Revisiting LS I +61 303 with VLBI astrometry

TL;DR

This study uses VLBA astrometry over multiple epochs to analyze the precessing radio jet of LS I +61 303, accurately determining its precession period, jet geometry, and system motion, revealing insights into its formation and dynamics.

Contribution

The paper provides the first precise measurement of the precession period and jet geometry of LS I +61 303 using multi-epoch VLBA observations, improving understanding of its system dynamics.

Findings

Precession period of 26.926 days determined.

Jet trajectory repeats after 9-year gap.

System has very low kick velocity (<20 km/s).

Abstract

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.