A propeller model for the sub-luminous accretion disk state of transitional millisecond pulsars

Transitional millisecond pulsars are quickly rotating neutron stars that perform transitions between a radio pulsar state and an accretion regime, on time scales of the order of a few days. X-ray observations performed by XMM-Newton revealed a wealth of information about the properties of the accretion flows on these pulsars. X-ray pulsations were detected both at a luminosity typical of accreting ms pulsars (few x 1E36 erg/s), and unexpectedly also at a much lower level (few x 1E33 erg/s), at which accretion onto the neutron star surface is expected to be completely inhibited by the centrifugal barrier set by the quickly rotating magnetosphere. Together with the peculiar X-ray flux variability on both short (less than 1 s) and long (up to 10 hr) time scales, and the relatively bright gamma-ray emission observed at GeV energies, these properties suggest that a significant fraction of the in-flowing mass is ejected from the disk-magnetosphere boundary. I will present a model of the high energy emission of transitional ms pulsars based on the hypothesis that these systems are found in a propeller state, and discuss the possibility of identifying low luminosity X-ray sources with a bright gamma-ray counterpart as ms pulsars.