AstroNote 2022-133

On 2022-06-21 05:28 UT (MJD 59751.2279) ZTF detected a new optical transient (ZTF22aaosjja) with a magnitude of g = 18.44 +/- 0.08.  The last secure ZTF nondetection was approximately two days prior (MJD 59749.2643) with a 5-sigma limit of g > 20.51 mag.  The source was detected the previous night by ATLAS (c = 19.32 at MJD 59750.2978) and reported to TNS as AT2022ncc (ATLAS22rbf; Tonry et al., TNS report ID 150642).  The source is very blue (g-i ~ -0.6 mag from ZTF imaging) and rising rapidly (>1 mag over the 25 hours between the last upper limit and ATLAS detection and another 1 mag over the 22 hours between the ATLAS and ZTF observations).  It is coincident with a faint, blue galaxy seen in SDSS, PS1, and Legacy Survey imaging; the galaxy has quoted photometric redshifts of 0.177, 0.219, and 0.128 by each of these surveys, respectively (albeit with large uncertainties in each case).

We triggered imaging with LT (IO:O) and Swift (UVOT+XRT), and spectroscopy with LT (SPRAT) and Gemini-North (GMOS-N).  The flux continued to rise over the 24 hours since the first ZTF detection but the light curve has since flattened, suggesting it is already near peak (rise time of ~3 days).  The LT and Gemini spectra show a purely featureless blue continuum.  Weak narrow emission lines from the host galaxy are visible in the Gemini spectrum at a redshift of z=0.0542.  This indicates a peak absolute magnitude of M_g ~ -19 (assuming no further brightening).   The source is not detected in Swift/XRT observations.

The source is somewhat less luminous than known radio-loud fast blue optical transients such as AT2018cow and no multiwavelength emission has yet been detected, but an association as a member of this class has not yet been ruled out.  Alternatively, it may be a fast-rising interaction-dominated supernova such as a SN Ibn, or an exceptionally luminous young core-collapse SN.  Further observations are ongoing.

This report is based 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.