We performed fits assuming absorption by a column density ${N_{\textrm{H}}}= {2.7 \times 10^{23}}$ [cm$^{-2}$]{} [@beckmann2004] since this value is not constrained by the data above 20 keV.

Fitting the IBIS data with a power law, we obtain a poor fit (${\chi^{2}/\textrm{dof}}=17/10$) with negative residuals above 100 keV (Fig. \[fig:ratios\], first panel). To reproduce this curvature, we first tried to include Compton reflection, replacing the cut-off power law with [[pexrav]{}]{} [@pexrav]. This model includes Compton reflection off a neutral slab of infinite column density. We fixed the inclination angle at 60 deg since the fit was not sensitive to this parameter. We first fixed the reflection fraction ($\mathcal{R} = \Omega / 2 \pi$, where $\Omega$ is the solid angle subtended by the reflector) at 0.5. The model also includes an exponential high-energy cut-off, which we left free to vary. This yields a slightly better fit (${\chi^{2}/\textrm{dof}}=14/9$), but still with the indication of a curvature (Fig. \[fig:ratios\], second panel). Indeed, leaving $\mathcal{R}$ free to vary, the value is pegged at zero with an upper limit of 0.3. Then, we replaced [[pexrav]{}]{} with a cut-off power law (equivalent to [[pexrav]{}]{} with $\mathcal{R}=0$) finding a good fit (${\chi^{2}/\textrm{dof}}=8/9$) and a cut-off energy of [$300^{+ 