2013ApJ...770...20K

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 1x10⁴⁰ 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 power law. 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 M_☉. 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°, and rapid black hole spins. We find a_* > 0.98 and M > 154 M_☉ for a disk inclination of 60°. Higher inclinations produce higher masses and somewhat lower spins.