Mid-infrared Outbursts in Nearby Galaxies (MIRONG). II. Optical Spectroscopic Follow-up

Infrared echo has proven to be an effective means discover transient accretion events of supermassive black holes (SMBHs), such as tidal disruption (TDEs) and changing-look active galactic nuclei (AGNs), in dusty circumnuclear environments. To explore the populations SMBH events, we have constructed a large sample Mid-infrared Outbursts Nearby Galaxies (MIRONG) performed multiwavelength observations. Here present results multiepoch spectroscopic follow-up observations subsample 54 objects spanning time scale 4 yr. Emission-line variability was detected 22 them with either emergence or enhancement broad Balmer emission lines comparison pre-outburst spectra. Coronal lines, HeII{\lambda}4686 Bowen line NIII{\lambda}4640 appeared spectra nine,seven two sources, respectively. These suggest that MIRONG is mixed bag different sources. We tentatively classified into subclass according their spectral evolution light curves. Two sources been steady high H{\alpha} flux up latest observation might turn-on AGNs. Broad faded out remaining indicating ionizing source ignited by TDE sporadic gas accretion. Thirty-one do not show noticeable change respect They statistically redder MIR color lower luminosity outbursts,which are consistent heavily obscured events.