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

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 $2.3\times 10^{38}$erg\,s${}^{-1}$, which is similar to Swift J0243 (happened around $0.9\times 10^{38}$erg\,s${}^{-1}$). The profile of RX J0209 shows a similar 0.25 phase offset between high double-peak and low double-peak around $1.25\times 10^{38}$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 $\sim 4\times 10^{13}$ G and $2\times 10^{13}$ 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 $\dot{\hat{I}}½\propto L^{0.94\pm 0.03}$, while that of RX J0209 follows $\dot{\hat{I}}½\propto L^{1.00\pm 0.03}$, 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.

9 pages, 5 figs, to appear on MNRAS