TITLE: GCN CIRCULAR NUMBER: 36929 SUBJECT: IceCube-240721A: Two candidates from the Zwicky Transient Facility DATE: 24/07/24 07:02:46 GMT FROM: Jannis Necker at DESY <jannis.necker@desy.de> Sven Weimann (Ruhr University Bochum), Jannis Necker (DESY), Robert Stein (Caltech), Simeon Reusch (DESY), Anna Franckowiak (DESY/Ruhr University Bochum) report: On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: As part of the ZTF neutrino follow up program (Stein et al. 2023), we observed the localization region of the neutrino event IceCube-240721A (Zegarelli et. al, GCN 36918) with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started observations in the g- and r-band beginning at 2024-07-22 07:09 UTC, approximately 14.9 hours after event time. We covered 100.0% (5.1 sq deg) of the reported localization region. This estimate accounts for chip gaps. Each exposure was 300s with a typical depth of 21.0 mag. The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019, Stein et al. 2021) was used to search the alerts database for candidates. We reject stellar sources (Tachibana and Miller 2018) and moving objects, and apply machine learning algorithms (Mahabal et al. 2019). We are left with the following high-significance transient candidates by our pipeline, all lying within the 90.0% localization of the skymap. +--------------------------------------------------------------------------------+ | ZTF Name | IAU Name | RA (deg) | DEC (deg) | Filter | Mag | MagErr | +--------------------------------------------------------------------------------+ | ZTF23aaslxmh | AT2023qqp | 354.7409079 | +23.6986176 | g | 20.34 | 0.16 | | ZTF24aaugpvz | AT2024pge | 355.2587572 | +23.8561717 | g | 20.11 | 0.13 | +--------------------------------------------------------------------------------+ AT2023qqp was first detected on 2023-06-21. It appears to be nuclear and is slowly rising ever since its first detection. We detect a 0.5 mag flare around the time of neutrino arrival with a similar flare happening roughly 80 days before. This could indicate AGN activity, although the WISE color of the host W1-W2=-0.07 is not AGN-like (Stern et al. 2012). AT2024pge was first detected on 2024-07-10, about ten days before the neutrino arrival. The rise time of the lightcurve and the offset of ~ 1" to the host suggest a supernova origin. We will resume our regular follow-up observation schedule to obtain more observations of these objects. 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. GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949. Alert distribution service provided by DIRAC@UW (Patterson et al. 2019). Alert database searches are done by AMPEL (Nordin et al. 2019). Alert filtering is performed with nuztf (Stein et al. 2021, https://github.com/desy-multimessenger/nuztf ). View this GCN Circular online at https://gcn.nasa.gov/circulars/36929. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...