Annu. Rev. Astron. Astrophys. 1999. 37: 409-443 Copyright © 1999 by Annual Reviews. All rights reserved

12. CONCLUSIONS AND PERSPECTIVES

The study of relativistic jets from X-ray binaries in our own galaxy sets on a firmer basis the relativistic ejections seen elsewhere in the Universe. The analogy between quasars and microquasars led to the discovery of superluminal sources in our own galaxy, where it is possible to follow the motions of the two-sided ejecta. This permits astronomers to overcome the ambiguities that had dominated the physical interpretation of one-sided moving jets in quasars, and conclude that the ejecta consist mainly of matter moving with relativistic bulk motions, rather than waves propagating through a slowly moving jet. The Lorentz factors of the bulk motions in the jets from microquasars seem to be similar to those believed to be common in quasars. From the study of the two-sided moving jets in one microquasar, an upper limit for the distance to the source was derived, using constraints from special relativity.

Because of the relative short timescales of the phenomena associated with the flows of matter around stellar mass black holes, one can sample phenomena that we have not been able to observe in quasars. Of particular importance is to understand the connection between accretion flow instabilities observed in the X-rays, with the ejection of relativistic clouds of plasma observed in the radio, infrared, and possibly in the optical. The detection of synchrotron infrared flares implies that the ejecta in microquasars contain very energetic particles with particle Lorentz factors of at least 10³.

The discovery of microquasars opens several new perspectives that could prove to be particularly productive:

1. They provide a new method to determine distances using special relativity constraints. If the proper motions of the two-sided ejecta and the Doppler factor of a spectral line from one ejecta are measured, the distance to the source can be derived. With the rapid advance of technological capabilities in astronomy, this relativistic method to determine distances may be applied first to black hole jet sources in galactic binaries, and in the decades to come, to quasars.

(2) Microquasars are nearby laboratories that can be used to gain a general understanding of the mechanism of ejection of relativistic jets. The multiwavelength observations of GRS 1915+105 during large-amplitude oscillations suggest that the clouds are ejected during the replenishment of the inner accretion disk that follows its sudden disappearance beyond the last stable orbit around the black hole. In the context of these new data, the time seems to be ripe for new theoretical advances on the models of formation of relativistic jets.

(3) High sensitivity X-ray spectroscopy of jet sources with future X-ray space observatories may clarify the phenomena in accretion disks that are associated with the formation of jets.

(4) More microquasars will be discovered in the future. Among them, microblazars should appear as sources with fast and large amplitude variations in the observed flux. Depending on the beaming angle and bulk Lorentz factor they will be observed up to very high photon energies. Microquasars in our own Galaxy may be less extreme local analogs of the super-relativistic jets that seem to be associated with distant gamma-ray bursters.

(5) The spin of stellar mass black holes could be derived from the observed maximum stable frequency of the QPOs observed in the X-rays, provided the mass has been independently determined. However, theoretical work is needed to distinguish between the alternative interpretations that in the context of general relativity have been proposed for the maximum stable frequency of QPOs.

(6) Finally, microquasars could be test grounds for general relativity theory in the strong field limit. General relativity theory in weak gravitational fields has been successfully tested by observing in the radio wavelengths the expected decay in the orbit of a binary pulsar, an effect produced by gravitational radiation damping. We expect that phenomena observed in microquasars could be used in the future to investigate the physics of strong field relativistic gravity near the horizon of black holes.

ACKNOWLEDGMENTS

We are most grateful to Vivek Dhawan for permission to include in this review unpublished results from our VLBA observations, and to Sylvain Chaty and Josep Martí for help in producing Figure 7. We thank Ralph Spencer, Jacques Paul, Josep Martí, and Alan Harmon for comments on the original manuscript. We are also grateful to Philippe Durouchoux for the organization of the workshop on Relativistic Jet Sources in the Galaxy held in Paris on December 12-13, 1998, from which we have benefitted. During this work, LFR received partial support from CONACyT, México and DGAPA, UNAM.