A team of astronomers formed by Josep M. Paredes (Universitat de Barcelona), Josep Martí (Universidad de Jaén), Marc Ribó (Universitat de Barcelona) and Maria Massi (Max Planck Institut für Radioastronomie) has discovered the first microquasar showing high-energy gamma-ray emission. This discovery has been published in Science on its 2000 June 30 issue (vol. 288, page 2340), with the title Discovery of a High-Energy Gamma-Ray-Emitting Persistent Microquasar.

Only about a dozen of microquasars are currently known in the Galaxy.  These astronomical objects are X-ray binaries that behave as small scale analogs of extragalactic quasars. The authors of this research have discovered that the star LS 5039, located in the constellation of Scutum, is a new microquasar in our galaxy, with continuous ejection of relativistic plasma in the form of jets. These structures can be seen in the radio image obtained by this group using very long baseline interferometry, conducted with the VLBA and VLA instruments.

A microquasar is usually discovered after experiencing an outburst, which is detected by X-ray satellites. In contrast, the choice of LS 5039 as a microquasar candidate was the result of an accurate cross-identification of catalogs (in optical, radio and X-rays). The success of this method opens new perspectives regarding the possibility to discover new microquasars, which have not experienced to date any outburst allowing their discovery. This fact indicates that the microquasar phenomenon may not be as rare as it seems.

Moreover, this microquasar is the only X-ray source in the ROSAT bright source catalog that has a position in agreement with that of the high-energy gamma-ray source 3EG J1824-1514, which strongly suggests that we are dealing with the same object emitting at different bands of the electromagnetic spectrum. It must be noted that this is the first time that an association between a microquasar and a high-energy gamma-ray source has been found. This fact is important if we take into account that 63% of the 271 gamma-ray sources catalogued with the instrument EGRET (E > 100 Mev) are still unidentified. On the other hand, among the identified ones 5 are pulsars in our galaxy and the remaining ones are extragalactic sources. In consequence, this discovery reveals the existence of a new type of galactic source with gamma-ray emission, indicating that some of the still unidentified gamma-ray sources could belong to this new class of objects: gamma-ray microquasars.

Therefore, this discovery can represent an important step in the understanding of the high-energy gamma-ray sources, as well as in the understanding of microquasars.
 

For any question please contact Josep M. Paredes (josep@am.ub.es)