Optical to X-ray spectrum of GRB 170817A. XMM-Newton data points (blue, this work) and HST contemporaneous detection (red circle) are shown. The grey shaded region shows the 90% uncertainty on the fit of the XMM-Newton data alone (dot-dashed line). The fit obtained combining the de-reddened HST flux (from Margutti et al. 2018) and our XMM-Newton data results in a photon index of 1.6 (dotted red line) with an error of ± 0.05 (90% c.l. – yellow shaded region).