  12. Hillas, A. M. Can diffusive shock acceleration in supernova remnants account for high-energy Galactic cosmic rays? J. Phys. G 31, R95–R131 (2005).

    ADS  Google Scholar 

  13. Giacalone, J., Jokipii, J. R. & Kota, J. Ion injection and acceleration at quasi-perpendicular shocks. J. Geophys. Res. 99, 19351–19358 (1994).

    ADS  Google Scholar 

  14. Orusa, L. & Caprioli, D. Fast particle acceleration in 3D hybrid simulations of quasiperpendicular shocks. Phys. Rev. Lett. 131, 095201 (2023).

    ADS  Google Scholar 

  15. Vink, J. Supernova remnants: the X-ray perspective. Astron. Astrophys. Rev. 20, 49 (2012).

    ADS  Google Scholar 

  16. Bell, A. R., Schure, K. M., Reville, B. & Giacinti, G. Cosmic-ray acceleration and escape from supernova remnants. Mon. Not. R. Astron. Soc. 431, 415–429 (2013).

    ADS  Google Scholar 

  17. Cristofari, P. The hunt for PeVatrons: the case of supernova remnants. Universe 7, 324 (2021).

    ADS  Google Scholar 

  18. Cristofari, P., Blasi, P. & Amato, E. The low rate of Galactic pevatrons. Astropart. Phys. 123, 102492 (2020).

    Google Scholar 

  19. Gabici, S. & Aharonian, F. A. Searching for Galactic cosmic-ray PeVatrons with multi-TeV gamma rays and neutrinos. Astrophys. J. Lett. 665, L131–L134 (2007).

    ADS  Google Scholar 

  20. Bell, A. R., Schure, K. M. & Reville, B. Cosmic ray acceleration at oblique shocks. Mon. Not. R. Astron. Soc. 418, 1208–1216 (2011).

    ADS  Google Scholar 

  21. Bell, A. R., Matthews, J. H. & Blundell, K. M. Cosmic ray acceleration by shocks: spectral steepening due to turbulent magnetic field amplification. Mon. Not. R. Astron. Soc. 488, 2466–2472 (2019).

    ADS  Google Scholar 

  22. Malkov, M. A. & Aharonian, F. A. Cosmic-ray spectrum steepening in supernova remnants. I. Loss-free self-similar solution. Astrophys. J. 881, 2 (2019).

    ADS  Google Scholar 

  23. Hanusch, A., Liseykina, T. V., Malkov, M. & Aharonian, F. Steepening of cosmic-ray spectra in shocks with varying magnetic field direction. Astrophys. J. 885, 11 (2019).

    ADS  Google Scholar 

  24. Caprioli, D., Haggerty, C. C. & Blasi, P. Kinetic simulations of cosmic-ray-modified shocks. II. Particle spectra. Astrophys. J. 905, 2 (2020).

    ADS  Google Scholar 

  25. Xu, S. & Lazarian, A. Shock acceleration with oblique and turbulent magnetic fields. Astrophys. J. 925, 48 (2022).

    ADS  Google Scholar 

  26. Cao, Z. et al. Does or did the supernova remnant Cassiopeia A operate as a PeVatron? Astrophys. J. Lett. 961, L43 (2024).

  27. Aharonian, F., Yang, R. & de Oña Wilhelmi, E. Massive stars as major factories of Galactic cosmic rays. Nat. Astron. 3, 561–567 (2019).

    ADS  Google Scholar 

  28. Peron, G., Casanova, S., Gabici, S., Baghmanyan, V. & Aharonian, F. The contribution of winds from star clusters to the Galactic cosmic-ray population. Nat. Astron. https://doi.org/10.1038/s41550-023-02168-6 (2024).

  29. Morlino, G., Blasi, P., Peretti, E. & Cristofari, P. Particle acceleration in winds of star clusters. Mon. Not. R. Astron. Soc. 504, 6096–6105 (2021).

