The binary system LS I +61 303 is the first microquasar in our Galaxy in which variable very high energy gamma-ray emission has been found. The discovery was reported in Science magazine on 18 May, in an article written by an international team of experts who work with the Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC). Among the team members are the researchers Josep Maria Paredes, Marc Ribó, Valentí Bosch-Ramon and Pol Bordas, of the Department of Astronomy and Meteorology at the UB.

Microquasars are binary star systems formed by a normal star and a compact object (a black hole or neutron star) that display jets of relativistic particles. The observations carried out by MAGIC between October 2005 and March 2006 have shown that the microquasar LS I +61 303 emits very high energy gamma-ray emissions that reach Earth. The radiation detected has energies above 100 giga-electronvolts, that is, 100 billion times more than visible light. “One of the most important results,” says Josep Maria Paredes, “is that the intensity of the emission of the gamma rays of this microquasar is variable.” In the case of the microquasar LS I +61 303, the compact object is probably a neutron star, which completes an orbital period around the donor star every 26 days. The pattern of emission of these rays is repeated every 26 days, indicating that the gamma rays are produced by high energy processes inside the binary system. According to the authors of the paper, this emission is probably due to the scattering of the photons of the donor star by the relativist particles from the jets. Future observations of the microquasar LS I +61 303 with MAGIC will aid the theoretical interpretation of the current results and will shed light on the mechanisms of gamma ray emission in microquasars and in astrophysical objects displaying relativistic jets in general.

Very high energy gamma rays are related to violent high energy phenomena in the Universe, such as blazars. Only a few objects in our Galaxy can generate them: for example, supernovae and pulsars. In the year 2000, in Science magazine the researchers Josep Maria Paredes, Marc Ribó, Josep Martí and Maria Massi reported the discovery of the first microquasar with high energy gamma ray emission in our Galaxy: the microquasar LS 5039, for which Josep Maria Paredes was awarded the Ciutat de Barcelona scientific research prize. This discovery was the first observational evidence of microquasars as unidentified high energy gamma ray sources in our Galaxy, a hypothesis which has now been confirmed by the new article published in Science.

The study of microquasars is one of the main aims of the space observatories that explore the Universe in X- and gamma rays. Microquasars present two features of relativistic physics: black holes of stellar origin, identified by their X radiation, and jets of relativistic particles. These jets are clearly detected in the radio band and show phenomena that can be observed throughout the electromagnetic spectrum, from radio to very high energy gamma rays. For this reason, they are considered as genuine «galactic laboratories» that can help to broaden our understanding of the physics behind the most intense gravitational fields and similar phenomena such as relativistic jets and so on that take place in quasars and in active galactic nuclei at large cosmic distances. Recent findings suggest a relation between microquasars and other types of high energy astrophysical objects, such as unidentified gamma ray sources in our Galaxy and Gamma Ray Bursts, which are the most energetic phenomena known in the Universe after the Big Bang. Today, among all the areas in which modern physics and astronomy converge, this is one of the most productive.

The first clear signs of the existence of microquasars emerged in 1990, when the SIGMA coded-mask telescope for gamma rays went into orbit. The first observations of the centre of our Galaxy with this telescope allowed the detection of a compact source of matter-antimatter annihilation. The cosmic object was named the “great annihilator” in the editorial published in the US journal Physics Today. As the object appeared to be a miniature replica of the structures observed in quasars, in a letter to Nature (1992), the scientists Félix Mirabel – doctor honoris causa at the UB in 2004 – and Luís F. Rodríguez proposed that the great annihilator could be considered a genuine microquasar. This far, in our Galaxy some 20 microquasars have been discovered.

In a microquasar, the ordinary star gravitates around the other cosmic object. When the two bodies are very close to each other, the more massive object (the star) loses matter to the more compact object (the black hole or neutron star) due to gravitational attraction. This matter forms a disk around the compact object (known as the accretion disk) which produces two jets of relativistic plasma along the axis of rotation of the accretion disk. The physical mechanisms of accretion and ejection of matter are similar to those at work in quasars (Quasi Stellar Radio Sources) but on a scale that is millions of times smaller. All these findings suggest that the association between the bipolar jets and the accretion disks is a universal phenomenon in quasars and microquasars. The predominant idea is that the jets of matter are propelled by the enormous energy of rotation of the compact objects and the accretion disks that surround them.

The project Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC) works with the largest Cherenkov telescope in the world, with a 17 m diameter, which detects gamma rays of galactic or extragalactic origin. It is located at the observatory Roque de los Muchachos in Garafía, La Palma (Canary Islands). Cherenkov radiation telescopes detect the light produced by very high energy gamma radiation and are the only ones that can perform gamma astronomy from the Earth.

MAGIC is the result of international cooperation between various universities and research institutes. Representing the UB are the astrophysicists Josep Maria Paredes, Marc Ribó, Valentí Bosch-Ramon and Pol Bordas, researchers at the Department of Astronomy and Meteorology. Josep Maria Paredes is also a member of the MAGIC Collaboration board, the project’s decision-making body. In Spain, in addition to the UB, the Institute of High Energy Physics in Barcelona, the Autonomous University of Barcelona and the Complutense University of Madrid are also members of MAGIC. Other participating countries are Germany, Italy, Finland, Poland, Switzerland, Armenia, Russia and the US. Spain, Germany and Italy are in charge of the construction of the instrument. The scientific projects in which MAGIC participates deal with supernovas, pulsars, gamma ray bursts (GRB), active galactic nuclei, dark matter and binary systems such as microquasars.

Image sources: MAGIC, European Space Agency (ESA), NASA and Félix Mirabel