Long-term OVRO monitoring of LS i +61 ° 303: Confirmation of the two close periodicities

Abstract

Context. The gamma-ray binary LS I +61°303 shows multiple periodicities. The timing analysis of 6.7 yr of GBI radio data and of 6 yr of Fermi-LAT GeV gamma-ray data both have found two close periodicities P1,GBI = 26.49 ± 0.07 d, P2,GBI = 26.92 ± 0.07 d and P1,γ = 26.48 ± 0.08 d, P2,γ = 26.99 ± 0.08 d. Aims. 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 0.0006 d-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) 40 m telescope has been monitoring the source at 15 GHz 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.Methods. We analysed OVRO data with the Lomb-Scargle method. We also updated the timing analysis of Fermi-LAT observations.Results. The periodograms of OVRO data confirm the two periodicities P1,OVRO = 26.5 ± 0.1 d and P2,OVRO = 26.9 ± 0.1 d. Conclusions. 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.