Dust Reverberation Mapping of Type 2 AGN NGC 2110 Realized with X-ray
  and 3-5 $\mu$m IR monitoring
  The dust reverberation mapping is one of powerful methods to investigate the
structure of the dusty tori in AGNs, and it has been performed on more than a
hundred type 1 AGNs. However, no clear results have been reported on type 2
AGNs because their strong optical-UV extinction completely hides their
accretion disc emission. Here we focus on an X-ray-bright type 2 AGN, NGC 2110,
and utilize 2-20 keV X-ray variation monitored by MAXI to trace disc emission,
instead of optical-UV variation. Comparing it with light curves in the WISE
infrared (IR) W1 band ($\lambda = 3.4$ $\mu$m) and W2 band ($\lambda = 4.6$
$\mu$m) with cross-correlation analyses, we found candidates of the dust
reverberation time lag at $\sim60$ days, $\sim130$ days, and $\sim1250$ days
between the X-ray flux variation and those of the IR bands. By examining the
best-fitting X-ray and IR light curves with the derived time lags, we found
that the time lag of $\sim130$ days is most favoured. With this time lag, the
relation between the time lag and luminosity of NGC 2110 is consistent with
those in type 1 AGNs, suggesting that the dust reverberation in NGC 2110 mainly
originates in hot dust in the torus innermost region, the same as in type 1
AGNs. As demonstrated by the present study, X-ray and IR simultaneous
monitoring can be a promising tool to perform the dust reverberation mapping on
type 2 AGNs.