    ADS  Google Scholar 

  30. Vieu, T., Gabici, S., Tatischeff, V. & Ravikularaman, S. Cosmic ray production in superbubbles. Mon. Not. R. Astron. Soc. 512, 1275–1293 (2022).

    ADS  Google Scholar 

  31. Bykov, A. M. Nonthermal particles and photons in starburst regions and superbubbles. Astron. Astrophys. Rev. 22, 77 (2014).

    ADS  Google Scholar 

  32. Vieu, T., Reville, B. & Aharonian, F. Can superbubbles accelerate ultrahigh energy protons? Mon. Not. R. Astron. Soc. 515, 2256–2265 (2022).

    ADS  Google Scholar 

  33. Badmaev, D. V., Bykov, A. M. & Kalyashova, M. E. Inside the core of a young massive star cluster: 3D MHD simulations. Mon. Not. R. Astron. Soc. 517, 2818–2830 (2022).

    ADS  Google Scholar 

  34. Cheng, A. F. & Ruderman, M. A. Particle acceleration and radio emission above pulsar polar caps. Astrophys. J. 235, 576–586 (1980).

    ADS  Google Scholar 

  35. Goldreich, P. & Julian, W. H. Pulsar electrodynamics. Astrophys. J. 157, 869 (1969).

    ADS  Google Scholar 

  36. de Oña Wilhelmi, E., López-Coto, R., Amato, E. & Aharonian, F. On the potential of bright, young pulsars to power ultrahigh gamma-ray sources. Astrophys. J. Lett. 930, L2 (2022).

    ADS  Google Scholar 

  37. Vieu, T. & Reville, B. Massive star cluster origin for the galactic cosmic ray population at very-high energies. Mon. Not. R. Astron. Soc. 519, 136–147 (2023).

    ADS  Google Scholar 

  38. Aharonian, F. A. et al. A search for TeV gamma-ray emission from SNRs, pulsars and unidentified GeV sources in the Galactic plane in the longitude range between −2 deg and 85 deg. Astron. Astrophys. 395, 803–811 (2002).

    ADS  Google Scholar 

  39. H. E. S. S. Collaboration et al. The H.E.S.S. Galactic plane survey. Astron. Astrophys. 612, A1 (2018).

    Google Scholar 

  40. Popkow, A. & VERITAS Collaboration. The VERITAS survey of the Cygnus region of the Galaxy. In 34th International Cosmic Ray Conference (PoS, 2015).

  41. Cao, Z. et al. Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources. Nature 594, 33–36 (2021).

    ADS  Google Scholar 

  42. Aharonian, F. et al. Observation of the Crab Nebula with LHAASO-KM2A—a performance study. Chin. Phys. C 45, 025002 (2021).

    ADS  Google Scholar 

  43. Funk, S. & Hinton, J. A. Monte-Carlo studies of the angular resolution of a future Cherenkov gamma-ray telescope. AIP Conf. Proc. 1085, 878–881 (2008).

  44. Li, C. Detection of emission from Cygnus Cocoon above 100TeV with LHAASO. Proc. Sci. ICRC2021, 843 (2021).

    Google Scholar 

  45. Abeysekara, A. U. et al. HAWC observations of the acceleration of very-high-energy cosmic rays in the Cygnus cocoon. Nat. Astron. 5, 465–471 (2021).

    ADS  Google Scholar 

  46. Ackermann, M. et al. A cocoon of freshly accelerated cosmic rays detected by Fermi in the Cygnus superbubble. Science 334, 1103–1107 (2011).

  47. Abramowski, A. et al. Discovery of extended VHE γ-ray emission from the vicinity of the young massive stellar cluster Westerlund 1. Astron. Astrophys. 537, A114 (2012).

    Google Scholar 

  48. Aharonian, F. et al. A deep spectromorphological study of the γ-ray emission surrounding the young massive stellar cluster Westerlund 1. Astron. Astrophys. 666, A124 (2022).

    Google Scholar 

  49. Ackermann, M. et al. Detection of the characteristic pion-decay signature in supernova remnants. Science 339, 807–811 (2013).

