Polarization imaging of M 87 jets by general relativistic radiative transfer calculation based on GRMHD simulations

Abstract

Abstract The spectacular images of the M 87 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 structures 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 M 87, to obtain their linear and circular polarization images in the BH horizon-scale. We found that the linear polarization components originating from the jet base and inner accretion flow should experience Faraday rotation and depolarization when passing through magnetized plasmas around the BH, thus sensitively depending on the BH spin. Through the comparison with total intensity image at $1.3\:$mm by EHT and the polarization degree and the rotation measure (RM) measured at $1.3\:$mm with the Submillimeter Array, the model with the spin parameter of $a=0.9\, M_{\,\mathrm{BH}}$ (with $M_{\,\mathrm{BH}}$ being the BH mass) is favored over other models with $a = 0.5\, M_{\,\mathrm{BH}}$ or $0.99\, M_{\,\mathrm{BH}}$, though we need further systematic studies for confirmation. We also find in low-temperature models a 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 direction. 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.