2006ApJ...644L..63K

The discovery and study of highly transient sources, especially those that rise to high brightness and then fade to obscurity, has been a major part of modern astrophysics. Well-known examples include supernovae and novae. A by-product of the Deep Lens Survey was the discovery of three transients that varied on a timescale of less than an hour. All three had faint and red counterparts, the brightest of which was identified with an M star. However, the remaining two showed hints of an extragalactic origin; one had a spatially extended counterpart, and the other appeared in projection on the outskirts of a bright elliptical galaxy. If these two sources were really of extragalactic origin, then the two events represent a new class of exotic explosive transients. We undertook spectroscopic observations with the Keck telescope and find that the two counterparts are also late-type Galactic dwarfs. Our main conclusion is that flares from M dwarfs constitute a dense foreground fog and dominate over any plausible class of extragalactic fast transients by at least 2 orders of magnitude. Overcoming this fog will likely require dedicated surveys with careful optimization of target field location, filter(s), and cadence, presearch imaging to filter out late-type dwarfs, and a well-planned rapid follow-up plan.