The transient X-ray pulsar 4U 0115+63 from quiescence to outburst through the centrifugal transition

We report on a BeppoSAX observation of the transient X-ray pulsar 4U 0115+6 3, close to periastron. This led to the discovery of a dramatic luminosity variation from similar to2 x 10(34) to similar to5 x 10(36) ergs s(-1) (a f actor of greater than or similar to 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 similar to2 x 10(33) ergs s (-1). Direct accretion onto the neutron star surface encounters major diffi culties in explaining the source variability properties. When the different regimes expected for a rotating magnetic neutron star subject to a variabl e inflow of matter from its companion are taken into consideration, the res ults of the 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 cl ose to periastron is readily interpreted as the transition regime between d irect neutron star accretion and the propeller regime. In this centrifugal transition regime, small variations of the mass inflow rate give rise to ve ry large luminosity variations. We present a simple model for this transiti on, which we successfully apply to the X-ray flux and pulse fraction variat ions measured by BeppoSAX.