AstroNote 2020-192

The Zwicky Transient Facility (ZTF) detected a bright (r=16.3) optical transient in a setting field on 2020-10-06.  The transient (internal name: ZTF20abzumlr) was saved as part of nightly ZTF Bright Transient Survey (BTS) scanning activities.   It was previously reported to TNS by ATLAS as AT2020tcw / ATLAS20zst, who report an initial detection on 2020-09-12 at a magnitude of cyan=18.92.  The most recent ZTF upper limit was on 2020-09-05 (g > 19.5).  No observations of the field were conducted in ZTF public data between this nondetection observation and the recent detection, although detections in g, r, and i filters were recovered from ZTF partnership data on 2020-09-17 and 2020-09-19.

The location of the transient is coincident with an extreme starburst galaxy at a spectroscopic redshift (from SDSS) of z=0.0645, implying an absolute magnitude of -20.4 in the most recent observations.

A spectrum of the transient was acquired using the Spectral Energy Distribution Machine at the Palomar 60-inch telescope on 2020-10-14.  The spectrum contains strong narrow emission lines from the host galaxy as well as broad features consistent with SLSN-I templates around maximum light.  We therefore classify this as a Type I superluminous supernova.  While the nighttime observability of this source is currently poor, it will likely be amenable to late-time follow-up in future observing seasons, as well as for multi-wavelength follow-up.