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We report on a NuSTAR observation of the recently discovered bright blackhole candidate MAXI J1535-571. NuSTAR observed the source on MJD 58003 (fivedays after the outburst was reported). The spectrum is characteristic of ablack hole binary in the hard state. We observe clear disk reflection features,including a broad Fe K$\alpha$ line and a Compton hump peaking around 30 keV.Detailed spectral modeling reveals narrow Fe K$\alpha$ line complex centeredaround 6.5 keV on top of the strong relativistically broadened Fe K$\alpha$line. The narrow component is consistent with distant reflection frommoderately ionized material. The spectral continuum is well described by acombination of cool thermal disk photons and a Comptonized plasma with theelectron temperature $kT_{\rm e}=19.7\pm{0.4}$ keV. An adequate fit can beachieved for the disk reflection features with a self-consistent relativisticreflection model that assumes a lamp-post geometry for the coronal illuminatingsource. The spectral fitting measures a black hole spin $a>0.84$, inner diskradius $R_{\rm in}<2.01~r_{\rm ISCO}$, and a lamp-post height$h=7.2^{+0.8}_{-2.0} r_{\rm g}$ (statistical errors, 90% confidence),indicating no significant disk truncation and a compact corona. Although thedistance and mass of this source are not currently known, this suggests thesource was likely in the brighter phases of the hard state during this NuSTARobservation.

Reflection Spectra of the Black Hole Binary Candidate MAXI J1535-571 in the Hard State Observed by NuSTAR