Indirect Evidence of an Active Radio Pulsar in the Quiescent State of the Transient Millisecond Pulsar SAX J1808.4-3658
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 pulsar 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2004
- DOI:
- 10.1086/425495
- arXiv:
- arXiv:astro-ph/0408584
- Bibcode:
- 2004ApJ...614L..49C
- Keywords:
-
- Accretion;
- Accretion Disks;
- Stars: Binaries: Close;
- Stars: Neutron;
- Astrophysics
- E-Print:
- Accepted for publication on ApJL (3 pages +2 color figures, references added)