Pulse phase-resolved analysis of SMC X-3 during its 2016--2017 super-Eddington outburst

TL;DR

This study conducts a phase-resolved analysis of the ultraluminous outburst of SMC X-3, revealing changes in pulse profiles, spectral features, and accretion geometry across different luminosity states, bridging understanding between typical Be/X-ray binaries and ultraluminous X-ray pulsars.

Contribution

It provides the first detailed phase-resolved spectral analysis of SMC X-3 during its super-Eddington outburst, highlighting the evolution of pulse profiles and accretion geometry at various luminosities.

Findings

Pulse profile transitions from double-peak to single-peak with decreasing luminosity.

Iron line widths vary with phase, indicating complex accretion geometry.

Pulse fraction increases with energy at high luminosity, with additional thermal component at low energies.

Abstract

The Be X-ray pulsar SMC X-3 underwent an extra long and ultraluminous giant outburst from 2016 August to 2017 March. The peak X-ray luminosity is up to $\sim 10^{39}$ erg/s, suggesting a mildly super-Eddington accretion onto the strongly magnetized neutron star. It therefore bridges the gap between the Galactic Be/X-ray binaries ($L_{\rm X}^{\rm peak} \leq 10^{38}$ erg/s) and the ultraluminous X-ray pulsars ($L_{\rm X}^{\rm peak} \geq 10^{40}$ erg/s) found in nearby galaxies. A number of observations were carried out to observe the outburst. In this paper, we perform a comprehensive phase-resolved analysis on the high quality data obtained with the Nustar and XMM-Newton, which were observed at a high and intermediate luminosity levels. In order to get a better understanding on the evolution of the whole extreme burst, we take the Swift results at the low luminosity state into account as well. At the early stage of outburst, the source shows a double-peak pulse profile, the second main peak approaches the first one and merges into the single peak at the low luminosity. The second main peak vanishes beyond 20 keV, and its radiation becomes much softer than that of the first main peak. The line widths of fluorescent iron line vary dramatically with phases, indicating a complicated geometry of accretion flows. In contrast to the case at low luminosity, the pulse fraction increases with the photon energy. The significant small pulse fraction detected below 1 keV can be interpreted as the existence of an additional thermal component located at far away from the central neutron star.