Have superhumps been seen in black hole soft X-ray transients?

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Abstract
Yes. Superhumps are always seen in the superoutbursts of the SU UMa subclass of dwarf novae. Current understanding of the phenomenon as arising from tidal stressing of accretion discs in high-mass-ratio (q > 3) interacting binaries suggests that they should be detectable in other kinds of semidetached binaries with extreme mass ratios, including the black hole soft X-ray transients (SXTs). Superhumps in SXTs during outburst have been reported in the literature. These claims have usually been based on data sets acquired and analysed shortly after the discovery of the system while it is still in outburst. At such times, key parameters such as the binary orbital period are not known with adequate certainty. This paper presents a systematic re-analysis of all the available optical photometry of three SXTs during their main outbursts, concluding that superhumps have been seen in Nova Mus 1991 and GRO J0422 + 32, and probably in GS 2000 + 25. These conclusions are interpreted using the tidal theory of superhumps, including an explanation of the ‘glitches’ seen in the X-ray light curves of SXTs. Estimates of the mass ratio and compact object mass are made, and found to be in satisfactory agreement with measurements of these quantities made by other means.

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