Acceleration of petaelectronvolt protons in the Galactic Centre

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Abstract

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.

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Figure 1: VHE γ-ray image of the Galactic Centre region.
Figure 2: Spatial distribution of the cosmic-ray density versus projected distance from Sgr A*.
Figure 3: VHE γ-ray spectra of the diffuse emission and HESS J1745−290.

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Acknowledgements

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).

Author information

F.A., S.G., E.M. and A.V. analysed and interpreted the data, and prepared the manuscript. The whole HESS collaboration contributed to the publication, with involvement at various stages ranging from the design, construction and operation of the instrument, to the development and maintenance of all software for data handling, data reduction and data analysis. All authors reviewed, discussed and commented on the present results and on the manuscript.

Correspondence to HESS Collaboration.

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Extended data figures and tables

Extended Data Figure 1 Cooling times of electrons in the Galactic Centre as a function of energy.

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.

Extended Data Table 1 γ-ray luminosities and masses in different regions of the central molecular zone
Extended Data Table 2 Cosmic-ray energy densities in different regions of the central molecular zone
Extended Data Table 3 Power-law spectral indices of the γ-ray energy spectrum in different regions of the central molecular zone

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Abramowski, A., Aharonian, F., Benkhali, F. et al. Acceleration of petaelectronvolt protons in the Galactic Centre. Nature 531, 476–479 (2016) doi:10.1038/nature17147

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