Scientists from the University of the Witwatersrand (Wits) form part of a international team of astronomers who have found three very bright gamma ray sources in the Large Magellanic Cloud, using the High Energy Stereoscopic System (HESS) observatory in Namibia. This is the first time that individual sources of very high-energy gamma rays have been isolated (“resolved”) in a galaxy outside the Milky Way. “This is a very important breakthrough for the team,” affirmed Wits School of Physics professor Sergio Colafrancesco. (He holds the Department of Science and Technology/National Research Foundation Square Kilometre Array Research Chair at the School.)
HESS is located in the Khomas Highland, some 100 km south-west of the Namibian capital of Windhoek. It is composed of four Phase 1 Imaging Atmospheric Cherenkov telescopes, which have all been operational since December 2013, and one, much larger, Phase 2 telescope of the same type, operational since July 2012. HESS is operated by an international group of more than 170 scientists from 32 scientific institutions in 12 countries. In addition to Namibia and South Africa, these are Armenia, Australia, Austria, the Czech Republic, France, Germany, Ireland, Poland, Sweden and the UK.
Imaging Air Cherenkov telescopes work as follows: when high-energy gamma rays reach the earth, they interact with the particles that make up the upper atmosphere and create showers of secondary particles. The numbers of secondary particles in these showers reach their maximum at an altitude of around 10 000 m. The showers then die out as they enter the lower atmosphere. But, as the shower particles are effectively travelling at the speed of light, they emit a faint blue light known as Cherenkov light. This light develops as a cone, following the direction of the original gamma ray which created the secondary particle shower. This cone forms a pool of light on the ground (invisible to the human eye), which can be detected by a telescope which lies within that pool, provided it has a big enough mirror area to collect the requisite number of photons.
HESS is an array because, with a single telescope, it is difficult to reconstruct the geometry of the secondary particle shower. With an array, the shower can be observed from different positions and the shower geometry can be stereoscopically reconstructed. This allows the course of the original gamma ray to be determined and tracked back to its source. The intensity of the image received indicates the energy of the gamma ray. The shape of the image distinguishes gamma ray sources from secondary particle showers generated by other phenomena, such as cosmic rays.
Gamma rays are important because they are the best means of identifying supernovae remnants, including pulsar wind nebulae. A supernova is the gigantic explosion of a massive star. This usually results in the creation of an incredibly dense and small neutron star. (In some cases, black holes are created.) Neutron stars spin very rapidly and emit narrow beams of intense energy at radio and other wavelengths. If these beams are pointed towards earth, they can be detected and their source stars are called pulsars. Neutron stars, including pulsars, lose energy over time because they suffer from a magnetic and particle outflow, known as the pulsar wind. The interaction between this pulsar wind and the environment around the pulsar itself creates the pulsar wind nebula.
The Large Magellanic Cloud is a dwarf galaxy that is a satellite of our own galaxy, the Milky Way. It is some 170 000 light years from earth. The three gamma ray sources were discovered following 210 hours of observation by the HESS telescope of the Tarantula Nebula within the cloud. “It paves the way to study external galaxies with very high-E [energy] telescopes such as HESS and then later with the planned Cherenkov Telescope Array in Namibia,” highlighted Colafrancesco. “It will lead us to re-examine galaxy evolution and answer questions such as how high-E particles can affect the evolution of cosmic structures in the universe, principally galaxies, and the life cycles of matter in galaxies.”
The three sources are very diverse: one is one of the most powerful supernova remnants, another is very powerful pulsar wind nebula and the third is a 270-light year diameter superbubble. A superbubble is a shell of interstellar gas and particles created by stellar winds from numerous stars and by shock waves from supernovae.