- Physical Review Journals
All JournalsPhysics Magazine
Access Provided by Xinjiang University

Physical Review D

Export citation

Export citation

Choose format for download:

Download Citation
  • Access by Xinjiang University

Spectral shapes of Galactic gamma-ray sources

Paolo Lipari*

Silvia Vernetto†

  • INFN, Sezione Roma “Sapienza,” piazzale A.Moro 2, 00185 Roma, Italy and IHEP, Key Laboratory of Particle Astrophysics, Chinese Academy of Sciences, Beijing, China

  • *Contact author: paolo.lipari@roma1.infn.it
  • †Contact author: vernetto@to.infn.it
PDF

Phys. Rev. D 111, 123040 – Published 25 June, 2025

DOI: https://doi.org/10.1103/k5dp-5str

Abstract

Recent observations with ground-based telescopes have measured the spectra of a large number of gamma-ray sources in the TeV and PeV energy ranges. Photons of such high energies are strongly absorbed as they propagate over extragalactic distances, and most of these sources are Galactic. Combining these new results with those obtained by Fermi-LAT at lower energies it is possible to study the ensemble of the Galactic gamma-ray sources in a very large energy range extending for seven decades from 0.1 GeV to 106  GeV. An interesting quantity is the cumulative spectrum obtained summing the fluxes of all resolved Galactic sources. This spectrum is not a simple power-law, but has a smooth shape that softens gradually with energy. This spectral shape can be used as a first approximation for the shape of the flux of the ensemble of the unresolved Galactic gamma-ray sources, that must be subtracted from the observations to obtain the diffuse gamma-ray flux produced in the collisions of cosmic rays with interstellar gas. The shape of cumulative Galactic sources spectrum emerges from the sum of contributions that can have very different spectral shapes, and cannot be considered as a good model for the emission of any individual source. Understanding the origin of the spectral shapes of the Galactic gamma-ray sources remains a crucial challenge for high-energy astrophysics.

View figure in article

Physics Subject Headings (PhySH)

Article Text

References (21)

  1. S. Abdollahi et al. (Fermi-LAT Collaboration), Fermi large area telescope fourth source catalog, Astrophys. J. Suppl. Ser. 247, 33 (2020).
  2. S. Abdollahi et al. (Fermi-LAT Collaboration), Incremental Fermi large area telescope fourth source catalog, Astrophys. J. Suppl. Ser. 260, 53 (2022).
  3. H. Abdalla et al. (HESS Collaboration), The H.E.S.S. Galactic plane survey, Astron. Astrophys. 612, A1 (2018).
  4. A. Albert et al. (HAWC Collaboration), 3HWC: The third HAWC catalog of very-high-energy gamma-ray sources, Astrophys. J. 905, 76 (2020).
  5. Z. Cao et al. (LHAASO Collaboration), The first LHAASO catalog of gamma-ray sources, Astrophys. J. Suppl. Ser. 271, 25 (2024).
  6. M. Ackermann et al. (Fermi-LAT Collaboration), The spectrum of isotropic diffuse gamma-ray emission between 100 MeV and 820 GeV, Astrophys. J. 799, 86 (2015).
  7. F. Acero et al. (Fermi-LAT Collaboration), Development of the model of Galactic interstellar emission for standard point-source analysis of Fermi large area telescope data, Astrophys. J. Suppl. Ser. 223, 26 (2016).
  8. M. Amenomori et al. (Tibet ASgamma Collaboration), First detection of sub-PeV diffuse gamma rays from the Galactic disk: Evidence for ubiquitous Galactic cosmic rays beyond PeV energies, Phys. Rev. Lett. 126, 141101 (2021).
  9. Z. Cao et al. (LHAASO Collaboration), Measurement of ultra-high-energy diffuse gamma-ray emission of the Galactic plane from 10 TeV to 1 PeV with LHAASO-KM2A, Phys. Rev. Lett. 131, 151001 (2023).
  10. R. Alfaro et al. (HAWC Collaboration), Galactic gamma-ray diffuse emission at TeV energies with HAWC data, Astrophys. J. 961, 104 (2024).
  11. P. Lipari and S. Vernetto, Diffuse Galactic gamma ray flux at very high energy (LV-2018), Phys. Rev. D 98, 043003 (2018).
  12. C. Steppa and K. Egberts, Modelling the Galactic very-high-energy γ-ray source population, Astron. Astrophys. 643, A137 (2020).
  13. M. Cataldo, G. Pagliaroli, V. Vecchiotti, and F. L. Villante, The TeV gamma-ray luminosity of the Milky Way and the contribution of H.E.S.S. unresolved sources to very high energy diffuse emission, Astrophys. J. 904, 85 (2020).
  14. P. D. Luque, D. Gaggero, D. Grasso, O. Fornieri, K. Egberts, C. Steppa, and C. Evoli, Galactic diffuse gamma rays meet the PeV frontier, Astron. Astrophys. 672, A58 (2023).
  15. M. G. Aartsen et al. (IceCube Collaboration), Evidence for high-energy extraterrestrial neutrinos at the IceCube detector, Science 342, 1242856 (2013).
  16. M. G. Aartsen et al. (IceCube Collaboration), Observation of high-energy astrophysical neutrinos in three years of IceCube data, Phys. Rev. Lett. 113, 101101 (2014).
  17. M. G. Aartsen et al. (IceCube Collaboration), Time-integrated neutrino source searches with 10 years of IceCube data, Phys. Rev. Lett. 124, 051103 (2020).
  18. R. Abbasi et al. (IceCube Collaboration), Observation of high-energy neutrinos from the Galactic plane, Science 380, adc9818 (2023).
  19. P. Lipari, The origin of the power–law form of the extragalactic gamma–ray flux, Astropart. Phys. 125, 102507 (2021).
  20. P. Lipari, Spectral features in the cosmic ray fluxes, Astropart. Phys. 97, 197 (2018).
  21. P. Lipari and S. Vernetto, Resolved and unresolved Galactic gamma-ray sources, Phys. Rev. D 111, 063035 (2025).

Outline

Information

Phys. Rev. D 111, 123040– Published 25 June, 2025
  • Received 16 February 2025
  • Accepted 20 May 2025
Crossmark - Check for updates button

© 2025 American Physical Society

Sign In to Your Journals Account

Filter

Filter

Article Lookup

Enter a citation