Revisiting LS I +61 303 with VLBI astrometry
  We conducted multi-epoch VLBA phase reference observations of LS I +61 303 in
order to study its precessing radio jet. Compared to similar observations in
2006, we find that the observed elliptical trajectory of emission at 8.4 GHz
repeats after the 9-year gap. The accurate alignment of the emission patterns
yields a precession period of 26.926 +- 0.005 d, which is consistent with that
determined by Lomb-Scargle analysis of the radio light curve. We analytically
model the projection on the sky plane of the peak position of a precessing,
synchrotron-emitting jet, which traces an elliptical trajectory on the sky.
Comparing the simulation with the VLBA astrometry we improve our knowledge of
the geometry of the system.We measure the LS I +61 303 absolute proper motion
to be -0.150 +- 0.006 mas/yr eastward and -0.264 +- 0.006 mas/yr northward.
Removing Galactic rotation, this reveals a small, < 20 km/s, non-circular
motion, which indicates a very low kick velocity when the black hole was
formed.
