2019MNRAS.484.4507V

We present results of the Sky2Night project: a systematic, unbiased search for fast optical transients with the Palomar Transient Factory. We have observed 407deg² in R-band for eight nights at a cadence of 2 h. During the entire duration of the project, the 4.2 m William Herschel Telescope on La Palma was dedicated to obtaining identification spectra for the detected transients. During the search, we found 12 supernovae, 10 outbursting cataclysmic variables, nine flaring M-stars, three flaring active galactic nuclei, and no extragalactic fast optical transients. Using this systematic survey for transients, we have calculated robust observed rates for the detected types of transients, and upper limits of the rate of extragalactic fast optical transients of R< 37×10^–4deg^–2 d^–1 and R< 9.3×10^–4deg^–2 d^–1 for time-scales of 4 h and 1 d and a limiting magnitude of R ≃ 19.7. We use the results of this project to determine what kind of and how many astrophysical false positives we can expect when following up gravitational wave detections in search for kilonovae.