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[17.16] Optical/UV Spectroscopy of Ultracompact Low-Mass X-Ray Binaries

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 (P_orb < 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, 35#2
© 2003. The American Astronomical Soceity.