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Indirect evidence for an active radio pulsar in the quiescent state of the transient ms pulsar SAX J1808.4-3658

S. Campana·P. D'Avanzo·J. Casares·S. Covino·G. L. Israel·G. Marconi·Rob Hynes·P. Charles·L. Stella
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Abstract

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.

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S. Campana, P. D'Avanzo, J. Casares, S. Covino, G. L. Israel, G. Marconi, Rob Hynes, P. Charles, L. Stella. 2004-09-06. Indirect evidence for an active radio pulsar in the quiescent state of the transient ms pulsar SAX J1808.4-3658. https://doi.org/10.1086/425495

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