    ADS  Google Scholar 

  50. Cardillo, M., Amato, E. & Blasi, P. Supernova remnant W44: a case of cosmic-ray reacceleration. Astron. Astrophys. 595, A58 (2016).

    ADS  Google Scholar 

  51. HESS Collaboration et al. Acceleration of petaelectronvolt protons in the Galactic Centre. Nature 531, 476–479 (2016).

    ADS  Google Scholar 

  52. H. E. S. S. Collaboration et al. Characterising the VHE diffuse emission in the central 200 parsecs of our Galaxy with H.E.S.S. Astron. Astrophys. 612, A9 (2018).

    Google Scholar 

  53. MAGIC Collaboration et al. MAGIC observations of the diffuse γ-ray emission in the vicinity of the Galactic Center. Astron. Astrophys. 642, A190 (2020).

    Google Scholar 

  54. Parsons, R. D. & Ohm, S. Background rejection in atmospheric Cherenkov telescopes using recurrent convolutional neural networks. Eur. Phys. J. C 80, 363 (2020).

    ADS  Google Scholar 

  55. Shilon, I. et al. Application of deep learning methods to analysis of imaging atmospheric Cherenkov telescopes data. Astropart. Phys. 105, 44–53 (2019).

    ADS  Google Scholar 

  56. Olivera-Nieto, L., Ren, H. X., Mitchell, A. M. W., Marandon, V. & Hinton, J. A. Background rejection using image residuals from large telescopes in imaging atmospheric Cherenkov telescope arrays. Eur. Phys. J. C 82, 1118 (2022).

    ADS  Google Scholar 

  57. Olivera-Nieto, L., Mitchell, A. M. W., Bernlöhr, K. & Hinton, J. A. Muons as a tool for background rejection in imaging atmospheric Cherenkov telescope arrays. Eur. Phys. J. C 81, 1101 (2021).

    ADS  Google Scholar 

  58. Harding, P. et al. The HAWC ultra-high-energy gamma-ray map with more than 5 years of data. Proc. Sci. ICRC2023, 698 (2023).

    Google Scholar 

  59. Yun Carcamo, S. L., Smith, A. & HAWC Team. Performance of HAWC with the improved reconstruction algorithm. In APS April Meeting 2022 Vol. 67, 6 (APS, 2022).

  60. Marandon, V., Jardin-Blicq, A. & Schoorlemmer, H. Latest news from the HAWC outrigger array. In 36th International Cosmic Ray Conference (PoS, 2019).

  61. Acero, F. et al. Sensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic supernova remnants. Astropart. Phys. 150, 102850 (2023).

    Google Scholar 

  62. Conceição, R. The Southern Wide-field Gamma-ray Observatory. Proc. Sci. ICRC2023, 963 (2023).

    Google Scholar 

  63. Aharonian, F. et al. Construction and on-site performance of the LHAASO WFCTA camera. Eur. Phys. J. C 81, 657 (2021).

    ADS  Google Scholar 

  64. Cardillo, M. The ASTRI mini-array: in the search for hidden PeVatrons. In Proceedings of 7th Heidelberg International Symposium on High-Energy Gamma-Ray Astronomy (PoS, 2023).

  65. Subieta Vasquez, M. A. Overview status of the ALPACA experiment. Proc. Sci. ICRC2023, 767 (2023).

    Google Scholar 

  66. Sako, T. Mega ALPACA to explore multi-PeV gamma-ray sky in the Southern Hemisphere. Proc. Sci. ICRC2023, 632 (2023).

    Google Scholar 

  67. Neilson, N. K. Highlights from the IceCube Neutrino Observatory. Proc. Sci. ICRC2023, 017 (2023).

    Google Scholar 

  68. Kouchner, A. From Antares to KM3NeT: the adventure of neutrino detection in the Mediterranean Sea. Proc. Sci. ICRC2023, 013 (2023).

