Swift observations of SMC X-3 during its 2016-2017 super-Eddington outburst

Abstract

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 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.