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

Torque reversals and wind variations of X-ray pulsar Vela X-1