The quiescent X-ray emission of three transient X-ray pulsars

We report on BeppoSAX and Chandra observations of three hard X-ray transients in quiescence containing fast-spinning (P < 5 s) neutron stars: A0538-66, 4U 0115+63, and V0332+53. These observations allowed us to study these transients at the faintest flux levels thus far. The spectra are remarkably different from the ones obtained at luminosities more than a factor of 10 higher, testifying that the quiescent emission mechanism is different. Pulsations were not detected in any of the sources, indicating that accretion of matter down to the neutron star surface has ceased. We conclude that the quiescent emission of the three X-ray transients likely originates from accretion onto the magnetospheric boundary in the propeller regime and/or from deep crustal heating resulting from pycnonuclear reactions during the outbursts.