Di Salvo, T.T.Di SalvoSanna, A.A.SannaBurderi, L.L.BurderiPAPITTO, ALESSANDROALESSANDROPAPITTOIaria, R.R.IariaGambino, A. F.A. F.GambinoRiggio, A.A.Riggio2021-02-102021-02-1020190035-8711http://hdl.handle.net/20.500.12386/30275The first discovered accreting millisecond pulsar, SAX J1808.4-3658, went into X-ray outburst in 2015 April. We triggered a 100 ks XMM-Newton ToO, taken at the peak of the outburst, and a 55 ks NuSTAR ToO, performed 4 d apart. We report here the results of a detailed spectral analysis of both the XMM-Newton and NuSTAR spectra. While the XMM-Newton spectrum appears much softer than in previous observations, the NuSTAR spectrum confirms the results obtained with XMM-Newton during the 2008 outburst. We find clear evidence of a broad iron line that we interpret as produced by reflection from the inner accretion disc. For the first time, we use a self-consistent reflection model to fit the reflection features in the NuSTAR spectrum; in this case, we find a statistically significant improvement of the fit with respect to a simple Gaussian or diskline model to fit the iron line, implying that the reflection continuum is also significantly detected. Despite the differences evident between the XMM-Newton and NuSTAR spectra, the smearing best-fitting parameters found for these spectra are consistent with each other and are compatible with previous results. In particular, we find an upper limit to the inner disc radius of ∼12R_g. In all the cases, a high inclination angle (>50°) of the system is required. This inclination angle, combined with measurements of the radial velocity of the optical companion, results in a low value for the neutron star mass (< 0.8 M_☉), a result that deserves further investigation.STAMPAenArticle10.1093/mnras/sty29742-s2.0-85063425146000462251000052https://academic.oup.com/mnras/article/483/1/767/5159480https://arxiv.org/abs/1811.009402019MNRAS.483..767DFIS/05 - ASTRONOMIA E ASTROFISICA