Mid-InfraRed Outburst in Nearby Galaxies (MIRONG) I: Sample Selection
  and Characterization
  The optical time-domain astronomy has grown rapidly in the past decade but
the dynamic infrared sky is rarely explored. Aiming to construct a sample of
mid-infrared outburst in nearby galaxies (MIRONG), we have conducted a
systematical search of low-redshift ($z<0.35$) SDSS spectroscopic galaxies that
have experienced recent MIR flares using their Wide-field Infrared Survey
Explorer (WISE) light curves. A total of 137 galaxies have been selected by
requiring a brightening amplitude of 0.5 magnitude in at least one WISE band
with respect to their quiescent phases. Only a small faction (10.9%) has
corresponding optical flares. Except for the four supernova (SNe) in our
sample, the MIR luminosity of remaining sources ($L_{\rm 4.6\mu m}>10^{42}~\rm
erg~s^{-1}$) are markedly brighter than known SNe and their physical locations
are very close to the galactic center (median <0.1"). Only four galaxies are
radio-loud indicating that synchrotron radiation from relativistic jets could
contribute MIR variability. We propose that these MIR outburst are dominated by
the dust echoes of transient accretion onto supermassive black holes, such as
tidal disruption events (TDEs) and turn-on (changing-look) AGNs. Moreover, the
inferred peak MIR luminosity function is generally consistent with the X-ray
and optical TDEs at high end albeit with large uncertainties. Our results
suggest that a large population of transients have been overlooked by optical
surveys, probably due to dust obscuration or intrinsically optical weakness.
Thus, a search in the infrared band is crucial for us to obtain a panoramic
picture of nuclear outburst. The multiwavength follow-up observations of the
MIRONG sample are in progress and will be presented in a series of subsequent
papers.
