Enhanced Polarized Emission from the One-Parsec-Scale Hotspot of 3C 84 as a Result of the Interaction with Clumpy Ambient Medium

TL;DR

This study uses VLBA polarimetric observations at 43 GHz to analyze the hotspot of 3C 84's jet, revealing enhanced polarized emission due to interaction with a clumpy ambient medium, and measuring a high and variable Faraday rotation measure.

Contribution

It provides the first detailed polarization and Faraday rotation measurements of the innermost jet hotspot in 3C 84, suggesting interaction with a clumpy ambient medium as the cause of observed phenomena.

Findings

Detected significant polarized emission at the hotspot.

Measured a high and variable rotation measure up to (6.3±1.9)×10^5 rad/m^2.

Estimated electron density of about 2×10^4 cm^-3 consistent with narrow line region clouds.

Abstract

We present Very Long Baseline Array polarimetric observations of the innermost jet of 3C$\sim$84 (NGC$\sim$1275) at 43$\sim$GHz. A significant polarized emission is detected at the hotspot of the innermost re-started jet, which is located $\sim$1 pc south from the radio core. While the previous report presented a hotspot at the southern end of the western limb, the hotspot location has been moved to the southern end of the eastern limb. Faraday rotation is detected within an entire bandwidth of the 43-GHz band. The measured rotation measure (RM) is at most (6.3$\pm$1.9)$\times10^{5}$$\sim$rad$\sim$m$^{-2}$ and might be slightly time variable on the timescale of a month by a factor of a few. Our measured RM and the RM previously reported by the CARMA and SMA observations cannot be consistently explained by the spherical accretion flow with a power-law profile. We propose that a clumpy/inhomogeneous ambient medium is responsible for the observed rotation measure. Using equipartition magnetic field, we derive the electron density of $2\times10^{4}$$\sim$cm$^{-3}$. Such an electron density is consistent with the cloud of narrow line emission region around the central engine. We also discuss the magnetic field configuration from black hole scale to pc scale and the origin of low polarization.