to obtain the same number of data points as in the X-ray spectrum. The statistical errors were calculated from the variance in the spectrum. A systematic error introduced by the response function was calculated by measuring the amplitude of the spurious wiggles introduced by the flux calibration process. This uncertainty and the error due to the calibration against the VLT photometric measurement were then added in quadrature to the formal error. The total resulting error on the optical spectrum is about 8 per cent of the flux. The optical-to-X-ray SED is shown in Fig. \[fig6\].\
We model the SED from the optical to the X-rays with a single power law (PL), a broken power law (BPL) and a tied broken power law (TBPL), where the spectral slopes are tied to differ by $\Delta\beta=0.5$ to reproduce the spectral break (cooling frequency) expected for a synchrotron spectrum. The fit was performed with the Interactive Spectral Interpretation System [ISIS; @Houck00] software, which allows all the data to be compared directly in count space. Working in count space has the advantage of not requiring any a priori model for the X-rays, otherwise needed for the conversion of the X-rays into flux [see e.g @Starling07]. 