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.

Di Salvo, T., Sanna, A., Burderi, L., Papitto, A., Iaria, R., Gambino, A.F., et al. (2019). NuSTAR and XMM–Newton broad-band spectrum of SAX J1808.4–3658 during its latest outburst in 2015. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 483(1), 767-779 [10.1093/mnras/sty2974].

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

Di Salvo, T
Writing – Original Draft Preparation
;
Burderi, L
Membro del Collaboration Group
;
Iaria, R
Writing – Review & Editing
;
Gambino, A F;
2019-01-01

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.
2019
Settore FIS/05 - Astronomia E Astrofisica
Di Salvo, T., Sanna, A., Burderi, L., Papitto, A., Iaria, R., Gambino, A.F., et al. (2019). NuSTAR and XMM–Newton broad-band spectrum of SAX J1808.4–3658 during its latest outburst in 2015. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 483(1), 767-779 [10.1093/mnras/sty2974].
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10447/350528
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