The optical counterpart to 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. (2001). This star shows a 6% semi-amplitude sinusoidal modulation of its flux at the orbital period of the system. It was proposed that the modulation arises from X-ray irradiation of the intrinsically faint companion by a remnant accretion disk, and that the bulk of the optical emission arises from viscous dissipation in the 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 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. Our computations indicate that the 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, a key phase for understanding the evolution of this class of objects.