Origin of Two Types of X-Ray Outbursts in Be/X-Ray Binaries. I. Accretion Scenarios

We propose the new scenario for X-ray outbursts in Be/X-ray binaries that normal and giant outbursts are respectively caused by radiatively inefficient accretion flows (RIAFs) and Bondi-Hoyle-Lyttleton (BHL) accretion of the material transferred from the outermost part of a Be disk misaligned with the binary orbital plane. Based on simulated mass-transfer rates from misaligned Be disks, together with simplified accretion flow models, we show that mass-accretion rates estimated from the luminosity of the normal X-ray outbursts are consistent with those obtained with advection-dominated accretion flows, not with the standard, radiative-cooling dominated, accretion. Our RIAF scenario for normal X-ray outbursts resolves problems that have challenged the standard disk picture for these outbursts. When a misaligned Be disk crosses the orbit of the neutron star, e.g., by warping, the neutron star can capture a large amount of mass via BHL-type accretion during the disk transit event. We numerically show that such a process can reproduce the X-ray luminosity of giant X-ray outbursts. In the case of very high Be disk density, the accretion flow associated with the disk transit becomes supercritical, giving rise to the luminosity higher than the Eddington luminosity.