    Google Scholar 

  69. Piro, L. et al. Athena synergies in the multi-messenger and transient universe. Exp. Astron. 54, 23–117 (2022).

    ADS  Google Scholar 

  70. Giuliani, A. et al. Neutral pion emission from accelerated protons in the supernova remnant W44. Astrophys. J. Lett. 742, L30 (2011).

    ADS  Google Scholar 

  71. Jogler, T. & Funk, S. Revealing W51C as a cosmic ray source using Fermi-LAT data. Astrophys. J. 816, 100 (2016).

    ADS  Google Scholar 

  72. Ambrogi, L. et al. Spectral and morphological study of the gamma radiation of the middle-aged supernova remnant HB 21. Astron. Astrophys. 623, A86 (2019).

    Google Scholar 

  73. Finger, M. R. Reconstruction of Energy Spectra for Different Mass Groups of High-energy Cosmic Rays. PhD thesis, Karlsruher Institut für Technologie (2011).

  74. Budnev, N. M. et al. The primary cosmic-ray energy spectrum measured with the Tunka-133 array. Astropart. Phys. 117, 102406 (2020).

    Google Scholar 

  75. Amenomori, M. et al. The all-particle spectrum of primary cosmic rays in the wide energy range from 1014 to 1017 eV observed with the Tibet-III air-shower array. Astrophys. J. 678, 1165–1179 (2008).

    ADS  Google Scholar 

  76. Matthews, J. & Telescope Array Collaboration. Highlights from the Telescope Array Experiment. In 35th International Cosmic Ray Conference Vol. 301, 1096 (PoS, 2017).

  77. Grebenyuk, V. et al. Energy spectra of abundant cosmic-ray nuclei in the NUCLEON experiment. Adv. Space Res. 64, 2546–2558 (2019).

    ADS  Google Scholar 

  78. Apel, W. D. et al. KASCADE-Grande measurements of energy spectra for elemental groups of cosmic rays. Astropart. Phys. 47, 54–66 (2013).

  79. Aartsen, M. G. et al. Cosmic ray spectrum and composition from PeV to EeV using 3 years of data from IceTop and IceCube. Phys. Rev. D 100, 082002 (2019).

    ADS  Google Scholar 

  80. Alfaro, R. et al. All-particle cosmic ray energy spectrum measured by the HAWC experiment from 10 to 500 TeV. Phys. Rev. D 96, 122001 (2017).

    ADS  Google Scholar 

  81. Verzi, V. Measurement of the energy spectrum of ultra-high energy cosmic rays using the Pierre Auger Observatory. In 36th International Cosmic Ray Conference (PoS, 2019).

  82. Evoli, C. The cosmic-ray energy spectrum. Zenodo https://doi.org/10.5281/zenodo.4396125 (2020).

  83. Maier, G. Hillas plot. Zenodo https://doi.org/10.5281/zenodo.6037985 (2022).

  84. Pueschel, E. & Maier, G. Extremely accelerating. Phys. J. https://pro-physik.de/zeitschriften/physik-journal/2022-1/ (2022).

  85. Atwood, W. et al. Pass 8: toward the full realization of the Fermi-LAT scientific potential. Preprint at https://doi.org/10.48550/arXiv.1303.3514 (2013).

  86. Aleksić, J. et al. The major upgrade of the MAGIC telescopes, Part II: a performance study using observations of the Crab Nebula. Astropart. Phys. 72, 76–94 (2016).

    ADS  Google Scholar 

  87. Holler, M. E. A. Observations of the Crab Nebula with H.E.S.S. Phase II. Proc. Sci. ICRC2015, 847 (2016).

    Google Scholar 

  88. Abeysekara, A. U. et al. Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory. Astrophys. J. 843, 39 (2017).

    ADS  Google Scholar 

  89. Albert, A. & Harding, J. P. Science Case for a Wide Field-of-View Very-High-Energy Gamma-ray Observatory in the Southern Hemisphere Report No. LA-UR-19-29118 (US Department of Energy Office of Scientific and Technical Information, 2019).

  90. Cao, Z. et al. The Large High Altitude Air Shower Observatory (LHAASO) science book (2021 edition). Chin. Phys. 46, 035001–035007 (2022).

