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We report on a NICER observation of the Galactic X-ray binary andstellar-mass black hole candidate, MAXI J1535-571. The source was likelyobserved in an "intermediate" or "very high" state, with importantcontributions from both an accretion disk and hard X-ray corona. The 2.3-10 keVspectrum shows clear hallmarks of relativistic disk reflection. Fits with asuitable 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). In addition to therelativistic spectrum from the innermost disk, a relatively narrow Fe Kemission line is also required. The resolution of NICER reveals that the narrowline may be asymmetric, indicating a specific range of emission radii. Fitswith a relativistic line model suggest an inner radius of r = 144 (+140,-60)GM/c^2 for the putative second reflection geometry; full reflection modelssuggest that radii a few times larger are possible. The origin of the narrowline is uncertain but a warp likely provides the most physically plausibleexplanation. We discuss our results in terms of the potential for NICER toreveal new features of the inner and intermediate accretion disk around blackholes.

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