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

We propose a new scenario for X-ray outbursts in Be/X-ray binaries that normal and giant outbursts are, respectively, caused by radiatively inefficient accretion flows (RIAF's) and Bondi-Hoyle-Lyttleton (BHL) accretion of 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 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 a 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 have numerically shown that such a process can reproduce the X-ray luminosity of giant X-ray outbursts. In the case of a very high Be disk density, the accretion flow associated with the disk transit becomes supercritical, giving rise to a luminosity higher than the Eddington luminosity.