A Precessing Stellar Disk Model for Superorbital Modulations of the Gamma-Ray Binary LS I+61° 303

Abstract

Gamma-ray binary LS I+61° 303 consists of a neutron star orbiting around a Be star with a period of P _orb ≃ 26.5 days. Apart from orbital modulations, the binary shows long-term flux variations with a superorbital period of ${P}_{\sup }\simeq 4.6\,{\rm{y}}{\rm{r}}$ as seen in nearly all wavelengths. The origin of this superorbital modulation is still not well understood. Under the pulsar wind–stellar outflow interaction scenario, we propose that the superorbital modulations of LS I+61° 303 could be caused by the precession of the Be disk. Assuming X-rays arise from synchrotron radiation of the intrabinary shock, we develop an analytical model to calculate expected flux modulations over the orbital and superorbital phases. The asymmetric two-peak profiles in orbital light curves and sinusoidal-like long-term modulations are reproduced under the precessing disk scenario. The observed orbital phase drifting of the X-ray peak and our fitting of long-term X-ray data indicate that the neutron star is likely orbiting around the star with a small eccentricity and periastron phase around Φ _p ∼ 0.6. We compare the Corbet diagrams of LS I+61° 303 with other Be/X-ray binaries, and the linear correlation in the ${P}_{\sup }\mbox{--}{P}_{\mathrm{orb}}$ diagram suggests that the precession of the Be disk in LS I+61° 303 is induced by the tidal torque of its neutron star companion.

Keywords

• Gamma-ray sources
• X-ray binary stars
• Compact binary stars
• X-ray sources