An Optically Led Search for Kilonovae to z$\sim$0.3 with the Kilonova and Transients Program (KNTraP)

TL;DR

This paper reports on the first observational run of the KNTraP program using the Dark Energy Camera, aiming to detect kilonovae in wide-field optical surveys without gravitational wave triggers, and sets constraints on their rate.

Contribution

First wide-field optical search for kilonovae without GW triggers, demonstrating real-time data processing and setting new upper limits on kilonova rates.

Findings

No kilonovae detected in the first run.

Three fast-rising candidates identified but not consistent with kilonova models.

Established an upper limit on kilonova rate of 1.8×10^5 Gpc^-3 yr^-1.

Abstract

Compact binary mergers detectable in gravitational waves can be accompanied by a kilonova, an electromagnetic transient powered by radioactive decay of newly synthesised r-process elements. A few kilonova candidates have been observed during short gamma-ray burst follow-up, and one found associated with a gravitational wave detection, GW170817. However, robust kilonova candidates are yet to be found in un-triggered, wide-field optical surveys, that is, a search not requiring an initial gravitational wave or gamma-ray burst trigger. Here we present the first observing run for the Kilonova and Transients Program (KNTraP) using the Dark Energy Camera. The first KNTraP run ran for 11 nights, covering 31 fields at a nightly cadence in two filters. The program can detect transients beyond the LIGO/Virgo/KAGRA horizon, be agnostic to the merger orientation, avoid the Sun and/or Galactic plane, and produces high cadence multi-wavelength light curves. The data were processed nightly in real-time for rapid identification of transient candidates, allowing for follow-up of interesting candidates before they faded away. Three fast-rising candidates were identified in real-time, however none had the characteristics of the kilonova AT2017gfo associated with GW170817 or with the expected evolution for kilonovae from our fade-rate models. After the run, the data were reprocessed, then subjected to stringent filtering and model fitting to search for kilonovae offline. Multiple KNTraP runs (3+) are expected to detect kilonovae via this optical-only search method. No kilonovae were detected in this first KNTraP run using our selection criteria, constraining the KN rate to $R < 1.8\times10^{5}$ Gpc$^{-3}$ yr$^{-1}$.