Using the H.E.S.S. (High Energy Stereoscopic System) Observatory, a system of four imaging atmospheric Cherenkov telescopes in Namibia, astronomers have detected a very powerful source of cosmic rays with energies never observed before in our Galaxy.
Artist’s impression of the giant molecular clouds surrounding the Milky Way’s center, bombarded by very high energy protons accelerated in the vicinity of Sagittarius A*. Image credit: Mark A. Garlick / H.E.S.S. Collaboration.
Particles that bombard our planet from anywhere beyond its atmosphere are known as cosmic rays. Some of these particles originate from the Sun, but most come from sources outside the Solar System and are known as Galactic cosmic rays.
About 85% of Galactic cosmic rays are protons (nuclei of hydrogen atoms), with about 12% consisting of alpha particles (helium nuclei); the remainder are electrons and nuclei of heavier atoms. These particles have probably been accelerated within the last few million years, and have traveled many times across our Milky Way Galaxy, trapped by its magnetic field.
As they travel through the very thin gas of interstellar medium, some of the cosmic rays interact and emit gamma rays, which is how we know that they pass through the Milky Way. These gamma rays travel in straight lines, undeflected by magnetic fields, and can therefore be traced back to their origin.
When a very-high-energy gamma ray reaches the Earth, it interacts with a molecule in the upper atmosphere, producing a shower of secondary particles that emit a short pulse of the so-called Cherenkov light. By detecting these flashes of light, more than 100 sources of very-high-energy gamma rays have been identified over the past 30 years.
“Today we know that cosmic rays with energies up to 100 teraelectronvolts (TeV) are produced in our Galaxy, by objects such as supernova remnants and pulsar wind nebulae,” said University of Hamburg astronomer Dr. Attila Abramowski and his colleagues from the H.E.S.S. Collaboration.
“Theoretical arguments and direct measurements of cosmic rays reaching the Earth indicate, however, that the cosmic-ray factories in our Galaxy should be able to provide particles up to one petaelectronvolt (PeV) at least.”
“While many multi-TeV accelerators have been discovered in recent years, so far the search for the sources of the highest energy Galactic cosmic rays has been unsuccessful,” they explained.
New observations of the Galactic center region, reported yesterday in the journal Nature, provide “direct indications for such PeV cosmic-ray acceleration.”
With unique measurements, Dr. Abramowski and co-authors were able to pinpoint the source of these very-high-energy particles.
“Somewhere within the central 33 light-years of the Milky Way there is an astrophysical source capable of accelerating protons to energies of about one petaelectronvolt, continuously for at least 1,000 years,” said team member Dr. Emmanuel Moulin, from CEA Saclay, France.
In analogy to the ‘TeVatron,’ the first human-built accelerator that reached energies of 1 TeV, the newly-discovered cosmic accelerator has been dubbed the ‘PeVatron.’
“We propose that the supermassive black hole Sagittarius A* is linked to this PeVatron,” the scientists said.
“Sagittarius A* went through active phases in the past, as demonstrated by X-ray outbursts and an outflow from the Galactic center.”
“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 ten million years, and therefore should be considered as a viable alternative to supernova remnants as a source of petaelectronvolt Galactic cosmic rays,” they said.
_____
A. Abramowski et al. (HESS Collaboration). Acceleration of petaelectronvolt protons in the Galactic Centre. Nature, published online March 16, 2016; doi: 10.1038/nature17147
