Constraining Black Hole Spin via X‐Ray Spectroscopy

We present an analysis of the observed broad iron line feature and putativewarm absorber in the long 2001 XMM-Newton observation of the Seyfert-1.2 galaxyMCG-6-30-15. The new "kerrdisk" model we have designed for simulating lineemission from accretion disk systems allows black hole spin to be a freeparameter in the fit, enabling the user to formally constrain the angularmomentum of a black hole, among other physical parameters of the system. In animportant extension of previous work, we derive constraints on the black holespin in MCG-6-30-15 using a self-consistent model for X-ray reflection from thesurface of the accretion disk while simultaneously accounting for absorption bydusty photoionized material along the line of sight (the warm absorber). Evenincluding these complications, the XMM-Newton/EPIC-pn data require extremerelativistic broadening of the X-ray reflection spectrum; assuming no emissionfrom within the radius of marginal stability, we derive a formal constraint onthe dimensionless black hole spin parameter of a > 0.987 at 90% confidence. Theprincipal unmodeled effect that can significantly reduce the inferred blackhole spin is powerful emission from within the radius of marginal stability.Although significant theoretical developments are required to fully understandthis region, we argue that the need for a rapidly spinning black hole is robustto physically plausible levels of emission from within the radius of marginalstability. In particular, we show that a non-rotating black hole is stronglyruled out.