VLBA Observations of Sub-Parsec Structure in Mrk 231: Interaction between a Relativistic Jet and a BAL Wind

TL;DR

This study presents high-frequency VLBI observations of Mrk 231, revealing a relativistic jet interacting with a dense BAL wind, with evidence of a compact radio lobe and jet activity influenced by surrounding gas.

Contribution

First high-frequency VLBI imaging of Mrk 231 showing jet-wind interaction and detailed analysis of jet components and absorption features.

Findings

Detected a compact double radio source with a core flare exceeding 150% flux increase.

Observed slow movement of a secondary component over 9.4 years, indicating jet dynamics.

Identified free-free absorption consistent with dense gas associated with the BAL wind.

Abstract

We report on the first high frequency VLBI observations of the nearby broad absorption line quasar (BALQSO), Mrk 231. Three epochs of observations were achieved at 15 GHz and 22 GHz, two of these included 43 GHz observations as well. The nuclear radio source is resolved as a compact double. The core component experienced a strong flare in which the flux density at 22 GHz increased by $> 150%$ (45 mJy) in three months. Theoretical models of the flare imply that the emission is likely enhanced by very strong Doppler boosting of a highly relativistic ejecta with a kinetic energy flux, $Q \sim 3 \times 10^{43} \mathrm{ergs/sec}$. Combining our data with two previous epochs of 15 GHz data, shows marginal evidence for the slow advance of the secondary component (located $\approx 0.97$ pc from the core) over a 9.4 year span. We estimate that the long term time averaged kinetic energy flux of the secondary at $\bar{Q}\approx 10^{42}\mathrm{ergs/sec}$. Low frequency VLBA observations indicate that the secondary is seen through a shroud of free-free absorbing gas with an emission measure of $\approx 10^{8} \mathrm{cm}^{-6}\mathrm{pc}$. The steep spectrum secondary component appears to be a compact radio lobe that is associated with a working surface between the ram-pressure confined jet, and a dense medium that is likely to be the source of the free-free absorption. The properties of the dense gas are consistent with the temperatures, displacement from the nucleus and the column density of total hydrogen commonly associated with the BAL wind.