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Black holes of stellar mass and neutron stars in binary systems are first detected as hard X-ray sources using high-energy space telescopes. Relativistic jets in some of these compact sources are found by means of multiwavelength observations with ground-based telescopes. The X-ray emission probes the inner accretion disk and immediate surroundings of the compact object, whereas the synchrotron emission from the jets is observed in the radio and infrared bands, and in the future could be detected at even shorter wavelengths. Black-hole Xray binaries with relativistic jets mimic, on a much smaller scale, many of the phenomena seen in quasars and are thus called microquasars. Because of their proximity, their study opens the way for a better understanding of the relativistic jets seen elsewhere in the Universe. From the observation of two-sided moving jets it is inferred that the ejecta in microquasars move with relativistic speeds similar to those believed to be present in quasars. The simultaneous multiwavelength approach to microquasars reveals in short timescales the close connection between instabilities in the accretion disk seen in the X-rays, and the ejection of relativistic clouds of plasma observed as synchrotron emission at longer wavelengths. Besides contributing to a deeper comprehension of accretion disks and jets, microquasars may serve in the future to determine the distances of jet sources using constraints from special relativity, and the spin of black holes using general relativity.

Radio imaging of the microquasar GRS 1915+105 with the Very Long Baseline Array (VLBA) over a range of wavelengths (13, 3.6, 2.0 and 0.7 cm), in different states of the black hole binary, always resolves the nucleus as a compact jet of length ∼10λ cm AU. The nucleus is best imaged at the shorter wavelengths, on scales of 2.5 -7 AU (0.2 -0.6 mas resolution). The brightness temperature of the core is T B ≥10 9 K, and its properties are better fit by a conically expanding synchrotron jet model, rather than a thermal jet. The nuclear jet varies in ∼30 min during minor X-ray/radio outbursts, and re-establishes within ∼18 hours of a major outburst, indicating the robustness of the X-ray/radio (or disk/jet) system to disruption.At lower resolution (80-240 AU), more extended ejecta are imaged at ∼500 AU separation from the stationary core. Time-lapse images clearly detect the superluminal motion of the ejecta in a few hours. The measured velocity is 1.5±0.1 c (D/12 kpc) for the approaching component, and is consistent with ballistic motion of the ejecta from 500 AU outwards, perhaps even since birth.The axis of the ejecta differs by ≤12 • clockwise from the axis of the AU-scale jet, measured in the same observation. Both axes are stable in time (±5 • ), the AU scale for two years, and the large scale for over four years. Astrometry over two years relative to an extragalactic reference locates the black hole to ±1.5 mas, and its secular parallax due to Galactic rotation is 5.8±1.5 mas yr −1 , consistent with a distance of 12 kpc. Finally, a limit of ≤100 km s −1 is placed on its proper-motion with respect to its neighbourhood.Some accreting black holes of stellar mass (e.g. Cyg X-1, 1E 1740-2942, GRS 1758 and supermassive black holes at the centre of galaxies (e.g. Sgr A * ) lack evidence of large flares and discrete transient ejecta, -3but have compact radio cores with steady, flat-spectrum 'plateau' states, like GRS 1915+105. Until now GRS 1915+105 is the only system where both AUscale steady jets and large-scale superluminal ejections have been unambiguously observed. Our observations suggest that the unresolved flat-spectrum radio cores of accreting black holes are compact quasi-continuous synchrotron jets.

The non-thermal 3.6 cm radio continuum emission from the young stars S1 and DoAr21 in the core of Ophiuchus, has been observed with the Very Long Baseline Array (VLBA) at 6 and 7 epochs, respectively, between June 2005 and August 2006. The typical separation between successive observations was 2 to 3 months. Thanks to the remarkably accurate astrometry delivered by the VLBA, the trajectory described by both stars on the plane of the sky could be traced very precisely, and modeled as the superposition of their trigonometric parallax and a uniform proper motion. The best fits yield distances to S1 and DoAr21 of 116.9 +7.2 −6.4 pc and 121.9 +5.8 −5.3 pc, respectively. Combining these results, we estimate the mean distance to the Ophiuchus core to be 120.0 +4.5 −4.2 pc, a value consistent with several recent indirect determinations, but with a significantly improved accuracy of 4%. Both S1 and DoAr21 happen to be members of tight binary systems, but our observations are not frequent enough to properly derive the corresponding orbital parameters. This could be done with additional data, however, and would result in a significantly improved accuracy on the distance determination.

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