  We report on a BeppoSAX observation of the transient X-ray pulsar 4U 0115+63
close to periastron. This led to the discovery of a dramatic luminosity
variation from ~2x10^34 erg/s to ~5x10^36 erg/s (factor ~250) in less than 15
hr. The variation was accompanied by only minor (if any) changes in the emitted
spectrum and pulse fraction. On the contrary an observation near apastron
detected the source in a nearly constant state at a level of ~2x10^33 erg/s.
Direct accretion onto the neutron star surface encounters major difficulties in
explaining the source variability properties. When the different regimes
expected for a rotating magnetic neutron star subject to a variable inflow of
matter from its companion are taken into consideration, the results of BeppoSAX
observations of 4U 0115+63 can be explained naturally. In particular close to
apastron, the regime of centrifugal inhibition of accretion applies, whereas
the dramatic source flux variability observed close to periastron is readily
interpreted as the transition regime between direct neutron star accretion and
the propeller regime. In this centrifugal transition regime small variations of
the mass inflow rate give rise to very large luminosity variations. We present
a simple model for this transition, which we successfully apply to the X-ray
flux and pulse fraction variations measured by BeppoSAX.
