Indirect evidence for an active radio pulsar in SAX J1808.4-3658 during quiescence

S. Campana, Osservatorio Astronomico di Brera
P. D'Avanzo, Osservatorio Astronomico di Brera
L. Stella, Osservatorio Astronomico di Roma
G. L. Israel, Osservatorio Astronomico di Roma
G. Marconi, Osservatorio Astronomico di Roma
J. Casares, Instituto Astrofisico de Canarias
Rob Hynes, The University of Texas at Austin
Phil Charles, University of Southampton


Millisecond radio pulsars are neutron stars that have been spun-up by the transfer of angular momentum during the low-mass X-ray binary phase. The transition from an accretion-powered to a rotation-powered pulsar takes place on evolutionary timescales at the end of the accretion process, however it may also occur sporadically in systems undergoing transient X-ray activity. We have obtained the first optical spectrum of the low mass transient X-ray pulsar SAX J1808.4-3658 in quiescence. Similar to the black widow millisecond pulsar B1957+20, this X-ray pulsar shows a large optical modulation at the orbital period due to an irradiated companion star. Using the brightness of the companion star as a bolometer, we conclude that a very high irradiating luminosity, a factor of ∼ 100 larger than directly observed, must be present in the system. This most likely derives from a rotation-powered neutron star that resumes activity during quiescence. © 2005 American Institute of Physics.