Context :

Aims : We investigate the spectral and temporal behavior of the high mass X-ray binary Vela X-1 during a phase of high activity , with special focus on the observed giant flares and off states .

Methods : INTEGRAL observed Vela X-1 in a long almost uninterrupted observation for two weeks in 2003 Nov/Dec .
The data were analyzed with OSA 7.0 and FTOOLS 6.2 .
We derive the pulse period , light curves , spectra , hardness ratios , and hardness intensity diagrams , and study the eclipse .

Results : In addition to an already high activity level , Vela X-1 exhibited several intense flares , the brightest ones reaching a maximum intensity of more than 5 Crab in the 20–40 keV band and several off states where the source was no longer detected by INTEGRAL .
We determine the pulse period to be 283.5320 \pm 0.0002 s , which is stable throughout the entire observation .
Analyzing the eclipses provided an improvement in the ephemeris .
Spectral analysis of the flares indicates that there appear to be two types of flares : relatively brief flares , which can be extremely intense and show spectral softening , in contrast to high intensity states , which are longer and show no softening .

Conclusions : Both flares and off states are interpreted as being due to a strongly structured wind of the optical companion .
When Vela X-1 encounters a cavity with strongly reduced density , the flux will drop triggering the onset of the propeller effect , which inhibits further accretion , giving rise to off states .
The sudden decrease in the density of the material required to trigger the propeller effect in Vela X-1 is of the same order as predicted by theoretical papers about the densities in OB star winds .
A similarly structured wind can produce giant flares when Vela X-1 encounters a dense blob in the wind .