# The Palomar Transient Factory Sky2Night programme

## TL;DR

The Sky2Night project systematically searched for fast optical transients over 407 deg$^2$ with the Palomar Transient Factory, identifying various transient types and setting upper limits on extragalactic fast transient rates, aiding gravitational wave follow-up strategies.

## Contribution

This study provides the first systematic, unbiased survey for fast optical transients, establishing robust transient rates and false positive expectations for gravitational wave counterpart searches.

## Key findings

- Detected 12 supernovae and other transient types.
- Set upper limits on extragalactic fast transient rates.
- Provided data to improve gravitational wave follow-up strategies.

## 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 $\mathcal{R}<37\times 10^{-4}$deg$^{-2}$d$^{-1}$ and $\mathcal{R}<9.3\times 10^{-4}$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.