Title: SPIRITS: Uncovering Unusual Infrared Transients With Spitzer,
Abstract: We present an ongoing, systematic search for extragalactic infrared
transients, dubbed SPIRITS --- SPitzer InfraRed Intensive Transients Survey. In
the first year, using Spitzer/IRAC, we searched 190 nearby galaxies with
cadence baselines of one month and six months. We discovered over 1958
variables and 43 transients. Here, we describe the survey design and highlight
14 unusual infrared transients with no optical counterparts to deep limits,
which we refer to as SPRITEs (eSPecially Red Intermediate Luminosity Transient
Events). SPRITEs are in the infrared luminosity gap between novae and
supernovae, with [4.5] absolute magnitudes between -11 and -14 (Vega-mag) and
[3.6]-[4.5] colors between 0.3 mag and 1.6 mag. The photometric evolution of
SPRITEs is diverse, ranging from < 0.1 mag/yr to > 7 mag/yr. SPRITEs occur in
star-forming galaxies. We present an in-depth study of one of them, SPIRITS
14ajc in Messier 83, which shows shock-excited molecular hydrogen emission.
This shock may have been triggered by the dynamic decay of a non-hierarchical
system of massive stars that led to either the formation of a binary or a
proto-stellar merger.