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[Abridged] We present a search for fast optical transients (~0.5 hr-1 day)using repeated observations of the Pan-STARRS1 Medium-Deep Survey (PS1/MDS)fields. Our search takes advantage of the consecutive g/r-band observations(16.5 min in each filter), by requiring detections in both bands, withnon-detections on preceding and subsequent nights. We identify 19 transientsbrighter than 22.5 AB mag (S/N>10). Of these, 11 events exhibit quiescentcounterparts in the deep PS1/MDS templates that we identify as M4-M9 dwarfs.The remaining 8 transients exhibit a range of properties indicative ofmain-belt asteroids near the stationary point of their orbits. Withidentifications for all 19 transients, we place an upper limit ofR_FOT(0.5hr)<0.12 deg^-2 d^-1 (95% confidence level) on the sky-projected rateof extragalactic fast transients at <22.5 mag, a factor of 30-50 times lowerthan previous limits; the limit for a timescale of ~day is R_FOT<2.4e-3 deg^-2d^-1. To convert these sky-projected rates to volumetric rates, we explore theexpected peak luminosities of fast optical transients powered by variousmechanisms, and find that non-relativistic events are limited to M~-10 mag(M~-14 mag) for a timescale of ~0.5 hr (~day), while relativistic sources(e.g., GRBs, magnetar-powered transients) can reach much larger luminosities.The resulting volumetric rates are <13 (M~-10 mag), <0.05 (M~-14 mag) and <1e-6Mpc^-3 yr^-1 (M~-24 mag), significantly above the nova, supernova, and GRBrates, respectively, indicating that much larger surveys are required toprovide meaningful constraints. Motivated by the results of our search wediscuss strategies for identifying fast optical transients in the LSST mainsurvey, and reach the optimistic conclusion that the veil of foregroundcontaminants can be lifted with the survey data, without the need for expensivefollow-up observations.

astrophysical journal/the astrophysical journal

A Search for Fast Optical Transients in the Pan-STARRS1 Medium-Deep  Survey: M Dwarf Flares, Asteroids, Limits on Extragalactic Rates, and  Implications for LSST