Paper detail

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

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.