GRANDMA Observations of ZTF/Fink Transients during Summer 2021

TL;DR

This paper details GRANDMA's coordinated follow-up observations of ZTF transients in 2021, employing multiple selection methods to identify kilonova candidates, and highlights the role of amateur astronomers in rapid classification, though no kilonovae were confirmed.

Contribution

It introduces three new selection methods for kilonova candidates and demonstrates the effectiveness of a large, coordinated telescope network including amateurs for rapid transient follow-up.

Findings

Six transients followed-up, mostly classified as asteroids or supernovae.

No kilonovae confirmed during the campaign.

Rapid decay rate measurement within 1.5 days post-discovery.

Abstract

We present our follow-up observations with GRANDMA of transient sources revealed by the Zwicky Transient Facility (ZTF). Over a period of six months, all ZTF triggers were examined in real time by a dedicated science module implemented in the Fink broker, which will be used for the data processing of the Vera C. Rubin Observatory. In this article, we present three selection methods to identify kilonova candidates. Out of more than 35 million candidates, a hundred sources have passed our selection criteria. Six were then followed-up by GRANDMA (by both professional and amateur astronomers). The majority were finally classified either as asteroids or as supernovae events. We mobilized 37 telescopes, bringing together a large sample of images, taken under various conditions and quality. To complement the orphan kilonova candidates (those without associated gamma-ray bursts, which were all), we included three additional supernovae alerts to conduct further observations of during summer 2021. We demonstrate the importance of the amateur astronomer community that contributed images for scientific analyzes of new sources discovered in a magnitude range r'=17-19 mag. We based our rapid kilonova classification on the decay rate of the optical source that should exceed 0.3 mag/day. GRANDMA's follow-up determined the fading rate within 1.5+/-1.2 days post-discovery, without waiting for further observations from ZTF. No confirmed kilonovae were discovered during our observing campaign. This work will be continued in the coming months in the view of preparing for kilonova searches in the next gravitational-wave observing run O4.