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