An image of the sky with the Milky Way and Magellanic Clouds and magnification of the Large Magellanic Cloud seen in infrared with gamma-ray emission map obtained via H.E.S.S. Milky Way image: © H.E.S.S. Collaboration, optical: SkyView, A. Mellinger. Photograph: © H.E.S.S. Collaboration.

According to the latest issue of the Science magazine, the High Energy Stereoscopic System  (H.E.S.S.) made it possible to discover three exceptionally bright high-energy gamma-ray emission sources in  the Large Magellanic Cloud, a nearby dwarf spiral galaxy. The discovery is particularly interesting due to the fact that all of those sources are in some way unique, as they include the brightest known pulsar wind nebula, the brightest gamma-ray emitting supernova residue, and a superbubble – a giant, spherical structure with the span of 270 light years located within an interstellar medium.

The discovery is all the more important, since never before have we observed a gamma-ray emission source of such high energy, and the superbubble represents a whole new class of high-energy gamma-ray emission sources, not present in our galaxy.

High-energy gamma-rays are created by cosmic particle accelerators, often much more powerful than CERN's Large Hadron Collider near Geneva. In sources of this type, charged particles are accelerated to extreme levels, and when they collide with photons or atoms of surrounding gas, they emit high-energy radiation (in this case, dozens of teraelectronvolts; 1 TeV = 10^12 eV).

High-energy gamma-rays are observed on Earth through the use of large optical telescopes with high-speed cameras registering Cherenkov radiation emitted by air showers generated in the higher layers of atmosphere by gamma photons. Located in Namibia, the international H.E.S.S. observatory is comprised of 4 medium-sized telescopes (12-metre diameter mirrors) and 1 large one (28-metre diameter mirror). The last one was constructed in collaboration with the consortium of Polish Academy of Sciences institutes and other higher education institutions, including the Jagiellonian University.

The Large Magellanic Cloud is a satellite of the Milky Way. It is a small galaxy, which is located about 170 light years away. Massive stars are created in the Large Magellanic Cloud very rapidly, and the frequency stars exploding as supernovas is five times higher than in our galaxy. In 1987, the youngest remains of a supernova in the Local Cluster, visible on the southern hemisphere even to the naked eye. Therefore, H.E.S.S. devoted most of the time to the search of high-energy gamma-rays from that galaxy. For over 200 hours, on clear and moonless nights, H.E.S.S observed the largest region of star formation, called the Tarantula Nebula. This resulted in the article published in Science. For the first time, three different particle accelerators, sources of high-energy gamma-ray radiation, were found outside of our galaxy.

The 30 Dor C superbubble was formerly a giant, almost-spherical structure. It seems it was created by a few supernovas which exploded during the last several thousands of years, and by strong stellar winds. According to H.E.S.S., if such structures are strong gamma-ray emitters, they must also be bubbles of cosmic radiation, i.e. accelerated, high-energy particles.

Pulsars are very rapidly rotating neutron stars possessing a strong magnetic field, which, apart from creating radio impulses, generates pulsar winds of charged high-energy particles, thus creating magnetised pulsar nebulae around them. The most famous object of this kind is the pulsar in the Crab Nebula, one of the brightest sources of gamma radiation in the sky. The PSR J0537-6910 pulsar discovered by H.E.S.S. in the Large Magellanic Cloud is very similar to that one, although ten times brighter.

The third discovered object, the residue left after the explosion of N132D supernova, seems to be one of the oldest and most powerful known objects emitting very high-energy gamma rays. Since its age is estimated to be between 2500 and 6000 years, it is brighter than any supernova remains in our galaxy.

JU astrophysicists – Dr Marek Jamrozy, Dr hab. Michał Ostrowski and Dr hab. łukasz Stawarz are co-authors of these three discoveries made by H.E.S.S.