  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.
