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We re-examine archival Ginga data for the black hole binary system GS1124-683, obtained when the system was undergoing its 1991 outburst. Ouranalysis estimates the dimensionless spin parameter a=cJ/GM^2 by fitting theX-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 a=-0.25(-0.64, +0.05) (90% confidence), implying that the disk is retrograde (i.e.rotating antiparallel to the spin axis of the black hole). We note that thismeasurement would be better constrained if the distance to the binary and themass of the black hole were more accurately determined. This result isunaffected by the model used to fit the hard component of the spectrum. Inorder to be able to recover a prograde spin, the mass of the black hole wouldneed to be at least 15.25 Msun, or the distance would need to be less than 4.5kpc, both of which disagree with previous determinations of the black hole massand distance. If we allow f_col to be free, we obtain no useful spinconstraint. We discuss our results in the context of recent spin measurementsand implications for jet production.

The Spin of the Black Hole GS 1124-683: Observation of a Retrograde Accretion Disk?