AstroNote 2020-199

During the night of 2020 Oct 12 (UT), the Zwicky Transient Facility (ZTF; Kulkarni et al., ATEL #11266; Bellm et al. 2019a, PASP, 131, 18002) detected the new optical transient ZTF20acigmel / AT2020xnd.  The ZTF light curve rose to a peak and then began a rapid decline within 4 days of discovery.  Follow-up observations with the Liverpool Telescope showed evidence for a very blue color (u-g = -0.4 mag AB) during the decline phase.  Spectroscopy at Keck Observatory confirmed the source to be a featureless and extremely luminous transient at intermediate redshift (z=0.244, peak absolute magnitude -20.9).  A summary of these initial observations is presented in ATEL #14105.

We report here further follow-up photometry with the Palomar 60-inch telescope (using the Rainbow Camera on the SED Machine) showing a continued rapid decay in flux.   As of JD 2459143.6030 we measure r = 21.29 +/- 0.14 mag.

Follow-up is continuing but is hampered by poor weather at La Palma and the rapid fading of the source.  Deep optical imaging over the coming 1-2 weeks would be useful to confirm or rule out the presence of a radioactively-powered underlying supernova, which was not observed in AT2018cow (Perley et al. 2019, MNRAS, 484, 1031).

ZTF is a project led by PI S. R. Kulkarni at Caltech, and includes IPAC; WIS, Israel; OKC, Sweden; JSI/UMd, USA; UW,USA; DESY, Germany; NRC, Taiwan; UW Milwaukee, USA and LANL USA. ZTF acknowledges the generous support of the NSF under AST MSIP Grant No 1440341. Alert distribution service provided by DIRAC@UW. Alert filtering is being undertaken by the GROWTH marshal system, supported by NSF PIRE grant 1545949.  The SED Machine is based upon work supported by the NSF under grant 1106171.