Galactic cosmic rays reach energies of at least a few petaelectronvolts1 (of the order of 1015 electronvolts). This implies that our Galaxy contains petaelectronvolt accelerators (‘PeVatrons’), but all proposed models of Galactic cosmic-ray accelerators encounter difficulties at exactly these energies2. Dozens of Galactic accelerators capable of accelerating particles to energies of tens of teraelectronvolts (of the order of 1013 electronvolts) were inferred from recent γ-ray observations3. However, none of the currently known accelerators—not even the handful of shell-type supernova remnants commonly believed to supply most Galactic cosmic rays—has shown the characteristic tracers of petaelectronvolt particles, namely, power-law spectra of γ-rays extending without a cut-off or a spectral break to tens of teraelectronvolts4. Here we report deep γ-ray observations with arcminute angular resolution of the region surrounding the Galactic Centre, which show the expected tracer of the presence of petaelectronvolt protons within the central 10 parsecs of the Galaxy. We propose that the supermassive black hole Sagittarius A* is linked to this PeVatron. Sagittarius A* went through active phases in the past, as demonstrated by X-ray outbursts5and an outflow from the Galactic Centre6. Although its current rate of particle acceleration is not sufficient to provide a substantial contribution to Galactic cosmic rays, Sagittarius A* could have plausibly been more active over the last 106–107 years, and therefore should be considered as a viable alternative to supernova remnants as a source of petaelectronvolt Galactic cosmic rays.
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Berezinskii, V. S., Bulanov, S. V., Dogiel, V. A. & Ptuskin, V. S. in Astrophysics of Cosmic Rays (ed. Ginzburg, V. L. ) 33–38 (North-Holland, 1990)
Malkov, M. A. & Drury, L. O. Nonlinear theory of diffusive acceleration of particles by shock waves. Rep. Prog. Phys. 64, 429–481 (2001)
Hillas, A. M. Evolution of ground-based gamma-ray astronomy from the early days to the Cherenkov Telescope Arrays. Astropart. Phys. 43, 19–43 (2013)
Aharonian, F. A. Gamma rays from supernova remnants. Astropart. Phys. 43, 71–80 (2013)
Clavel, M. et al. Echoes of multiple outbursts of Sagittarius A* revealed by Chandra. Astron. Astrophys. 558, A32 (2013)
Su, M., Slatyer, T. R. & Finkbeiner, D. P. Giant gamma-ray bubbles from Fermi-LAT: active galactic nucleus activity or bipolar galactic wind? Astrophys. J. 724, 1044–1082 (2010)
Aharonian, F. et al. Discovery of very-high-energy γ-rays from the Galactic Centre ridge. Nature 439, 695–698 (2006)
Yang, R., Jones, D. I. & Aharonian, F. Fermi-LAT observations of the Sagittarius B complex. Astron. Astrophys. 580, A90 (2015)
Aharonian, F. Very High Energy Cosmic Gamma Radiation: A Crucial Window on the Extreme Universe (World Scientific, 2004)
Strong, A. W., Moskalenko, I. V. & Ptuskin, V. S. Cosmic-ray propagation and interactions in the Galaxy. Annu. Rev. Nucl. Part. Sci. 57, 285–327 (2007)
Aharonian, F. et al. Very high-energy gamma rays from the direction of Sagittarius A*. Astron. Astrophys. 425, L13–L17 (2004)
Kosack, K. et al. TeV gamma-ray observations of the Galactic Center. Astrophys. J. 608, L97–L100 (2004)
Tsuchiya, K. et al. Detection of sub-TeV gamma rays from the Galactic center direction by CANGAROO-II. Astrophys. J. 606, L115–L118 (2004)
Albert, J. et al. Observation of gamma rays from the Galactic center with the MAGIC telescope. Astrophys. J. 638, L101–L104 (2006)
Aharonian, F. & Neronov, A. High-energy gamma rays from the massive black hole in the Galactic center. Astrophys. J. 619, 306–313 (2005)
Wang, Q. D., Lu, F. J. & Gotthelf, E. V. G359.95–0.04: pulsar candidate near Sgr A*. Mon. Not. R. Astron. Soc. 367, 937–944 (2006)
