  In classical supergiant X-ray binaries (SgXBs), the Bondi-Hoyle-Lyttleton
wind accretion was usually assumed, and the angular momentum transport to the
accretors is inefficient. The observed spin-up/spin-down behavior of the
neutron star in SgXBs is not well understood. In this paper, we report an
extended low state of Vela X-1 (at orbital phases 0.16-0.2), lasting for at
least 30 ks, observed with Chandra during the onset of an unusual spin-up
period. During this low state, the continuum fluxes dropped by a factor of 10
compared to the preceding flare period, and the continuum pulsation almost
disappeared. Meanwhile, the Fe K$\alpha$ fluxes of the low state were similar
to the preceding flare period, leading to an Fe K$\alpha$ equivalent width (EW)
of 0.6 keV, as high as the Fe K$\alpha$ EW during the eclipse phase of Vela
X-1. Both the pulsation cessation and the high Fe K$\alpha$ EW indicate an
axisymmetric structure with a column density larger than $10^{24}\rm cm^{-2}$
on a spatial scale of the accretion radius of Vela X-1. These phenomena are
consistent with the existence of an accretion disk that leads to the following
spin-up of Vela X-1. It indicates that disk accretion, although not always,
does occur in classical wind-fed SgXBs.
