NuSTAR and XMM–Newton broad-band spectrum of SAX J1808.4–3658 during its latest outburst in 2015

The first discovered accreting millisecond pulsar, SAX J1808.4-3658, wentinto X-ray outburst in April 2015. We triggered a 100 ks XMM-Newton ToO, takenat the peak of the outburst, and a 55 ks NuSTAR ToO, performed four days apart.We report here the results of a detailed spectral analysis of both theXMM-Newton and NuSTAR spectra. While the XMM-Newton spectrum appears muchsofter than in previous observations, the NuSTAR spectrum confirms the resultsobtained with XMM-Newton during the 2008 outburst. We find clear evidence of abroad iron line that we interpret as produced by reflection from the inneraccretion disk. For the first time, we use a self-consistent reflection modelto fit the reflection features in the NuSTAR spectrum; in this case we find astatistically significant improvement of the fit with respect to a simpleGaussian or diskline model to fit the iron line, implying that the reflectioncontinuum is also significantly detected. Despite the differences evidentbetween the XMM-Newton and NuSTAR spectra, the smearing best-fit parametersfound for these spectra are consistent with each other and are compatible withprevious results. In particular, we find an upper limit to the inner diskradius of $sim 12~R_g$. In all the cases, a high inclination angle($>50^circ$) of the system is required. This inclination angle, combined withmeasurements of the radial velocity of the optical companion, results in a lowvalue for the neutron star mass ($<0.8,M_odot$), a result that deservesfurther investigation.