Today, in Science Magazine, the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) Telescope reports the discovery of variable very high energy (VHE) gamma-ray emission from a microquasar.

Microquasars are binary star systems consisting of a massive ordinary star -much more massive than the Sun- and a compact object that is either a neutron star or a black hole (Fig. 1). The stars are gravitationally bound to each other, orbiting around a common center, and when they are close enough, the effect of the mutual tides can cause a burst of mass transfer from the normal star that falls onto the compact companion. A part of the gravitational energy released by this exchange is transformed into mechanical work, producing jets of particles that are ejected at close to the speed of light along with spectacular emission of radiation. Microquasars appear to be scaled-down versions of active galactic nuclei, in particular quasars. These galaxies also emit jets of relativistic particles, but their central engine is a supermassive black hole having millions of solar masses. For quasars the formation and ejection of the jets takes many years while for microquasars these changes happen in intervals of days. It is thus possible, on ordinary human timescales, to study the individual episodes. Microquasars are also implicated as a source of the highest energy particles, cosmic rays, the origin of which has remained a mystery since their discovery almost a hundred years ago.

Fig. 1. Artist view of a microquasar. A compact object (a black hole or a neutron star) orbits around a massive normal star. The latter loses matter into an accretion disk that surrounds the compact object. The matter falling into the compact object is ejected in the form of jets of relativistic particles. In the case of LS I +61 303 the compact object is likely a neutron star that completes an orbit around the massive star every 26 days. It always displays jets.

The study of microquasars represents one of the most important additions to the recently born field of VHE gamma-ray astrophysics. VHE gamma-rays are the most energetic radiation observed. The emission is produced in the most violent phenomena of our Universe, such as supernova explosions and quasars. They can reach the Earth, albeit at a very low rate (typically less than one gamma-ray per square meter per week). MAGIC detects gamma-rays through the short light flashes that they produce as they enter the atmosphere. With a 17 m diameter mirror, MAGIC is the largest telescope exploiting this experimental technique. It is located at the Roque de los Muchachos observatory site on the Canary Island of La Palma (Spain). The MAGIC team is composed by more than 130 scientists from Germany, Spain, Italy, Switzerland, Poland, Armenia, Finland, Bulgaria and USA. A second MAGIC telescope is under construction at a site around 80m from the first telescope (Fig. 2).

Fig. 2. The second MAGIC telescope is under construction in the observatory Roque de los Muchachos, in the Canary Island of La Palma (Spain). The first MAGIC telescope is seen at the background. Picture: D. Dorner.

Between October 2005 and March 2006, MAGIC observed one of the approximately 20 known microquasars, called LS I +61 303, detecting the source at a rate of one gamma-ray per square meter per month. More important, the MAGIC observations show that LS I +61 303 varies with time (see Fig. 3). The binary system was observed at different moments around the orbit and clear variability has been found that is consistent with the orbital changes in aspect of the compact object with even a hint of periodicity. This shows that the VHE emission is directly related to the interaction between the two stars. This new discovery indicates that gamma-ray production may be a common property of microquasars.

Future observations of LS I +61 303 with MAGIC, together with theoretical interpretation of the present results, will further our understanding of the mechanisms for gamma-ray production and absorption in microquasars and in other objects displaying relativistic jets.

Fig. 3. Map of gamma-rays measured by MAGIC around the location of LS I +61 303 at two different moments along the orbital cycle. (A) When the two stars are closest to one another (periastron passage). (B) One third of an orbit away from the periastron passage.

The MAGIC Telescope is on the Roque de los Muchachos observatory, on La Palma, and it is operated by an international group including scientists from the following Spanish research institutions:

· Institut de Física d’Altes Energies (IFAE)

· Universitat Autònoma de Barcelona (UAB)

· Universidad Complutense de Madrid (UCM)

· Universitat de Barcelona (UB)

MAGIC is the largest telescope of its class, and includes many technological innovations in its design. This has rendered possible the present discovery. The camera that records the signals from the gamma ray has been designed, build and it is maintain by the team of researchers, engineers and technicians from IFAE.

The research that has led to the present results has been carried out at IFAE. It has been directed by Dr. Juan Cortina (Ramón y Cajal researcher) and Dr. Javier Rico (Juan de la Cierva researcher) and is part of Mrs. Nuria Sidro PhD thesis. The three of them work in the research group directed by Dr. Manel Martínez (Investigador Catedràtic at IFAE and MAGIC’s spokesperson from several years). Other researchers having actively participated in this work are Dr. Diego Torres (Ramón y Cajal at Institut d’Estudis Espacials de Catalunya/Centro Superior de Investigaciones Científicas) and the group of Dr. Josep María Paredes (Profesor titular at UB).