Rapid Luminosity Decline and Subsequent Reformation of the Innermost
  Dust Distribution in the Changing-look AGN Mrk 590
  We examine the long-term optical/near-infrared (NIR) flux variability of a
"changing-look" active galactic nucleus (AGN) Mrk 590 between 1998 and 2007.
Multi-band multi-epoch optical/NIR photometry data from the SDSS Stripe 82
database and the Multicolor Active Galactic Nuclei Monitoring (MAGNUM) project
reveal that Mrk 590 experienced a sudden luminosity decrease during the period
from 2000 to 2001. Detection of dust reverberation lag signals between $V$- and
$K$-band light curves obtained by the MAGNUM project during the faint state in
$2003-2007$ suggests that the dust torus innermost radius $R_\text{dust}$ of
Mrk 590 had become very small [$R_\text{dust} \simeq 32$~ light-days (lt-days)]
by the year 2004 according to the aforementioned significant decrease in AGN
luminosity. The $R_\text{dust}$ in the faint state is comparable to the
H$\beta$ broad line region (BLR) radius of $R_{\text{H}\beta, \text{BLR}}
\simeq 26$ lt-days measured by previous reverberation mapping observations
during the bright state of Mrk 590 in $1990-1996$. These observations indicate
that the innermost radius of the dust torus in Mrk 590 decreased rapidly after
the AGN ultraviolet-optical luminosity drop, and that the replenishment time
scale of the innermost dust distribution is less than 4 years, which is much
shorter than the free fall time scale of BLR gas or dust clouds. We suggest
that rapid replenishment of the innermost dust distribution can be accomplished
either by new dust formation in radiatively-cooled BLR gas clouds or by new
dust formation in the disk atmosphere and subsequent vertical wind from the
dusty disk as a result of radiation pressure.
