{
  "bibcode": "2020GCN.28757....1S",
  "body": "Robert Stein (DESY), Simeon Reusch (DESY), Sven Weimann (Ruhr University Bochum) and Michael Coughlin (University of Minnesota) report,\n\nOn behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: \n\nWe observed the localization region of the neutrino event IceCube-201021A (Lagunas et. al, GCN 28715) 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 2020-10-23 02:21 UTC, approximately 43.7 hours after event time. We covered 6.3 sq deg at least twice, corresponding to 95.7% of the reported localization region. This estimate accounts for chip gaps. Each exposure was 300s with a typical depth of 21.0 mag. \n \nThe 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. 2020) 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 one high-significance transient candidate by our pipeline, lying within the 90.0% localization of the skymap.\n\n+---------------------------------------------------------------------------------+\n| ZTF Name     | IAU Name   | RA (deg)    | DEC (deg)   | Filter | Mag   | MagErr |\n+---------------------------------------------------------------------------------+\n| ZTF20acmxnpa |  AT2020ybb | 260.4617133 | +14.7743083 | g      | 20.94 | 0.17   |\n+---------------------------------------------------------------------------------+\n\nZTF20acmxnpa/AT2020ybb was first detected on 2020-10-13 02:37 UTC, and is a candidate supernova around peak. This would be consistent with a CSM-interaction supernova neutrino production model. We encourage spectroscopic follow-up of this object to confirm its nature.\n\nWe were unable to retrieve the optical source reported by Im et al. (GCN 28756) with forced photometry of the ZTF science images (at SNR=3). However, our limiting magnitude is comparable to the reported flux level.\n\nZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC, USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY, Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan; IITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd, Australia.\n\nZTF acknowledges the generous support of the NSF under AST MSIP Grant No 1440341. \nGROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949.\nAlert distribution service provided by DIRAC@UW (Patterson et al. 2019).\nAlert database searches are done by AMPEL (Nordin et al. 2019).\nAlert filtering is performed with the AMPEL Follow-up Pipeline (Stein et al. 2020).",
  "circularId": 28757,
  "createdOn": 1603470516000,
  "email": "robert.stein@desy.de",
  "subject": "IceCube-201021A: One candidate counterpart from the Zwicky Transient Facility",
  "submitter": "Robert Stein at DESY  <robert.stein@desy.de>",
  "eventId": "IceCube-201021A"
}