Fan beamed X-ray emission from 1 keV to above 130 keV from the ultraluminous X-ray pulsar RX J0209.6-7427 in the Small Magellanic Cloud

Abstract

We present detailed timing and spectral analyses of the transient X-ray pulsar RX J0209.6$-$7427 in the Small Magellanic Cloud during its 2019 giant outburst. With a better known distance than most galactic X-ray pulsars, its peak luminosity is determined to be $(1.11\pm0.06)\times 10^{39}\, \rm erg\ s^{-1}$; it is thus a {\it bonda fide} pulsating ultraluminous X-ray source (PULX). Owing to the broad energy band of \textit{Insight}-HXMT, its pulsed X-ray emission was detected from 1 keV up to the 130$-$180 keV band, which is the highest energy emission detected from any PULXs outside the Milky Way. This allows us to conclude that its main pulsed X-ray emission is from the "fan beam" of the accretion column, and its luminosity is thus intrinsic. We also estimate its magnetic field of (4.8$-$8.6)$\times10^{12}$ G or (1.7$-$2.2)$\times10^{13}$ G, from its spin evolution or transition in the accretion column structure during the outburst; we suggest that the two values of the magnetic field strength correspond to the dipole and multipole magnetic fields of the neutron star, similar to the recent discovery in the Galactic PULX Swift J0243.6+6124. Therefore, the nature of the neutron star and its ULX emission can be understood within the current theoretical frame of accreting neutron stars. This may have implications for understanding the nature of those farther away extragalactic PULXs.