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We present simultaneous NuSTAR and Swift observations of the black holetransient IGR J17091-3642 during its 2016 outburst. By jointly fitting sixNuSTAR and four Swift spectra, we found that during this outburst the sourceevolves from the hard to the hard/soft intermediate and back to the hard state,similar to the 2011 outburst. Unlike in the previous outburst, in this case weobserved both a broad emission and an moderately broad absorption line in ourobservations. Our fits favour an accretion disc with an inclination angle of$\sim$$45\deg$ with respect to the line of sight and a high iron abundance of$3.5\pm0.3$ in units of the solar abundance. We also observed heartbeatvariability in one NuSTAR observation. We fitted the phase-resolved spectra ofthis observation and found that the reflected emission varies independentlyfrom the direct emission, whereas in the fits to the average spectra these twoquantities are strongly correlated. Assuming that in IGR J17091-3642 the innerradius of the disc both in the average and the phase-resolved spectra islocated at the radius of the innermost stable circular orbit, with 90%confidence the spin parameter of the black hole in this system is $-0.13\leqa_{*}\leq0.27$.

The reflection component in the average and heartbeat spectra of the black-hole candidate IGR J17091-3642 during the 2016 outburst