  It has long been expected that some massive stars produce stellar mass black
holes (BHs) upon death. Unfortunately, the observational signature of such
events has been unclear. It has even been suggested that the result may be an
"unnova," in which the formation of a BH is marked by the disappearance of a
star rather than an electromagnetic outburst. I argue that when the progenitor
is a red supergiant, evidence for BH creation may instead be a ~3-10 day
optical transient with a peak luminosity of ~10^{40}-10^{41} erg s^{-1}, a
temperature of ~10^4 K, slow ejection speeds of ~200 km s^{-1}, and a spectrum
devoid of the nucleosynthetic products associated with explosive burning. This
signal is the breakout of a shock generated by the hydrodynamic response of a
massive stellar envelope when the protoneutron star loses ~few*0.1Msun to
neutrino emission prior to collapse to a BH. Current and future wide-field,
high-cadence optical surveys make this an ideal time to discover and study
these events. Motivated by the unique parameter space probed by this scenario,
I discuss more broadly the range of properties expected for shock breakout
flashes, with emphasis on progenitors with large radii and/or small shock
energies. This may have application in a wider diversity of explosive events,
from pair instability supernovae to newly discovered but yet to be understood
transients.
