  The erratic spin history of Vela X-1 shows some continuous spin-up/spin-down
trend over tens of days. We study the orbital profile and spectral property of
Vela X-1 in these spin-up/spin-down intervals, using the spin history monitored
by Fermi/GBM and light curve from Swift/BAT and MAXI/GSC. The BAT fluxes in the
spin-up intervals are about 1.6 times those of the spin-down intervals for
out-of-eclipse orbital phases. The spin-up intervals also show a higher column
density than the spin-down intervals, indicating there are more material on the
orbital scale for the spin-up intervals. It could be due to the variation of
the stellar wind of the optical star (HD 77581) on tens of days. The varying
wind could lead to alternating prograde/retrograde accreting flow to the
neutron star, which dominates the transfer of the angular momentum to Vela X-1,
but not the total observed luminosity.
