2015A&A...575L...9M

The gamma-ray binary LS I +61°303 shows multiple periodicities. The timing analysis of 6.7yr of GBI radio data and of 6yr of Fermi-LAT GeV gamma-ray data both have found two close periodicities P_1,GBI=26.49±0.07d, P_2,GBI=26.92±0.07d and P_1,γ=26.48±0.08d, P_2,γ=26.99±0.08d. The system LS I +61°303 is the object of several continuous monitoring programs at low and high energies. The frequency difference between ν₁ and ν₂ of only 0.0006d^–1 requires long-term monitoring because the frequency resolution in timing analysis is related to the inverse of the overall time interval. The Owens Valley Radio Observatory (OVRO) 40m telescope has been monitoring the source at 15GHz for five years and overlaps with Fermi-LAT monitoring. The aim of this work is to establish whether the two frequencies are also resolved in the OVRO monitoring. We analysed OVRO data with the Lomb-Scargle method. We also updated the timing analysis of Fermi-LAT observations. The periodograms of OVRO data confirm the two periodicities P₁,OVRO₌ 26.5±0.1d and P₂,x OVRO₌ 26.9±0.1d. The three independent measurements of P₁ and P₂ with GBI, OVRO, and Fermi-LAT observations confirm that the periodicities are permanent features of the system LS I +61°303. The similar behaviours of the emission at high (GeV) and low (radio) energy when the compact object in LS I +61°303 is toward apastron suggest that the emission is caused by the same periodically (P₁) ejected population of electrons in a precessing (P₂) jet.