AstroNote 2021-155

ZTF21abaiono (SN 2021lwz) was first detected by the Zwicky Transient Facility (ZTF; Bellm et al. 2019; Graham et al. 2019) in public imaging from 2021-05-10 with a discovery magnitude of g=18.76 (MJD 59344.1809). It was saved as a SN candidate as part of scanning for the Bright Transient Survey (Fremling et al. 2020; Perley et al. 2020).  It was also identified as a fast transient candidate on the basis of its rapid rise (the most recent g-band upper limit is from 2 days before the detection; g>20.57 mag at MJD 59324.22) and coincidence with a dim, blue, extended counterpart in pre-explosion survey imaging from Pan-STARRS, SDSS, and the Legacy Survey.

A spectrum was obtained on 2021-05-11 with the SED Machine IFU at the Palomar 60-inch telescope (Blagorodnova et al. 2018, Rigault et al. 2019).  Only a featureless blue continuum is evident in this observation.  We triggered Swift and the Liverpool Telescope for additional photometry, and obtained a short spectrum (300s) with the Low-Resolution Imaging Spectrometer (LRIS) on Keck I at UT 2021-05-13 05:40.  The Keck spectrum is largely featureless in the red arm, but shows probable narrow emission from host-galaxy H-alpha at a redshift of z=0.065, along with (marginal) detections of narrow [OIII] at consistent redshift.  The blue arm shows several weak broad continuum features similar to those seen in pre-max Type I superluminous supernovae (e.g. SN 2005ap, PTF10cwr) but at higher velocities (~ 21000 km/s).

At this redshift, the magnitude at the time of the most recent ZTF observation (g = 17.58 at MJD 59348.9) implies an absolute magnitude of approximately -19.8.  The source is very blue and still rising in all optical bands.  We suggest that this is likely a superluminous supernova (SLSN-I) caught very early in its evolution.  Follow-up observations across the electromagnetic spectrum are encouraged.

This report is based in part on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW.