A State Transition of the Luminous X-Ray Binary in the Low-metallicity Blue Compact Dwarf Galaxy I Zw 18
Abstract
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 1 × 1040 erg s-1 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2013
- DOI:
- 10.1088/0004-637X/770/1/20
- arXiv:
- arXiv:1304.6731
- Bibcode:
- 2013ApJ...770...20K
- Keywords:
-
- black hole physics;
- galaxies: individual: blue compact dwarf I Zw 18;
- X-rays: binaries;
- X-rays: galaxies;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 5 pages, to appear in ApJ, references revised