A New Puzzling Periodic Signal in GeV Energies of the $\gamma$-Ray Binary LS I +61$^\circ$303

Abstract

LS I + 61$^\circ$303 is a high-mass X-ray binary system comprising a massive Be star and a rapidly rotating neutron star. Its spectral energy distribution across multi-wavelengths categorizes it as a $\gamma$-ray binary system. In our analysis of LS I + 61$^\circ$303 using Fermi-LAT observations, we not only confirmed the three previously discussed periodicities of orbital, superorbital, and orbital-superorbital beat periods observed in multi-wavelength observations, but also identified an additional periodic signal. This newly discovered signal exhibits a period of $\sim$26.3 day at a $\sim7\sigma$ confidence level. Moreover, the power spectrum peak of the new signal gradually decreases as the energy increases across the energy ranges of 0.1-0.3, 0.3-1.0, and 1.0-500.0 GeV. Interestingly, a potential signal with a similar period was found in data obtained from the Owens Valley Radio Observatory 40 m telescope. We suggest that the newly discovered periodic signal may originate from a coupling between the orbital period and the retrograde stellar precession period.