fit

As a second step, we performed a broad-band fit including the lower energy data from [[*XMM–Newton*]{}]{} and [[*NuSTAR*]{}]{}. We used the XMM1, XMM2, and NUS spectra, that is to say those in good agreement with IBIS. XMM3, despite having the deepest exposure, likely represents a different spectral state compared with the average and is discussed in Appendix \[appendix\].

First, we fitted the data with a phenomenological model. Following [@kamraj2017], we used a model consisting of a cut-off power law plus a Gaussian [Fe K$\alpha$]{} line, modified by partial covering absorption. The model reads <span style="font-variant:small-caps;">pcfabs(cutoffpl+zgauss)</span> in [[xspec]{}]{} terminology. We included a cross-normalisation constant free to vary among the XMM1, XMM2, and NUS observations, to account for the flux variations. We first assumed all the other parameters to be constant, meaning that they were tied among the different spectra. We obtained a good fit (${\chi^{2}/\textrm{dof}}=455/435$), but with some residuals near 6.4 keV indicating variability of the iron line flux not related to the primary continuum. We thus left the normalisation of the Gaussian line free to vary among the different spectra. We obtained an excellent fit (${\chi^{2}/\textrm{dof}}=433/432$), with $\Gamma \simeq 1.6$ and a well constrained cut-off energy of [$200^{+ 75}_{- 