Hinton, J. A. & Aharonian, F. Inverse Compton scenarios for the TeV gamma-ray emission of the Galactic centre. Astrophys. J. 657, 302–307 (2007)
Belikov, A. V., Zaharijas, G. & Silk, J. Study of the gamma-ray spectrum from the Galactic Center in view of multi-TeV dark matter candidates. Phys. Rev. D 86, 083516 (2012)
Aharonian, F. et al. Spectrum and variability of the Galactic center VHE γ-ray source HESS J1745–290. Astron. Astrophys. 503, 817–825 (2009)
Archer, A. et al. Very-high energy observations of the Galactic center region by VERITAS in 2010–2012. Astrophys. J. 790, 149 (2014)
HESS Collaboration. Localising the VHE γ-ray source at the Galactic Centre. Mon. Not. R. Astron. Soc. 402, 1877–1882 (2010)
Crocker, R. M. et al. γ-rays and the far-infrared-radio continuum correlation reveal a powerful Galactic Centre wind. Mon. Not. R. Astron. Soc. 411, L11–L15 (2011)
Bykov, A. M. Nonthermal particles and photons in starburst regions and superbubbles. Astron. Astrophys. Rev. 22, 1–54 (2014)
Bell, A., Schure, K., Reville, B. & Giacinti, G. Cosmic ray acceleration and escape from supernova remnants. Mon. Not. R. Astron. Soc. 431, 415–429 (2013)
Istomin, Y. N. On the origin of galactic cosmic rays. New Astron. 27, 13–18 (2014)
Atoyan, A. & Dermer, C. D. TeV emission from the Galactic center black hole plerion. Astrophys. J. 617, L123–L126 (2004)
Genzel, R., Eisenhauer, F. & Gillessen, S. The Galactic Center massive black hole and nuclear star cluster. Rev. Mod. Phys. 82, 3121–3195 (2010)
Cristofari, P., Gabici, S., Casanova, S., Terrier, R. & Parizot, E. Acceleration of cosmic rays and gamma-ray emission from supernova remnants in the Galaxy. Mon. Not. R. Astron. Soc. 434, 2748–2760 (2013)
Parizot, E. Cosmic ray origin: lessons from ultra-high-energy cosmic rays and the Galactic/extragalactic transition. Nucl. Phys. B 256–257 (Suppl.), 197–212 (2014)
Tsuboi, M., Handa, T. & Ukita, N. Dense molecular clouds in the Galactic Center region. I. Observations and data. Astrophys. J. 120 (Suppl.), 1–39 (1999)
Aharonian, F. et al. Observations of the Crab Nebula with HESS. Astron. Astrophys. 457, 899–915 (2006)
de Naurois, M. & Rolland, L. A high performance likelihood reconstruction of γ-rays for imaging atmospheric Cherenkov telescopes. Astropart. Phys. 32, 231–252 (2009)
Berge, D., Funk, S. & Hinton, J. A. Background modelling in very-high-energy γ-ray astronomy. Astron. Astrophys. 466, 1219–1229 (2007)
Piron, F. et al. Temporal and spectral gamma-ray properties of Mkn 421 above 250 GeV from CAT observations between 1996 and 2000. Astron. Astrophys. 374, 895–906 (2001)
Kelner, S., Aharonian, F. & Bugayov, V. Energy spectra of gamma-rays, electrons and neutrinos produced at proton-proton interactions in the very high energy regime. Phys. Rev. D 74, 034018 (2006); erratum 79, 039901 (2009)
Oka, T. et al. A large-scale CO survey of the Galactic center. Astrophys. J. 118 (Suppl.), 455–515 (1998)
Jones, P. et al. Spectral imaging of the Central Molecular Zone in multiple 3-mm molecular lines. Mon. Not. R. Astron. Soc. 419, 2961–2986 (2012)
Ferrière, K., Gillard, W. & Jean, P. Spatial distribution of interstellar gas in the innermost 3 kpc of our Galaxy. Astron. Astrophys. 467, 611–627 (2007)
Crocker, R. M., Jones, D. I., Melia, F., Ott, J. & Protheroe, R. J. A lower limit of 50 microgauss for the magnetic field near the Galactic Centre. Nature 463, 65–67 (2010)
Vissani, F., Aharonian, F. & Sahakyan, N. On the detectability of high-energy galactic neutrino sources. Astropart. Phys. 34, 778–783 (2011)
Heard, V. & Warwick, R. S. XMM-Newton observations of the Galactic Centre region – I. The distribution of low-luminosity X-ray sources. Mon. Not. R. Astron. Soc. 428, 3462–3477 (2013)
Büsching, I., de Jager, O. C. & Snyman, J. Obtaining cosmic-ray propagation parameters from diffuse very high energy gamma-ray emission from the galactic center ridge. Astrophys. J. 656, 841–846 (2007)
