Spectral evolution of IGR J17091-3642 during its 2016 outburst and the heartbeats

We present simultaneous NuSTAR and SWIFT observations of the black hole transient IGR J17091-3642 during its 2016 outburst.By jointly fitting six NuSTAR and four SWIFT average spectra, we found that during this outburst the source evolves from the hard to the hard/soft intermediate and back to the hard state, similar to the 2011 outburst. One difference with respect to the previous outburst is that in this case we observed both a broad emission and a narrow absorption line in those observations. Our fits favour an accretion disc with relatively low inclination angle of ˜45^o with respect to the line of sight and a high iron abundance of ˜3.5 in units of the solar abundance. This high iron abundance, however, is likely model-dependent.We also observed heartbeat variability in one NuSTAR observation. We fitted the phase-resolved spectra of this observation and found that the direct emission varies independently from the reflection component, which suggests that during the heartbeat state the corona is very far away from the disc in IGR J17091-3642. Assuming that in IGR J17091-3642 the inner radius of the disc both in the average and the phase-resolved spectra is located 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≤ a_{*}≤0.27.