AstroNote 2020-143

We report the classification of the optical transient ZTF20abisysx / AT2020nov (RA=16:58:12.99, Dec=+02:07:03.2, z = 0.087), first detected in the Zwicky Transient Facility (ZTF; Bellm et al. 2019; Graham et al. 2019) MSIP Northern Sky Survey with an apparent g-band magnitude of 19.67+/- 0.16 on 2020 Jun 27 (JD: 2459027.73). The transient passed the Nuclear Transients GROWTH Marshal filter with coordinates consistent with the nucleus of the host galaxy within 0.12''. The object was publicly announced by ALeRCE and AMPEL (Nordin et al. 2019) via the TNS. It has continued to rise monotonically for ~25 days by ~1.5 mag while maintaining a constant g-r color. The ZTF Nuclear Transients Program triggered a follow-up observation with the SED Machine IFU Spectrograph mounted on the Palomar 60-inch (Blagorodnova et al. 2018), and we obtained a spectrum taken 20 days after discovery on 2020 Jul 16. The follow-up spectrum displayed a steep blue continuum, a broad H-alpha+[N II] line complex with FWHM ~ 125 Å, as well as a strong broad He II 4686 line profile. We report the classification of the follow-up spectrum of this transient as a likely tidal disruption event (TDE) candidate, joining the large sample of ZTF TDEs reported in van Velzen et al. 2020. The WISE color of the host galaxy WISEA J165812.98+020703.0 was not clearly AGN-like (W1-W2=0.23+/-0.04), although it displayed variability in WISE in the past. We triggered high cadence monitoring by the Neil Gehrels Swift Telescope to determine whether UV/X-rays are concurrent with the transient, and additional spectroscopic observations are ongoing. We encourage further spectroscopic follow-up observations of this TDE candidate, which is still pre-peak.

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 (Kasliwal et al. 2019), supported by NSF PIRE grant 1545949.