mm and sub-mm polarimetry of accretion flow towards M 87

Abstract

Mass accretion rate on the SMBHs is one of the fundamental parameters used to investigate AGNs. Faraday Rotation Measure (RM) observations at mm/sub-mm wavelengths is one of the powerful methods to derive the mass accretion rate of hot accretion flows towards our galactic center, Sgr A* (e.g., Marrone et al. 2006). Based on this scheme, we conducted an SMA observation to apply this method to M 87, which is one of the primary target for our submm VLBI observations, in February 2013. We succeeded to derive an RM of (2.1 ± 1.8) × 10^5 rad m^-2, it gives the range of the mass accretion rate (M_dot) between 0 and 9.2 × 10^-4 M_sun yr^-1 at the distance of 21 rs from the SMBH. Our estimated M.is already two orders of magnitude smaller than the M_dot at the outer part of the accretion flow (~10^5 r_s) of 0.1 M_sun yr^-1 determined by X-ray observations (Di Matteo et al. 2003). This significant suppression of the M_dot at the inner region is expected with the radiatively inefficient accretion flow (RIAF) model. With future submm VLBI polarimetry towards jetted sources including M 87, we will derive the profile of accretion flow along the jet. It is very important itself for the study of the accretion process onto the SMBH, but also provide fundamental properties to derive BH parameters from the BH shadow imaging.