GR-RMHD Simulations of Super-Eddington Accretion Flows onto a Neutron Star with Dipole and Quadrupole Magnetic Fields (Inoue et al. 2024, ApJ, 977, 10)

  We performed general relativistic radiation magnetohydrodynamic (GR-RMHD) simulations of super-Eddington accretion flows onto a neutron star with both dipole and quadrupole magnetic fields. The right figure shows the resulting gas density distribution. Cyan lines denote the magnetic field lines. When the dipole field is dominant over the quadrupole field, the gas accretes along the dipole field lines (model D_d001, DQ_d001, QD_d001, D_d01, and DQ_d01). On the other hand, when the quadrupole field dominates over the dipole field, the accretion flow mainly accretes through the equatorial region (model Q_d001, QD_d01, and Q_d01). Based on our findings, we conclude that the neutron star in Swift J0243.6+6124 possesses a significant quadrupole magnetic component in addition to a dipole field.

Modeling of Thermal Emission from ULX Pulsar Swift J0243.6+6124 with General Relativistic Radiation MHD simulations (Inoue et al. 2023, ApJ, 952, 62)

  We performed general relativistic radiation magnetohydrodynamic (MHD) simulations of the super-Eddington accretion flows onto a neutron star with dipole magnetic fields. Our simulations showed accretion columns near the magnetic poles, an accretion disk outside the magnetosphere, and optically thick outflows from the disk. We revealed that the optically thick outflows can explain the observed thermal emission at ~1e+7 K. Such outflows are generated if the mass accretion rate is much higher than the Eddington rate and if the magnetospheric radius is smaller than the spherization radius. By comparing our models with observations, we restricted the magnetic field strength of the neutron star in Swift J0243.6+6124. The right figure shows the phase diagram of the ULX diversity (see, our paper in details).

Pulsed fraction of super-critical column accretion flows onto neutron stars (Inoue et al. 2020, PASJ, 72, 34)

  We performed post-processing general relativistic radiative transfer simulations and obtained pulse profiles from the supercritical accretion columns. Our simulations are based on the radiation hydrodynamics models in Kawashima et al. 2017. Pulsed fractions (semi-amplitude of the pulse profile) are functions of the observer's viewing angle θobs and the offset angle between the rotation axis and magnetic axis θB.  We found that the pulsed fraction is ~50% at maximum.