We examined 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 ( 1 ) new dust formation in radiatively-cooled BLR gas clouds or ( 2 ) new dust formation in the disk atmosphere and subsequent vertical wind from the dusty disk launched by radiation pressure .