Yusef-Zadeh, F. et al. Interacting cosmic rays with molecular clouds: a bremsstrahlung origin of diffuse high energy emission from the inner 2° × 1° of the Galactic center. Astrophys. J. 762, 33 (2013)
Vink, J. Supernova remnants: the X-ray perspective. Astron. Astrophys. Rev. 20, 49 (2012)
Bell, A. R. Turbulent amplification of magnetic field and diffusive shock acceleration of cosmic rays. Mon. Not. R. Astron. Soc. 353, 550–558 (2004)
Yusef-Zadeh, F. et al. The origin of diffuse X-ray and γ-ray emission from the Galactic center region: cosmic ray particles. Astrophys. J. 656, 847–869 (2007)
Strong, A. W. & Moskalenko, I. V. Propagation of cosmic-ray nucleons in the Galaxy. Astrophys. J. 509, 212–228 (1998)
The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of HESS is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the German Research Foundation (DFG), the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the UK Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Czech Science Foundation, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation, and the University of Namibia. We thank the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay and Namibia for the construction and operation of the equipment. R.C.G.C. is funded by an EU FP7 Marie Curie grant (PIEF-GA-2012-332350), J. Conrad is a Wallenberg Academy Fellow, and F.R. is a Heisenberg Fellow (DFG).
The author declare no competing financial interests.
Extended data figures and tables
The cooling times (τcool) due to ionization (or Coulomb) losses and bremsstrahlung are inversely proportional to the gas density n; here n = 100 cm−3 is assumed. The cooling time of the synchrotron radiation is proportional to 1/B2, where B is the magnetic field. The synchrotron cooling times are given for magnetic fields B = 10 μG and B = 100 μG. The total energy densities of the cosmic microwave background and of local near-infrared (NIR) and far-infrared (FIR) infrared radiation fields used to calculated the cooling time due to the IC scattering are extracted from the GALPROP code47. The integrated densities are 17.0 eV cm3 and 1.3 eV cm3 for NIR and FIR, respectively.
Extended Data Figure 2 Broad-band spectral energy distribution of radiation by relativistic electrons.
The flux from synchrotron radiation, bremsstrahlung and IC scattering is compared to the fluxes of diffuse γ-ray emission measured by HESS (black points with vertical error bars). The flux of diffuse X-ray emission measured by XMM-Newton41 (black point with horizontal error bar) and integrated over the central molecular zone region is also shown. Inset (top right) shows a zoomed view of the spectral energy distribution in the VHE range (100 GeV–100 TeV). The vertical and horizontal error bars show the 1σ statistical errors and the bin size, respectively.
Extended Data Figure 3 The spectral energy distribution of high energy neutrinos—the counterparts of diffuse γ-rays from the Galactic Centre.
The energy spectrum of parent protons is derived from the γ-ray data. The three curves correspond to different values of the exponential cut-off in the proton spectrum: 1 PeV, 10 PeV and 100 PeV.
Extended Data Figure 4 The spectral energy distribution of synchrotron radiation of secondary electrons produced in pp interactions.
The spectra of protons are the same as in Extended Data Fig. 3. The magnetic field is assumed to be 100 μG. The flux of diffuse X-ray emission measured by XMM-Newton and integrated over the central molecular zone region is also shown. The horizontal error bar corresponds to the bin size.
About this article
Cite this article
HESS Collaboration. Acceleration of petaelectronvolt protons in the Galactic Centre. Nature 531, 476–479 (2016). https://doi.org/10.1038/nature17147
Nature Astronomy (2021)
Space Science Reviews (2020)
Astrophysics and Space Science (2020)