ABSTRACT:
The advent of wide-field synoptic imaging has re-invigorated the venerable field of time domain astronomy. Our framework of cosmic explosions no longer has a wide six-magnitude luminosity "gap" between the brightest novae and faintest supernovae. Multiple new and distinct classes of very rare explosions have been uncovered just in the past few years.
I review the surge of excitement (and debate) on the physics of these transients with unprecedented explosion signatures. "Gap transients" represent missing pieces in two fundamental pictures: the fate of massive stars and the evolution of compact binaries. Calcium-rich gap transients may even hold the key to solving a long-standing abundance problem in the intra-cluster medium. I conclude with the next frontier in gap transients --- discovering elusive binary neutron star mergers, a goal which may soon be within reach with coordination between the next generation of synoptic surveys and advanced gravitational wave interferometers.