What is the Hidden Depolarization Mechanism in Low Luminosity AGN?

Abstract

Millimeter wavelength polarimetry of accreting black hole systems can provide a tomographic probe of the accretion flow on a wide range of linear scales. We searched for linear polarization in two low luminosity active galactic nuclei (LLAGN), M81 and M84, using the Combined Array for Millimeter Astronomy (CARMA) and the Submillimeter Array (SMA). We find upper limits of $\sim 1-2\%$ averaging over the full bandwidth and with a rotation measure (RM) synthesis technique. These low polarization fractions, along with similar low values for LLAGN M87 and 3C84, suggest that LLAGN have qualitatively different polarization properties than radio-loud sources and Sgr A*. If the sources are intrinsically polarized and then depolarized by Faraday rotation then we place lower limits on the RM of a few times $10^7\mathrm{rad}{\mathrm{m}}^{-2}$ for the full bandwidth case and $\sim 10^9\mathrm{rad}{\mathrm{m}}^{-2}$ for the RM synthesis analysis. These limits are inconsistent with or marginally consistent with expected accretion flow properties. Alternatively, the sources may be depolarized by cold electrons within a few Schwarzschild radii from the black hole, as suggested by numerical models.

Accepted for publication in ApJL

References in corpus (6)

• An Unambiguous Detection of Faraday Rotation in Sagittarius A*
• Observational appearance of inefficient accretion flows and jets in 3D GRMHD simulations: Application to Sgr~A*
• The Disc-Jet Symbiosis Emerges: Modeling the Emission of Sagittarius A* with Electron Thermodynamics
• Results from an extensive simultaneous broadband campaign on the underluminous active nucleus M81*: further evidence for mass-scaling accretion in black holes
• Probing the Parsec-scale Accretion Flow of 3C 84 with Millimeter Polarimetry
• The Submillimeter Array Polarimeter

Cited by in corpus (6)

• Polarimetric properties of Event Horizon Telescope targets from ALMA
• Faraday rotation in the jet of M87 inside the Bondi radius: indication of winds from hot accretion flows confining the relativistic jet
• Jet collimation and acceleration in the giant radio galaxy NGC 315
• Decomposing the Internal Faraday Rotation of Black Hole Accretion Flows
• Relative depolarization of the black hole photon ring in GRMHD models of Sgr A* and M87*
• Low optical polarisation at the core of the optically-thin jet of M87