Journal
ASTROPHYSICAL JOURNAL LETTERS
Volume 679, Issue 1, Pages L37-L40Publisher
UNIV CHICAGO PRESS
DOI: 10.1086/588840
Keywords
binaries : general; black hole physics; galaxies : individual (M33); X-rays : binaries
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In prior work, Chandra and Gemini-North observations of the eclipsing X-ray binary M33 X-7 have yielded measurements of the mass of its black hole primary and the system's orbital inclination angle of unprecedented accuracy. Likewise, the distance to the binary is known to a few percent. In an analysis based on these precise results, 15 Chandra and XMM-Newton X-ray spectra, and our fully relativistic accretion disk model, we find that the dimensionless spin parameter of the black hole primary is a* = 0.77 +/- 0.05. The quoted 1 sigma error includes all sources of observational uncertainty. Four Chandra spectra of the highest quality, which were obtained over a span of several years, all lead to the same estimate of spin to within statistical errors (2%), and this estimate is confirmed by 11 spectra of lower quality. There are two remaining uncertainties: (1) the validity of the relativistic model used to analyze the observations, which is being addressed in ongoing theoretical work; and (2) our assumption that the black hole spin is approximately aligned with the angular momentum vector of the binary, which can be addressed by a future X-ray polarimetry mission.
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