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microquasar LS5039.
Computer illustration of microquasar LS5039. Image credit: PPARC.

An international team of Astronomers have discovered a strange source of high energy gamma rays. The object is called LS5039, and it's known as a microquasar because of the kinds of radiation it generates. Microquasars are binary systems, where a regular star orbits a dead star, like a neutron star or black hole. Material streams from the regular star to the neutron star in such quantity that it starts to back up, like too much water going down a drain. What's unusual about LS5039 is that it's producing more high energy gamma rays than should be possible.

In a recent issue of Science Magazine, the High energy Stereoscopic System (H.E.S.S.) team of international astrophysicists reports the discovery of another new type of very high energy (VHE) gamma ray source.



However, sometimes the compact object receives more matter than it can cope with. The material is then squirted away from the system in a jet of matter moving at speeds close to that of light, resulting in a microquasar. Only a few such objects are known to exist in our Galaxy and one of them, an object called LS5039, has now been detected by the H.E.S.S. team.

In fact, the real nature LS5039 is something of a mystery. It is not clear what the compact object is. Some of the characteristics suggest it is a Neutron star, some that it is a black hole. Not only that, but the jet isn't much of a jet; although it is moving at about 20% of the speed of light, which might seem a lot, in the context of these objects it's actually quite slow.




The results were obtained using the High energy Stereoscopic System (H.E.S.S.) Telescopes in Namibia, in South-West Africa. This system of four 13 m diameter Telescopes is currently the most sensitive detector of VHE gamma-rays - radiation that is a million, million times more energetic than the visible light. These high energy gamma rays are quite rare even for relatively strong sources; only about one gamma ray per month hits a square metre at the top of the Earth's atmosphere. Also, since they are absorbed in the atmosphere, a direct detection of a significant number of the rare gamma rays would require a satellite of huge size. The H.E.S.S. Telescopes employ a trick - they use the atmosphere as detector medium. When gamma rays are absorbed in the air, they emit short flashes of blue light, named Cherenkov light, lasting a few billionths of a second. This light is collected by the H.E.S.S. Telescopes with large mirrors and extremely sensitive cameras and can be used to create images of astronomical objects as they appear in gamma-rays.


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