Acceleration of petaelectronvolt protons in the Galactic Centre (1603.07730v1)

Abstract: Galactic cosmic rays reach energies of at least a few Peta-electronvolts (1 PeV =$10^\mathbf{15}$ electron volts). This implies our Galaxy contains PeV accelerators (PeVatrons), but all proposed models of Galactic cosmic-ray accelerators encounter non-trivial difficulties at exactly these energies. Tens of Galactic accelerators capable of accelerating particle to tens of TeV (1 TeV =$10^\mathbf{12}$ electron volts) energies were inferred from recent gamma-ray observations. None of the currently known accelerators, however, not even the handful of shell-type supernova remnants commonly believed to supply most Galactic cosmic rays, have shown the characteristic tracers of PeV particles: power-law spectra of gamma rays extending without a cutoff or a spectral break to tens of TeV. Here we report deep gamma-ray observations with arcminute angular resolution of the Galactic Centre regions, which show the expected tracer of the presence of PeV particles within the central 10~parsec of the Galaxy. We argue 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 outbursts and an outflow from the Galactic Centre. 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 $\gtrsim 10^{6-7}$ years, and therefore should be considered as a viable alternative to supernova remnants as a source of PeV Galactic cosmic rays.

• The paper identifies a central PeVatron in the Galactic Centre that persistently injects PeV protons, evidenced by a 1/r energy density profile.
• It leverages high-precision gamma-ray measurements from H.E.S.S. to correlate very-high-energy emissions with gas-rich complexes, supporting a hadronic origin.
• These findings challenge the supernova remnant paradigm, suggesting alternative cosmic accelerators linked to Sgr A* for sustained high-energy proton acceleration.

Acceleration of Petaelectronvolt Protons in the Galactic Centre

The paper "Acceleration of Petaelectronvolt protons in the Galactic Centre" by the H.E.S.S. Collaboration addresses a significant aspect of astrophysics concerning the origins and acceleration mechanisms of cosmic ray protons. This paper hinges on high-precision gamma-ray observations conducted by the High Energy Stereoscopic System (H.E.S.S.), providing crucial insights into the presence of Petaelectronvolt (PeV) particles in the Galactic Centre.

Key Findings and Data Analysis

The H.E.S.S. Collaboration conducted gamma-ray observations that revealed the presence of PeV particle activity within 10 parsecs of the Galactic Centre, an area densely packed with molecular gases. Analysis indicates a clear correlation between the emission of very-high-energy (VHE) gamma-rays and the distribution of massive gas-rich complexes. This suggests a hadronic origin for the observed diffuse emission, as relativistic protons interact with ambient gases, leading to gamma-ray production—a process further supported by the lack of significant distributions stemming from leptonic origins due to multi-TeV electrons' radiative losses, which inhibit propagation over the observed spatial extents.

The energy density profile determined from gamma-ray data displays an $E \geq 10$ TeV cosmic rays profile that varies as $1/r$, indicative of a quasi-continuous injection of protons by a centrally located particle accelerator. This provides substantial evidence against alternative cosmic-ray injection scenarios such as burst-like events or wind advection predictions, which present different radial dependency ($1/r^2$ or constant).

The data supports the existence of a central PeVatron, which likely entails an energetic link with Sagittarius A* (Sgr A*), the supermassive black hole at the Galactic Centre. This scenario is compelling given Sgr A*’s historical active phases that might have been conducive to heightened particle acceleration rates, although its current level of activity is insufficient to account for the entire cosmic ray population of our galaxy.

Implications and Theoretical Considerations

The implications of these findings suggest that within the Galactic Centre, intense PeV particle accelerators, potentially connected to massive astrophysical phenomena like Sgr A*, can exist. The paper posits that the Galactic Centre could potentially be regularly injected with multi-TeV protons, sustaining high cosmic-ray densities despite the relatively uniform distribution of lower energy cosmic rays throughout the Galaxy.

The suggestion of a viable PeVatron challenges existing paradigms which primarily attribute cosmic ray origins to supernova remnants (SNRs). Unlike traditional SNRs, which can momentarily accelerate particles to high energies, the observed $1/r$ profile denotes a capability for persistent acceleration over longer durations—an aspect lacking in SNR-driven models.

In addition, this paper raises the possibility of alternative PeV accelerators such as compact stellar clusters, indicating that the full range of potential cosmic accelerators remains to be determined.

Future Prospects

Future endeavors in the field of high-energy astrophysics will necessitate further investigations aligning observational advancements, particularly in neutrino and X-ray astronomy, to verify these findings. The identification of additional Galactic PeVatrons could elucidate the broader cosmic ray distribution and propagation dynamics within the galaxy and aid in understanding their role in shaping interstellar matter.

Further research into the specific mechanisms at Sgr A*, and analogous systems elsewhere in the universe, could provide profound insights into how massive galactic cores potentially influence cosmic particle distributions and contribute to the cosmic ray flux observed within our Milky Way.

Overall, the observations and hypotheses laid forth by the H.E.S.S. Collaboration mark a pivotal step in unraveling the complex mechanisms that govern cosmic ray acceleration and propagation, driving forward our comprehension of the universe's high-energy phenomena.