A NICER Spectrum of MAXI J1535-571: Near-Maximal Black Hole Spin and Potential Disk Warping

Understand

We report on a NICER observation of the Galactic X-ray binary and stellar-mass black hole candidate, MAXI J1535-571.

• The source was likely observed in an "intermediate" or "very high" state, with important contributions from both an accretion disk and hard X-ray corona.
• The 2.3-10 keV spectrum shows clear hallmarks of relativistic disk reflection.
• Fits with a suitable model strongly indicate a near-maximal spin parameter of a = cJ/GM^2 = 0.994(2) and a disk that extends close to the innermost stable circular orbit, r/r_ISCO = 1.08(8) (1-sigma statistical errors).