TITLE:   GCN CIRCULAR
NUMBER:  32357
SUBJECT: IceCube-220624A: Classification of AT2022nit as a type Ia supernova and AT2021bei as an AGN
DATE:    22/07/09 11:15:01 GMT
FROM:    Simeon Reusch at DESY  <simeon.reusch@desy.de>

Robert Stein (Caltech), Jannis Necker, Simeon Reusch (DESY), Shreya Anand, Kaustav Das (Caltech), Jesper Sollerman (Stockholm Uni), Viraj Karambelkar, Mansi Kasliwal (Caltech) and Anna Franckowiak (DESY/Ruhr University Bochum) report:

We observed neutrino IC220624A (Santander et. al, GCN 32260) with the Zwicky Transient Facility (Reusch et al., GCN 32270) as part of our ZTF neutrino follow-up program (Stein et al. 2022). Though no candidates were identified with our initial ToO observation, we continued our regular monitoring of the localisation with additional ToO and serendipitous observations. 

As part of these additional observations, we identified the transient ZTF22aaparxg/AT2022nit as a possible optical counterpart. We undertook spectroscopic observations of AT2022nit with the Low Resolution Imaging Spectrometer (LRIS, Oke et al. 95) at the Keck I Observatory.

Using SNID (Blondin et al. 2007), we classify AT2022nit as a type Ia supernova at redshift z=0.16 (see https://www.wis-tns.org/object/2022nit for the full spectrum). Given that type Ia supernovae are not predicted to emit high-energy neutrinos, we therefore exclude AT2022nit as a candidate counterpart to IC220624A.

We also identified ZTF19adgzidh/AT2021bei as another optical counterpart candidate and observed it with ALFOSC on the 2.5m NOT telescope at Roque de los Muchachos Observatory.

Using SNID (Blondin et al. 2007), we classify AT2021bei as flare in an AGN at redshift z=0.48 (see https://www.wis-tns.org/object/2021bei for the full spectrum). Because such flares occur frequently across the sky, we expect to find coincidences between AGN flares and neutrinos by random chance. Given the lack of any exceptional flaring activity in this source, we conclude that AT2021bei is not a promising counterpart to IC220624A.

We have not identified any other candidate optical counterparts with ZTF.

Based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias.

The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.