Recoiling supermassive black hole in changing-look AGN Mrk 1018
  The spectral type of Mrk 1018 changed from Type 1.9 to 1 and returned back to
1.9 over a period of 40 years. We have investigated physical mechanisms
responsible for the spectral change in Mrk 1018 by analyzing archival spectral
and imaging data. Two kinematically distinct broad-line components, blueshifted
and redshifted components, are found from spectral decomposition. The velocity
offset curve of the broad-line as a function of time shows a characteristic
pattern. An oscillating recoiled supermassive black hole (rSMBH) scenario is
proposed to explain the observed velocity offset in broad emission lines. A
Bayesian Markov-Chain Monte Carlo simulation is performed to derive the best
fit orbital parameters, we find that the rSMBH has a highly eccentric orbit
with a period of $\sim$29 years. The AGN activity traced by variation of broad
H$\beta$ emission line is found to increase and decrease rapidly at the start
and end of the cycle, and peaked twice at the start and near the end of the
cycle. Extinction at the start and end of the cycle (when its spectral type is
Type 1.9) is found to increase due to increased covering factor. Perturbations
of the accretion disk caused by pericentric passage can reasonably explain the
AGN activity and spectral change in Mrk 1018. Currently, the spectral type of
Mrk 1018 is Type 1.9, and we do not know if it will repeat a similar pattern of
spectral change in the future, but if it does, then spectral type will turn to
Type 1 around mid 2020's.
