One-sided jet at milliarcsecond scales in LS I+61°303

We present Very Long Baseline Interferometry (VLBI) observations of the high mass X-ray binary LS I +61 degrees 303, carried out with the European VLBI Network (EVN). Over the 11 hour observing run, performed similar to 10 days after a radio outburst, the radio source showed a constant flux density, which allowed sensitive imaging of the emission distribution. The structure in the map shows a clear extension to the southeast. Comparing our data with previous VLBI observations we interpret the extension as a collimated radio jet as found in several other X-ray binaries. Assuming that the structure is the result of an expansion that started at the onset of the outburst, we derive an apparent expansion velocity of 0.003 c, which, in the context of Doppler boosting, corresponds to an intrinsic velocity of at least 0.4 c for an ejection close to the line of sight. From the apparent velocity in all available epochs we are able to establish variations in the ejection angle which imply a precessing accretion disk. Finally we point out that LS I +61 degrees 303, like SS 433 and Cygnus X-1, shows evidence for an emission region almost orthogonal to the relativistic jet.