Comparing the Super-Eddington accretion of SMC X-3 and RX J0209.6-7427 with Swift J0243.6+6124

TL;DR

This study compares super-Eddington accretion behaviors of SMC X-3, RX J0209.6-7427, and Swift J0243.6+6124, revealing beam transition, magnetic field estimates, and RPD disk consistency, advancing understanding of ULXPs and neutron star accretion physics.

Contribution

It provides the first comparative analysis of giant outbursts in these sources, linking phase offsets to beam geometry changes and magnetic field strengths, and confirms RPD disk models at super-Eddington luminosities.

Findings

0.25 phase offset indicates beam transition from fan to pencil beam.

Magnetic fields estimated at ~4×10^13 G and ~2×10^13 G for SMC X-3 and RX J0209.

Spin-up rate follows a power-law relation with luminosity consistent with RPD disk predictions.

Abstract

We study the giant outbursts of SMC X-3 and RX J0209.6-7427 to compare their super-Eddington accretion regime with that of Swift J0243.6+6124. The high double-peak profile of SMC X-3 is found to be 0.25 phase offset from that below $2.3\times10^{38}$erg\,s$^{-1}$, which is similar to Swift J0243 (happened around $0.9\times10^{38}$erg\,s$^{-1}$). The profile of RX J0209 shows a similar 0.25 phase offset between high double-peak and low double-peak around $1.25\times10^{38}$erg\,s$^{-1}$. The 0.25 phase offset corresponds to a 90 degree angle change of the emission beam and strongly supports for a transition from a fan beam to a pencil beam. Their critical luminosities imply a surface magnetic field $\sim4\times10^{13}$ G and $2\times10^{13}$ G for SMC X-3 and RX J0209, respectively, based on the recently measured cyclotron line of Swift J0243. The spin-up rate and luminosity of SMC X-3 follows a relation of $\dot{\nu}\propto L^{0.94\pm0.03}$, while that of RX J0209 follows $\dot{\nu}\propto L^{1.00\pm0.03}$, which are similar to Swift J0243 and consistent with the prediction of a radiation-pressure-dominated (RPD) disk. These results indicate that accretion columns are indeed formed above Eddington luminosity, and the population of ULXPs likely corresponds to X-ray pulsars of highest magnetic field.