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TIZIANA DI SALVO

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

  • Autori: Di Salvo, T; Sanna, A; Burderi, L; Papitto, A; Iaria, R; Gambino, A F; Riggio, A
  • Anno di pubblicazione: 2019
  • Tipologia: Articolo in rivista (Articolo in rivista)
  • OA Link: http://hdl.handle.net/10447/350528

Abstract

The first discovered accreting millisecond pulsar, SAX J1808.4-3658, went into X-ray outburst in April 2015. We triggered a 100 ks XMM-Newton ToO, taken at 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 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 disk. 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-fit 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 disk radius of $sim 12~R_g$. In all the cases, a high inclination angle ($>50^circ$) 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 ($<0.8,M_odot$), a result that deserves further investigation.