GRANDMA Observations of ZTF/Fink Transients during Summer 2021

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, from 1 April to 30 September 2021, 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 promising 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 then demonstrate the added value of a substantial follow-up campaign of optical transients. 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 from one to 23 images within the first two days after the Fink alert 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 and the commissioning of the Vera C. Rubin Observatory.

The GRANDMA network in preparation for the fourth gravitational-wave observing run

“…To uniformly process the diverse set of images acquired by various telescopes, we have developed two dedicated data analysis tools: MUphoten ¶11, 12 and STDPipe ‖ . 12,13 They follow slightly different approaches, with the former being a ready-to-use set of scripts pre-configured for processing the data from selected instruments, while the latter is a library of both low-and high-level routines for quick creation of custom pipelines for the data from arbitrary telescopes and varying complexity of the analysis (e.g., taking into account spatial dependence of photometric zero points or color terms, using custom noise models, advanced filtering of detected transient candidates, etc.). We foresee them being used by both telescope teams lacking their in-house pipelines for quick-look data analysis, and for the final centralized data processing.…”

“…The results of these campaigns are presented in separate papers. 12 In this section we discuss some details of the campaigns and the lessons learned from them.…”

“…More details on the triggering criteria can be found in the paper that describes the campaign. 12 Only a handful of candidates were selected by more than one filter. The campaign followed six events for which a total of 180 photometric data points were obtained.…”