This proposal aims for completing spectroscopic follow-up observations for a sample of drastically fading quasars discovered by comparing SDSS and Subaru wide-field imaging (Suprime-Cam and HSC) photometry of the SDSS quasars. These quasars are among quasars with the top 0.01% largest declining variability and the best candidates in which AGN activity shuts down or becomes obscured. The ultimate goal of this proposal is to identify long-lasting shutting-down events in AGN, and to study in great detail what happens in this kind of rapid change in accretion state and their host galaxy properties. We here take optical spectra and imaging with Keck/LRIS of 5 fading quasars with decline by a factor of >6, selected using the archival Suprime-Cam data. We measure the flux of the remaining broad emission lines to obtain the accurate decline factor which is difficult to estimate in the faded phase because of the large contamination of the host galaxy fluxes to the total fluxes. The new data will enable us to examine whether or not obscuration of the accretion disk happens. These quasars are all in the northern hemisphere and will never be observed with Rubin/LSST and this study is the best use of the archival Suprime-Cam data in time-domain astronomy.
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