Non-periodic type I Be/X-ray binary outbursts

Abstract

Type I Be/X-ray binary outbursts are driven by mass transfer from a Be star decretion disc to a neutron star companion during each orbital period. Treiber et al. (2021) recently observed non-periodic type I outbursts in RX J0529.8-6556 that has unknown binary orbital properties. We show that non-periodic type I outbursts may be temporarily driven in a low eccentricity binary with a disc that is inclined sufficiently to be mildly unstable to Kozai-Lidov oscillations. The inclined disc becomes eccentric and material is transferred to the neutron star at up to three locations in each orbit: when the neutron star passes the disc apastron or one of the two nodes of the disc. The timing and magnitude of each vary with the disc argument of periapsis and longitude of the ascending node that precess in opposite directions. Calculating the orbital period of the RX J0529.8-6556 system is non-trivial but we suggest it maybe >300 day, longer than previous estimates.