-2 -Microquasars are stellar x-ray binaries that behave as a scaled down version of extragalactic quasars. The star LS 5039 is a new microquasar system with apparent persistent ejection of relativistic plasma at a 3 kiloparsec distance from the sun. It may also be associated with a γ-ray source discovered by the Energetic Gamma Ray Experiment Telescope (EGRET) on board the COMPTON-Gamma Ray Observatory satellite. Before the discovery of LS 5039, merely a handful of microquasars had been identified in the Galaxy, and none of them was detected in high-energy γ-rays.-3 -The V = 11.2 magnitude star LS 5039 (1) has been recently identified as a nearby high-mass x-ray binary with spectral type O7V((f)) (2) and persistent radio emission (3,4). Here, we report high-resolution radio observations with the Very Long Baseline Array (VLBA) and the Very Large Array (VLA) that reveal that LS 5039 is resolved into bipolar radio jets emanating from a central core.Because LS 5039 appeared unresolved (≤ 0.1 ′′ ) to the VLA alone, we proceeded to study this object with milliarc sec resolution using the VLBA at the frequency of 5 GHz (6 cm wavelength) on 8 May 1999. The VLA in its phased array mode, equivalent to a dish of 115 m diameter, also participated as an independent station, providing sensitive baselines with the VLBA antennas. The source 3C345 was used as a fringe-finder, whereas J1733−1304 was the phasing source for the VLA. The data were calibrated using standard procedures in unconnected radio interferometry. The resulting pattern of the observed visibility amplitudes, decaying as a function of baseline length, indicated that LS 5039 had structure at milliarc sec scales.The final synthesis map ( Fig. 1) shows that bipolar jets emerge from a central core. A deconvolved angular size of about 2 milliarc sec is estimated for the core. The jets extend over 6 milliarc sec on the sky oriented along a position angle (PA) of 125• with respect to the North, and they account for 20% of the total 16 mJy flux density. To obtain some order of magnitude estimates, we will assume that the overall size of the radio source is approximately 6 × 2 milliarc sec 2 . This implies a high brightness temperature of ∼ 9.4 × 10 7 K, indicating synchrotron radiation. The LS 5039 radio spectrum as a function of frequency ν, namely S ν ∝ ν α , often displays a negative spectral index α = −0.5 in agreement with a non-thermal optically thin emission mechanism (3,4). The detection of jets occurred at a time when the source was at its typical persistent level of radio emission, and only moderately variable, as inferred from concurrent radio monitoring by the Green Bank Interferometer (GBI) (Fig. 2).The absence of any precursor outburst for the radio jets strongly suggests that they are always present and continuously emanating from the core. The flux density ratio between X-ray binaries with collimated radio jets belong to the class of galactic microquasars.The production of jets is almost certainly related to the capture of matter from a normal star by a...

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Abstract. Here we discuss two consecutive MERLIN observations of the X-ray binary LS I +61• 303. The first observation shows a double-sided jet extending up to about 200 AU on both sides of a central source. The jet shows a bent S-shaped structure similar to the one displayed by the well-known precessing jet of SS 433. The precession suggested in the first MERLIN image becomes evident in the second one, showing a one-sided bent jet significantly rotated with respect to the jet of the day before. We conclude that the derived precession of the relativistic (β = 0.6) jet explains puzzling previous VLBI results. Moreover, the fact that the precession is fast could be the explanation of the never understood short term (days) variability of the associated gamma-ray source 2CG 135+01/3EG J0241+6103.

Abstract. We present here new observations conducted with the EVN and MERLIN of the persistent microquasar LS 5039 discovered by Paredes et al. (2000) with the VLBA. The new observations confirm the presence of an asymmetric two-sided jet reaching up to ∼1000 AU on the longest jet arm. The results suggest a bending of the jets with increasing distance from the core and/or precession. The origin and location of the high-energy gamma-ray emission associated with the system is discussed and an estimate of the magnetic field at the base of the jet given. Our results suggest a well collimated radio jet. We also comment on new observing strategies to be used with satellites and forthcoming detectors, since this persistent source appears to be a rather good laboratory to explore the accretion/ejection processes taking place near compact objects.