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Recently, compact black hole X-ray binaries XTE J 1118+480 and A0620-00 havebeen reported to be experiencing a fast orbital period decay, which is twoorders of magnitude higher than expected with gravitational wave radiation.Magnetic braking of an Ap/Bp star has been suggested to account for the periodchange when the surface magnetic field of the companion star $B_{\rm s}\ga10^{4}$ G. However, our calculation indicates that anomalous magnetic brakingcannot significantly contribute to the large orbital period decay ratesobserved in these two sources even if $B_{\rm s}\ga 10^{4}$ G. Observationshave provided evidence that circumbinary disks around two compact black holeX-ray binaries may exist. Our analysis shows that, for some reasonableparameters, tidal torque between the circumbinary disk and the binary canefficiently extract the orbital angular momentum from the binary, and result ina large orbital period change rate. Based on the circumbinary disk model, wesimulate the evolution of XTE J 1118+480 via a stellar evolution code. Ourcomputations are approximatively in agreement with the observed data (themasses of two components, donor star radius, orbital period, and orbital periodderivative). The mass transfer rate and circumbinary disk mass are obviouslyfar greater than the inferred values from observations. Therefore, it seemsthat the circumbinary disk is unlikely to be the main cause of the rapidorbital decay observed in some compact black hole X-ray binaries.

Orbital period decay of compact black hole x-ray binaries: the influence of circumbinary disks?