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We present a measurement of the X-ray spectrum of the luminous X-ray binaryin I Zw 18, the blue compact dwarf galaxy with the lowest known metallicity. Wefind the highest flux yet observed, corresponding to an intrinsic luminositynear 1E40 erg/s establishing it as an ultraluminous X-ray source (ULX). Theenergy spectrum is dominated by disk emission with a weak or absent Comptoncomponent and there is no significant timing noise; both are indicative of thethermal state of stellar-mass black hole X-ray binaries and inconsistent withthe Compton-dominated state typical of most ULX spectra. A previous measurementof the X-ray spectrum shows a harder spectrum that is well described by apowerlaw. Thus, the binary appears to exhibit spectral states similar to thoseobserved from stellar-mass black hole binaries. If the hard state occurs in therange of luminosities found for the hard state in stellar-mass black holebinaries, then the black hole mass must be at least 85 solar masses. Spectralfitting of the thermal state shows that disk luminosities for which thin diskmodels are expected to be valid are produced only for relatively high diskinclinations, >= 60 degrees, and rapid black hole spins. We find a_* > 0.98 andM > 154 solar masses for a disk inclination of 60 degrees. Higher inclinationsproduce higher masses and somewhat lower spins.

A State Transition of the Luminous X-ray Binary in the Low-Metallicity Blue Compact Dwarf Galaxy I Zw 18