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We report on a BeppoSAX observation of the transient X-ray pulsar 4U 0115+63close to periastron. This led to the discovery of a dramatic luminosityvariation from ~2x10^34 erg/s to ~5x10^36 erg/s (factor ~250) in less than 15hr. The variation was accompanied by only minor (if any) changes in the emittedspectrum and pulse fraction. On the contrary an observation near apastrondetected 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 inexplaining the source variability properties. When the different regimesexpected for a rotating magnetic neutron star subject to a variable inflow ofmatter from its companion are taken into consideration, the results of BeppoSAXobservations of 4U 0115+63 can be explained naturally. In particular close toapastron, the regime of centrifugal inhibition of accretion applies, whereasthe dramatic source flux variability observed close to periastron is readilyinterpreted as the transition regime between direct neutron star accretion andthe propeller regime. In this centrifugal transition regime small variations ofthe mass inflow rate give rise to very large luminosity variations. We presenta simple model for this transition, which we successfully apply to the X-rayflux and pulse fraction variations measured by BeppoSAX.Comment: 7 pages, 3 figures, 1 table (requires emulateapj macro). Accepted for  publication on Ap

The Transient X‐Ray Pulsar 4U 0115+63 from Quiescence to Outburst through the Centrifugal Transition