Paper detail

Polarization Imaging of M87 Jets by General Relativistic Radiative Transfer Calculation based on GRMHD Simulations

The spectacular images of the M87 black hole taken by the Event Horizon Telescope (EHT) have opened a new era of black hole research. One of the next issues is to take polarization images around the central black hole (BH). Since radio emission is produced by synchrotron process, polarization properties should vividly reflect the magnetic field structure at the jet base and thus provide good information regarding the magnetic mechanism of jet formation. With this kept in mind we perform general relativistic (GR) radiative transfer calculations of polarized light based on the GR magnetohydrodynamic (MHD) simulation data of accretion flow and outflow in M87, to obtain their linear and circular polarization images in the horizon-scale. We found that the linear polarization components from the jet base and inner accretion flow should experience Faraday rotation when passing through magnetized plasmas around the BH, thus sensitively depending on the BH spin. Through the comparison with intensity image at 1.3mm by EHT and the polarized degree and the rotation measure (RM) at 1.3mm with the Submillimeter Array, the model with the spin parameter of a=0.9M_BH (with M_BH being the BH mass) is favored over other models with a=0.5M_BH or 0.99M_BH, though we need further systematic studies for confirmation. We also find in low-temperature models clear ring-like image in the circular polarization map, which arises because of Faraday conversion of the linearly polarized synchrotron emission and is thus indicative of magnetic field. This occurs only when the emission region is threaded with well-ordered magnetic fields and hence no clear images are expected in high-temperature disk models, in which disk emission is appreciable. We will be able to elucidate the field configuration through the comparison between the simulated polarization images and future polarimetry with EHT and other VLBI observations.