Modeling the Luminosity-dependent Pulse Profile and Emission Geometry of SMC X-2 during a Giant Outburst

One of the brightest X-ray pulsars in the Small Magellanic Cloud is SMC X-2. During its most recent major outburst in 2015, this transient pulsar displayed significant changes in both its accretion state and magnetosphere, particularly when it entered the low-luminosity regime of subcritical accretion. Polestar is a pulse-profile modeling code that helps in delineating the geometry of the emission as the source evolves past outburst and toward lower-luminosity states. Applying Polestar to XMM-Newton and NuSTAR pulse profiles, we constrained the most likely inclination of the spin axis of the pulsar to be i = 87 degrees +/- 4 degrees. As the X-ray luminosity declined, an increase in the pulsed fraction was detected from Swift observations, which suggests a transition from fan- to pencil-beam emission during the later stages of the outburst. Additionally, we also performed analysis of the OGLE IV light curves, which showed strong modulation in the optical profiles during the outburst.

Automated summary

A recent study on the X-ray pulsar SMC X-2 reveals significant changes in its accretion state and magnetosphere during a major outburst, especially during the low-luminosity regime. By using the pulse-profile modeling code Polestar, researchers were able to determine the most likely inclination of the pulsar's spin axis and observe a transition in the emission pattern from fan- to pencil-beam. Analysis of the optical profiles also showed strong modulation during the outburst.