TITLE: GCN CIRCULAR NUMBER: 34837 SUBJECT: IC231004A: Classification of AT2023uqf/ZTF23abidzvf as a rapidly-evolving supernova with CSM interaction DATE: 23/10/18 23:57:27 GMT FROM: Robert Stein at Caltech <rdstein@astro.caltech.edu> Robert Stein (Caltech), Jannis Necker (DESY), Simeon Reusch (DESY), Anna Franckowiak (RUB), Vikram Ravi (Caltech), Anna Y. Q. Ho (Cornell), Daniel Perley (LJMU), Mansi Kasliwal (Caltech), Jesper Sollerman (OKC), Kaustav Das (Caltech) and Jean Somalwar (Caltech) report: AT2023uqf/ZTF23abidzvf was first reported by ZTF (Reusch et al., GCN 34810) as a possible optical counterpart to neutrino IC231004A (Lincetto et al., GCN 34797), as part of the broader ZTF neutrino follow-up program (Stein et al. 2023). After the initial report, our ZTF observations showed that AT2023uqf rapidly brightened, rising by ~1 mag in the first 2 days. There was a 5-day gap in ZTF observations, and the source has since been fading at a rate of > 0.1 mag per day. We obtained spectroscopic observations of this source at the Nordic Optical Telescope with ALFOSC (PI: Franckowiak), and the Low Resolution Imaging Spectrometer at the Keck observatory (LRIS; Oke et al 1995, PI: Ravi). Observations were conducted on 2013-10-14, 12 days after the transient was first detected. The spectra show a hot and mostly featureless blue continuum with numerous narrow host-galaxy emission features (H-alpha, H-beta, [OIII], [OII]) at a consistent redshift of z=0.1503) , At this redshift, the implied absolute g-band magnitude is at least M =-19.7. There are intermediate-width emission features at rest-frame wavelengths of ~4470 and ~5880 Angstroms consistent with redshifted He-I also visible, but no other individually-identifiable features from the transient. We highlight in particular the lack of any H lines in the spectrum (except those associated with the host galaxy). Based on these observations, and the general similarity of the spectra to Type Ibn events from the literature such as iPTF14aki (Hosseinzadeh et al. 2017) at a similar rest-frame epoch, we classify AT2023uqf as a Type Ibn supernova. The rapidly evolving light curve, blue colour and high peak luminosity of AT2023uqf is similar to that of other Type Ibn supernovae in the literature (e.g., Ho et al. 2023) as well as to luminous fast blue optical transients (LFBOTs) such as AT2020mrf (Yao et al. 2022). In any case, the narrow He lines present in the spectra of AT2023uqf confirm the presence of CSM interaction in this source. AT2023uqf thus unambiguously exhibits all the expected spectroscopic and temporal behaviour expected for the popular CSM neutrino production models (see e.g Murase et al. 2011, Zirakashvili & Ptuskin 2016, Petropoulou et al. 2017, Sarmah et al. 2022, Pitik et al. 2023). However, somewhat surprisingly, AT2023uqf does not belong to the canonical Type IIn class of CSM-interacting supernovae. Our spectra show that AT2023uqf experienced substantial mass loss prior to explosion, with the hydrogen shell being entirely stripped. The AT2023uqf supernova shock collided with a dense He-rich/H-poor CSM, rather than the H-rich CSM seen in Type IIn sources. We are planning multi-wavelength observations to better characterise AT2023uqf, and additional community follow-up is encouraged. ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC, USA; WIS, Israel; OKC, Sweden; JSI/UMd, USA; DESY, Germany; TANGO, Taiwan; UW Milwaukee, USA; LANL, USA; TCD, Ireland; IN2P3, France. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 non-profit organization 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 Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. 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 here were obtained in part with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOT. View this GCN Circular online at https://gcn.nasa.gov/circulars/34837. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...