Orbital variability of the optical polarization of γ-ray binary LS I +61 303

We present the results of BVR polarimetric and photometric observations of the γ-ray binary LS I +61 303, performed from November 2016 to January 2019. We detect statistically significant variability of the Stokes parameters with a period P = 13.25 d, close to half of the orbital period Porb = 26.496 d. Using a model of Thomson scattering by a cloud that orbits the Be star, we obtained constraints on the orbital parameters, including a small eccentricity e < 0.2 and periastron phase of φp ≈ 0.6, which coincides with the peaks in the radio, X-ray, and TeV emission. By folding the photometry data acquired during a three-year time span with the orbital period, we found a linear phase shift of the moments of the brightness maximum, confirming the possible existence of superorbital variability.
{"references":["Paredes-Fortuny et al, 2015, Astronomy & Astrophysics, Volume 575, id.L6, 4 pp","Piirola et al, 2014, Proceedings of the SPIE, Volume 9147, id. 91478I 9 pp."]}