The optical counterpart of SAX J1808.4-3658 in quiescence: evidence of an active radio pulsar?

The optical counterpart of the binary millisecond X-ray pulsar SAX J1808.4-3658 during quiescence was detected at V = 21.5 mag by Homer et al. [14]. It was proposed that the bulk of the optical emission arises from viscous dissipation in the innermost zones of a remnant disk. The serious difficulty in this scenario lies in the estimate of the irradiating luminosity required to match the observational data, that is a factor 10 - 50 higher than the observed quiescent X-ray luminosity of this source. To overcome this problem, we propose an alternative scenario, in which the irradiation is due to the release of rotational energy by the fast spinning neutron star, switched on, as magneto-dipole rotator (radio pulsar), during quiescence. In this scenario the companion behaves as a bolometer, reprocessing in optical part of the power emitted by the pulsar. The reprocessed fraction depends only on known binary parameters. Thus the blackbody temperature of the companion can be predicted and compared with the observations. Our computations indicate that the observed optical magnitudes are fully consistent with this hypothesis. In this case the observed optical luminosity may be the first evidence that a radio pulsar is active in this system in quiescence.