The Palomar Transient Factory Sky2Night programme

TLDR

The Sky2Night project systematically searches for fast optical transients with the Palomar Transient Factory to quantify expected astrophysical false positives in gravitational‑wave follow‑ups for kilonovae. We monitored 407 deg² in R‑band for eight nights with a two‑hour cadence and used the William Herschel Telescope to obtain spectra, enabling calculation of robust transient rates and upper limits on extragalactic fast optical transients. The survey detected 12 supernovae, 10 cataclysmic variables, 9 flaring M‑stars, 3 flaring AGN, and set upper limits of R<37×10⁻⁴ deg⁻² d⁻¹ (4 h) and R<9.3×10⁻⁴ deg⁻² d⁻¹ (1 d) for extragalactic fast optical transients at R≈19.7.

Abstract

We present results of the Sky2Night project: a systematic, unbiased search for fast optical transients with the Palomar Transient Factory. We have observed 407 deg$^2$ in $R$-band for 8 nights at a cadence of 2 hours. During the entire duration of the project, the 4.2m 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, 9 flaring M-stars, 3 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 $\mathcal{R}&lt;37\times 10^{-4}$deg$^{-2}$d$^{-1}$ and $\mathcal{R}&lt;9.3\times 10^{-4}$deg$^{-2}$d$^{-1}$ for timescales of 4h and 1d and a limiting magnitude of $R\approx19.7$. 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.