THE SPIN OF THE BLACK HOLE GS 1124−683: OBSERVATION OF A RETROGRADE ACCRETION DISK?
We re-examine archival Ginga data for the black hole binary system GS 1124−683, obtained when the system was undergoing its 1991 outburst. Our analysis estimates the dimensionless spin parameter a* = cJ/GM2 by fitting the X-ray continuum spectra obtained while the system was in the “thermal dominant” state. For likely values of mass and distance, we find the spin to be (90% confidence), implying that the disk is retrograde (i.e., rotating antiparallel to the spin axis of the black hole). We note that this measurement would be better constrained if the distance to the binary and the mass of the black hole were more accurately determined. This result is unaffected by the model used to fit the hard component of the spectrum. In order to be able to recover a prograde spin, the mass of the black hole would need to be at least 15.25 M☉, or the distance would need to be less than 4.5 kpc, both of which disagree with previous determinations of the black hole mass and distance. If we allow fcol to be free, we obtain no useful spin constraint. We discuss our results in the context of recent spin measurements and implications for jet production.