LIGO recently finished their third observing run (O3), in which they identified many new Gravitational Wave (GW) candidate events. Many of these mergers contained at least one neutron star, either as a binary pair (NSNS merger), or with a black hole (NSBH). Neutron star mergers can produce electromagnetic transients known as kilonovae.
LIGO GW events are published in realtime, so that astronomers can observe the direction of the GW event and seek to find a kilonova. We observed 13 of these mergers with the Zwicky Transient Facility (ZTF), an optical telescope on Mt Palomar, CA. Objects detected in these images are then processed by several pipelines, filtering out unrelated objects and leaving only strong kilonova candidates. These candidates are then prioritised for additional follow-up observations, to determine their nature.
One method for filtering ZTF objects is the AMPEL Follow-up Pipeline, a pipeline I developed to identify candidate electromagnetic counterparts to multi-messenger alerts. It rejects objects based on image quality, photometric properties and compatibility with a given trigger. It is built using AMPEL software framework for realtime astronomy.
Unfortunately, we did not identify a kilonova, and nor did any other teams. This was unlike in the previous LIGO O2 observing run when the kilonova AT2017gfo was identified coincident with BNS GW170817. Instead, using our ZTF non-detections, we set limits on the luminosity function of kilonovae, shown below alongside the Kasen and Bullia models. Hopefully we’ll be luckier with O4!
You can read the full article in ApJ.