AstroNote 2021-180

ZTF21abhywxv (AT2021qyd) is a young (<5 day) transient first detected by ZTF on UT 2021 June 25 and saved to the TNS by ALeRCE (Munoz-Arancibia et al. 2021).

The source was observed again the next night (June 26) as part of the ZTF high-cadence partnership survey. From the first to second night, the light curve appeared to fade in r-band. Due to the young age of the source, the fast fading, and the presence of a host galaxy, AT2021qyd passed two filters for rapidly evolving transients, the ZTF fast-transient filter pipeline developed by A. Ho and Y. Yao (Perley et al. 2021, arXiv:2103.01968) and the ZTFReST pipeline (Andreoni et al. 2021, arXiv:2104.06352).

LT observations obtained 1.6 days after the first ZTF detection confirmed the fast-fading nature, at 0.6 mag/day in g-band and 0.5 mag/day in r-band. The color is blue: g-r= -0.13 +/- 0.09 mag during the LT observation. 

The host galaxy is bright (g=18.45, r=17.89, z=17.50 in Legacy Survey DR9). The Legacy Survey photometric redshift is 0.03, with 68% bounds of [0.01, 0.14]. The implied peak absolute magnitude is -15.77 (68% interval [-14.23, -19.6]). 

Given the bright host galaxy and blue colors, this is likely to be shock-cooling emission from a nearby supernova, similar to fast-evolving Type IIb and Type Ib SNe observed by ZTF (Ho et al. 2021, arXiv:2105.08811). However, due to a lack of spectroscopy and host-galaxy redshift, we cannot rule out more exotic scenarios (e.g., transients like AT2018cow). 

We encourage spectroscopic classification. The most recent LT magnitudes are g=20.48 +/- 0.07 mag and r=20.61 +/- 0.06 mag (MJD 59391.9). 

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.