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[17.23] Superhumps and an extreme mass ratio in the ultracompact low-mass X-ray binary 4U 1820-30

Z. Wang, D. Chakrabarty (MIT)

The long history of X-ray observations of the low-mass X-ray binary (LMXB) 4U 1820-30 has shown that it is an ultracompact system having the shortest orbital period (685 s) among LMXBs. Here we report the detection of a slightly longer period from its ultraviolet time series data, obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Various timing analysis techniques, including epoch-folding, Fourier transform, phase dispersion minimization, are used to determine the period of the UV light curve. The resulting periods are consistently in a range between 690-693~s. We interpret this longer-period UV signal as a ``superhump oscillation'', arising from a tidal resonance in the accretion disk of an extreme-mass-ratio binary. The 1 percent period excess implies a neutron-star/white-dwarf mass ratio around 20. Our work strongly supports the suggestion that all neutron star LMXBs with orbital periods below 4 hours are potential superhump sources, as proposed by Haswell et al (2001).

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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.