Understanding the periodicities in radio and GeV emission from LS I +61°303

Abstract

Accretion models predict two ejections along the eccentric orbit of LS I +61 303: one major ejection at periastron and a second, lower ejection towards apastron. We develop a physical model for LS I +61 303 in which relativistic electrons are ejected twice along the orbit. The ejecta form a conical jet that is precessing with P2. The jet radiates in the radio band by the synchrotron process and the jet radiates in the GeV energy band by the external inverse Compton and synchrotron self-Compton processes. We compare the output fluxes of our physical model with two available large archives: OVRO radio and Fermi Large Area Telescope (LAT) GeV observations, the two databases overlapping for five years. The larger ejection around periastron passage results in a slower jet, and severe inverse Compton losses result in the jet also being short. While large gamma-ray emission is produced, there is only negligible radio emission. Our results are that the periastron jet has a length of 3.0 10^6 rs and a velocity beta ~ 0.006, whereas the jet at apastron has a length of 6.3 10^7 rs and beta ~ 0.5.