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