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Description
The gamma-ray binary LS I +61 303 is composed of a Be star and a compact companion orbiting in an eccentric orbit. Variable flux modulated with the orbital period of ~26.5d has been detected from radio to very high-energy gamma-rays. In addition, the system presents a superorbital variability of the phase and amplitude of the radio outbursts with a period of ~4.6yr. We present optical photometric observations of LS I +61 303 spanning ~1.5yr (table 1) and contemporaneous H-alpha equivalent width (EW) data (table 2). The optical photometry shows, for the first time, that the known orbital modulation suffers a positive orbital phase shift and an increase in flux for data obtained 1-yr apart. The orbital modulation of the EW presents, for the first time, a positive orbital phase shift. These orbital phase shifts of the thermal indicators are in line with the observed behavior for nonthermal indicators, such as X-ray or radio emission. This shows that there is a strong coupling between the thermal and nonthermal emission processes in the gamma-ray binary LS I +61 303.
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