Swift Observations of SMC X-3 during Its 2016–2017 Super-Eddington Outburst

Shan-Shan Weng, Ming-Yu Ge, Hai-Hui Zhao, Wei Wang, Shuang-Nan Zhang, Wei-Hao Bian, Qi-Rong Yuan
2017 Astrophysical Journal  
The Be X-ray pulsar, SMC X-3 underwent a giant outburst from 2016 August to 2017 March, which was monitored with the Swift satellite. During the outburst, its broadband flux increased dramatically, and the unabsorbed X-ray luminosity reached an extreme value of $\sim 10^{39}$ erg/s around August 24. Using the Swift/XRT data, we measure the observed pulse frequency of the neutron star to compute the orbital parameters of the binary system. After applying the orbital corrections to Swift
more » ... s to Swift observations, we find that the spin frequency increases steadily from 128.02 mHz on August 10 and approach to spin equilibrium $\sim 128.74$ mHz in 2017 January with an unabsorbed luminosity of $L_{\rm X} \sim 2\times10^{37}$ erg/s, indicating a strong dipolar magnetic field $B \sim 6.8\times10^{12}$ G at the neutron star surface. The spin-up rate is tightly correlated with its X-ray luminosity during the super-Eddington outburst. The pulse profile in the Swift/XRT data is variable, showing double peaks at the early stage of outburst and then merging into a single peak at low luminosity. Additionally, we report that a low temperature ($kT \sim 0.2$ keV) thermal component emerges in the phase-averaged spectra as the flux decays, and it may be produced from the outer truncated disk or the boundary layer between the exterior flow and the magnetosphere.
doi:10.3847/1538-4357/aa76ec fatcat:mod7idmbpjgq3gcz6uyvychhse