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Most powerful source of cosmic radiation

- Wits University

Super-massive black hole at center of Galaxy is likely to accelerate cosmic ray particles to energies 100 times larger than the Large Hadron Collider at CERN.

In yet another discovery emanating from detailed analysis of the latest data from the High Energy Stereoscopic System (H.E.S.S.) observatory in Namibia, an international team of scientists, including astrophysicists from the University of the Witwatersrand in Johannesburg, announced they have found the most powerful source of cosmic radiation at the center of our Galaxy.

The findings, published Wednesday evening (16 March 2016) in the scientific journal, Nature, 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 Professor Sergio Colafrancesco, DST/NRF SKA Research Chair in the School of Physics at Wits University, 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 super-massive 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.

Says 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 hence 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, since more than a century, the origin of the cosmic rays remains one of the most enduring mysteries of science.

Fortunately, cosmic rays interact with light and gas in the neighbourhood 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 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 “Cherenkov light”.

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 very-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.

A powerful cosmic Pevatron in the centre of the Milky Way: Artistic view of the gamma-ray emission coming from the interaction of relativistic protons, injected by the central super-massive black hole Sgr A*, with the giant clouds of the Central Molecular Zone.

About the latest discovery

Theoretical arguments and direct measurement of cosmic rays reaching the 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 very 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 1PeV

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.

Says Aion Viana (MPIK, Heidelberg): “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,” says Emmanuel Moulin (CEA, 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,” adds Stefano Gabici (CNRS, 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 the 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.

Publication: Acceleration of Petaelectronvolt protons in the Galactic Centre; H.E.S.S. collaboration; corresponding authors: F. Aharonian, S. Gabici, E. Moulin et A. Viana; Nature (16 March 2016).

South African involvement: The H.E.S.S. telescope is operated by an international collaboration of scientists with a strong involvement by South African universities, in particular Wits University, North West University, and the Universities of the Free State and Johannesburg. Wits physicists are particularly involved in data analysis techniques, the development of theoretical interpretation tools of both extragalactic and galactic sources, and in the operational shifts at the telescope location in Namibia.