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Z. Wang, D. Chakrabarty (Massachusetts Institute of Technology)
We present spectroscopic observations of the ultracompact low-mass X-ray binary (LMXB) pulsar 4U 1626-67, as well as several LMXBs which are likely ultracompact binaries (Porb < 80 min). The composite 1150-9000 A spectrum of 4U 1626-67 is well-fit by an X-ray-heated accretion disk model, and indicates that the inner edge of the accretion disk is truncated by the strong magnetic field of the pulsar at a radius of order 108 cm. No hydrogen lines are detected in any of these systems, consistent with the expected hydrogen-depleted donor and disk composition predicted for ultracompact binaries. We discuss several other similarities in the optical spectra which may provide a means for discriminating ultracompact LMXBs.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.