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