  91. Cherenkov Telescope Array Observatory & Cherenkov Telescope Array Consortium. CTAO Instrument Response Functions—prod5 version v0.1. Zenodo https://doi.org/10.5281/zenodo.5499840 (2021).

  92. Lombardi, S. et al. Performance of the ASTRI mini-array at the Observatorio del Teide. In 37th International Cosmic Ray Conference (PoS, 2022).

Download references

Acknowledgements

This article is the result of fruitful discussions during the 2nd HONEST (Hot Topics in High Energy Astrophysics) Workshop ‘PeVatrons and their environments’ (see https://indico.desy.de/event/34265/). We thank the scientific organizing committee of the workshop, composed of the authors of this Review, plus T. Bell (University of Oxford), D. Berge (DESY Zeuthen), J. Cortina (CIEMAT), P. Huentemeyer (Michigan Tech University), S. Recchia (INFN Torino) and R. Zanin (CTA Observatory); the Local Organizing Committee, J. Eckert, S. Patel, J. Kramer and M. I. Bernardos; and all the participants of the workshop, without whom the fruitful discussions would not have been possible. R.L.-C. acknowledges the Ramón y Cajal programme through grant RYC-2020-028639-I. R.L.-C. also acknowledges financial support from the Spanish ‘Ministerio de Ciencia e Innovación’ (MCIN/AEI/ 10.13039/501100011033) through the Center of Excellence Severo Ochoa award for the Instituto de Astrofísica de Andalucía-CSIC (CEX2021-001131-S), and through grants PID2019-107847RB-C44 and PID2022-139117NB-C44. E.A. acknowledges support by INAF under grant PRIN-INAF 2019, and by the European Union - Next Generation EU through grant PRIN-MUR 2022TJW4EJ. S.G. acknowledges support from Agence Nationale de la Recherche (project CRitiLISM, ANR-21-CE31-0028).

Author information

Authors and Affiliations

  1. Deutsches Elektronen-Synchrotron DESY, Zeuthen, Germany

    Emma de Oña Wilhelmi

  2. Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain

    Ruben López-Coto

  3. Max-Planck-Institut für Kernphysik, Heidelberg, Germany

    Felix Aharonian & Jim Hinton

  4. Dublin Institute for Advanced Studies, Dublin, Ireland

    Felix Aharonian

  5. Yerevan State University, Yerevan, Armenia

    Felix Aharonian

  6. INAF Osservatorio Astrofisico di Arcetri, Florence, Italy

    Elena Amato

  7. Università degli Studi di Firenze, Sesto Fiorentino, Italy

    Elena Amato

  8. Key Laboratory of Particle Astrophysics and Experimental Physics Division and Computing Center, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China

    Zhen Cao

  9. TIANFU Cosmic Ray Research Center, Chengdu, Sichuan, China

    Zhen Cao

  10. University of Chinese Academy of Sciences, Beijing, China

    Zhen Cao

  11. Université de Paris, CNRS, Astroparticule et Cosmologie, Paris, France

    Stefano Gabici

Authors
  1. Emma de Oña Wilhelmi
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Ruben López-Coto
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Felix Aharonian
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Elena Amato
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. Zhen Cao
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. Stefano Gabici
    View author publications

    You can also search for this author in PubMed Google Scholar

  7. Jim Hinton
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

E.d.O.W. and R.L.-C. coordinated the writing of the paper. All authors meet the journal’s authorship criteria and have reviewed, discussed and commented on the review content.

Corresponding authors

Correspondence to Emma de Oña Wilhelmi or Ruben López-Coto.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Astronomy thanks Kaya Mori and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

de Oña Wilhelmi, E., López-Coto, R., Aharonian, F. et al. The hunt for PeVatrons as the origin of the most energetic photons observed in the Galaxy. Nat Astron 8, 425–431 (2024). https://doi.org/10.1038/s41550-024-02224-9

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41550-024-02224-9

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing