Cosmic Explosions (optical Transients)

One of the principal motivations of wide-field and synoptic surveys is the search for and study of transients. By transients I mean those sources which arise from the background, are detected, and then fade away to oblivion. Transients in distant galaxies need to be sufficiently bright so as to be detectable and in almost all cases these transients are catastrophic events, marking the deaths of stars. Exemplars include supernovae and gamma-ray bursts. In our own Galaxy, the transients are, in almost all cases, cataclysmic rather than catastrophic, e.g. flares from M dwarfs, novae of all sorts (dwarf novae, recurrent novae, classical novae, X-ray novae) and instabilities in the surface layers (S Dor, eta Carina). In the nearby Universe (say out to the Virgo cluster) we have sufficient sensitivity to see classical novae. This paper is an extended summary of the talk I gave at IAU Symposium New Horizons in Time Domain Astronomy (Oxford, 2011). I first review the history of transients (which is intimately related to the advent of wide-field telescopic imaging; §1). In §2 I summarize wide field imaging projects. The motivations that led to the design of the Palomar Transient Factory (PTF) followed by a summary of the astronomical returns can be found in §3. In §4 I review the lessons learnt from PTF. I conclude that, during this decade, optical transient searches will continue to flourish and may even accelerate as surveys at other wavelengths – notably radio, UV and X-ray – come on line. As a result, I venture to suggest that specialized searches for transients will continue – even into the LSST era. I end the article by discussing the importance of follow-up telescopes for transient object studies – a topical issue given that in the US the Portfolio Review is under away (§5).