We present a newmethod for effectively selecting objects which may be low-mass active black holes (BHs) at galaxy centers using high-cadence optical imaging data, and our first spectroscopic identification of an active 2.7 106M BH at z = 0.164. This active BH was originally selected due to its rapid optical variability, from a few hours to a day, based on Subaru Hyper Suprime-Cam g-band imaging data taken with a 1 hr cadence. Broad and narrow H lines and many other emission ones are detected in our optical spectra taken with Subaru FOCAS, and the BH mass is measured via the broad H emission line width (1880kms1) and luminosity (4.2 1040 erg s1) after careful correction to the atmospheric absorption around 7580-7720 A° . We measure the Eddington ratio and find it to be as low as 0.05, considerably smaller than those in a previous SDSS sample with similar BH mass and redshift, which indicates one of the special potentials of our Subaru survey. The g r color and morphology of the extended component indicate that the host galaxy is a star-forming galaxy. We also show the effectiveness of our variability selection for low-mass active BHs.
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