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S. Campana (Osservatorio astronomico di Brera), F. Gastaldello (Universita` di Milano & Osservatorio astronomico di Brera), L. Stella, G.L. Israel (Osservatorio astronomico di Roma), M. Colpi, F. Pizzolato (Universita` di Milano-Bicocca), M. Orlandini, D. Dal Fiume (TeSRE C.N.R. Bologna)
Accretion onto magnetic stars is expected to be inhibited when the drag exerted by the rotating magnetosphere on the accreting matter gives rise to a centrifugal force that is locally stronger than gravity. The action of such a "centrifugal barrier" likely plays an important role in a variety of astrophysical conditions ranging from the decay and quiescent phase of X-ray transients hosting a magnetic neutron star, to peculiar magnetic white dwarves in cataclysmic variables, symbiotic and T Tauri stars. Yet the observational evidence in favor of the centrifugal barrier has so far been scarce and, at best, indirect.
Here we report on a BeppoSAX observation of the 3.6 s X-ray pulsar transient 4U 0115+63 in a low luminosity state. For the first time dramatic variations (up to factor of ~ 250 in 15 hr) in the accretion luminosity were seen, which were not accompanied by any significant variation in other source properties, such as the emitted spectrum, pulse shape and relative amplitude. This provide the first direct evidence of an accreting star (a neutron star in this case) right at the transition between the centrifugal inhibition and the accretion regime. The inferred dependence of the source luminosity on the mass inflow rate is exceptionally steep (power law dependence with index ~30), providing new important information on physics of the disk-magnetosphere interaction.
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