The Nature of the Deep Lens Survey Fast Transients

The discovery and study of highly transient sources, especially those whichrise to high brightness and then fade to obscurity, has been a major part ofmodern astrophysics. Well known examples include supernovae and novae. Abyproduct of the Deep Lens Survey was the discovery of three transients whichvaried on a timescale of less than an hour. All three had faint and redcounterparts, the brightest of which was identified with an M star. However,the remaining two showed hints of an extragalactic origin, one had a spatiallyextended counterpart and the other appeared in projection on the outskirts of abright elliptical galaxy. If these two sources were really of an extragalacticorigin then the two events represent a new class of exotic explosivetransients. We undertook spectroscopic observations with the Keck telescope andfind the two counterparts are also late type Galactic dwarfs. Our mainconclusion is that flares from M dwarfs constitute a dense foreground fog anddominate over any plausible class of extragalactic fast transients by at leasttwo orders of magnitude. Overcoming this fog will likely require dedicatedsurveys with careful optimization of target field location, filter(s) andcadence, pre-search imaging to filter out late type dwarfs and a well plannedrapid followup plan.Comment: 4 pages, 2 figures, ApJL in press, referee's comments implemente