The findings reveal for the first time a source of cosmic radiation at energies never observed before in the Milky Way.
The supermassive black hole at the center of our galaxy is likely to accelerate cosmic ray particles to energies 100 times larger than those achieved at the largest terrestrial particle accelerator, the Large Hadron Collider at European Organization for Nuclear Research (CERN) in Switzerland.
According to Sergio Colafrancesco from the University of the Witwaatersrand, the discovery sheds light simultaneously on two fundamental aspects of nature: the understanding of the origin of cosmic rays since the discovery of their extraterrestrial nature in 1912, and the ability of the supermassive black hole at the center of our galaxy (as in almost every other galaxy in the universe) to accelerate the most energetic particles produced in the universe.
Said Colafrancesco: “We are therefore able to use the center of our galaxy as a laboratory for testing the nature and the interaction properties of the most extreme particles in the universe, beyond the capability of any viable terrestrial accelerator.
“In future, our understanding of how cosmic rays travel in the galaxy on their path to the Earth and how they interact with the material of which our galaxy is made of, will also be further boosted by combining the H.E.S.S. gamma-ray measurements in the inner 30 light-years of our galaxy with the radio measurements of the magnetic field in the same region that will be produced by the Square Kilometre Array (SKA) and its precursor MeerKAT radio telescope.”
An enduring mystery
The Earth is constantly bombarded by high-energy particles (protons, electrons, and atomic nuclei) of cosmic origin, particles that comprise the so-called “cosmic radiation.” These “cosmic rays” are electrically charged and are strongly deflected by the interstellar magnetic fields that pervade our galaxy. Their path through the cosmos is randomized by these deflections, making it impossible to directly identify the astrophysical sources responsible for their production. Thus, for more than a century, the origin of the cosmic rays has remained one of the most enduring mysteries of science.
Fortunately, cosmic rays interact with light and gas in the neighborhood of their sources and thus produce gamma-rays. These gamma rays travel in straight lines, undeflected by magnetic fields, and can therefore be traced back to their origin. When a high-energy gamma ray reaches Earth, it interacts with a molecule in the upper atmosphere, producing a shower of secondary particles that emit a short pulse of “Cherenkov light.”
H.E.S.S. observations
By detecting these flashes of light using telescopes equipped with large mirrors, sensitive photo detectors, and fast electronics, more than 100 sources of high-energy gamma rays have been identified over the past three decades. The H.E.S.S. observatory represents the latest generation of such telescope arrays. It is operated by scientists from 42 institutions in 12 countries, including astrophysicists from the Wits School of Physics, with major contributions by MPIK Heidelberg, Germany, and CEA, CNRS, France.
Today we know that cosmic rays with energies up to approximately 100 tera-electronvolts (TeV) are produced in our galaxy by objects such as supernova remnants and pulsar wind nebulae.
About the latest discovery
Theoretical arguments and direct measurement of cosmic rays reaching Earth however indicate that the cosmic ray factories in our galaxy should be able to provide particles to at least one peta-electronvolt (PeV).
While many multi-TeV accelerators where discovered during the last 10 years, so far the search for the sources of the highest energy galactic cosmic rays remained unsuccessful.
Now this latest analysis by the H.E.S.S. Collaboration as described in their research letter, titled “Acceleration of Petaelectronvolt Protons in the Galactic Centre,” finally provide strong indications.
During the first three years of observations, the H.E.S.S. uncovered a powerful point source of gamma rays in the galactic center region, as well as diffuse gamma-ray emission from the giant molecular clouds that surround it in a region approximately 500 light-years across.
These molecular clouds are bombarded by cosmic rays moving at close to the speed of light, which produce gamma rays through their interactions with the clouds’ material. A remarkably good spatial coincidence between the observed gamma rays and the density of material in the clouds indicated the presence of one or more accelerators of cosmic rays in that region. However, the nature of the source remained a mystery.
Observing 1 PeV
Deeper observations obtained by H.E.S.S. between 2004 and 2013 shed new light on the processes powering the cosmic rays in this region.
Aion Viana from MPIK in Heidelberg said: “The unprecedented amount of data and progress made in analysis methodologies enables them to measure simultaneously the spatial distribution and the energy of the cosmic rays.”
With these unique measurements, the H.E.S.S. scientists are for the first time able to pinpoint the source of these 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 peta-electronvolt continuously for at least 1,000 years,” said Emmanuel Moulin from CEA in Saclay.
In analogy to the “Tevatron,” the first human-built accelerator that reached energies of 1 tera-electronvolt (TeV), this new class of cosmic accelerator has been dubbed a “Pevatron.”
“With H.E.S.S., we are now able to trace the propagation of PeV protons in the central region of the galaxy,” said Stefano Gabici from CNRS in Paris.
Supermassive black hole at the center of our galaxy
The center of our galaxy is home to many objects capable of producing cosmic rays of high-energy, including, in particular, a supernova remnant, a pulsar wind nebula, and a compact cluster of massive stars.
However, the supermassive black hole located at the center of the galaxy, called Sgr A*, is the most plausible source of the PeV protons. The scientists say several possible acceleration regions can be considered, either in the immediate vicinity of the black hole or further away where a fraction of the material falling into the black hole is ejected back into the environment and there initiates acceleration of particles.
Measurement of the energy spectrum of the gamma-ray emission by H.E.S.S. allows the spectrum of the protons that have been accelerated by the central black hole to be inferred. It turns out that Sgr A* is very likely accelerating protons to PeV energies.
However, it cannot account for the total flux of cosmic rays detected at Earth. The scientists argue that if it were more active in the past then it could indeed be responsible for the entire flux of today’s cosmic rays. And if true, this would put an end to the century-old debate about the origin of the galactic cosmic rays.
