AstroNote 2024-118

At UTC 2024-04-21 08:37:22, ZTF detected a new transient candidate (ZTF24aakwijy) at a projected offset of 0.3” from a galaxy in Pan-STARRS. The magnitude at this time was g = 20.1 +/- 0.3 mag. Two nights later (2024-04-23 08:05:35), the transient had brightened by one magnitude, and exhibited blue colors (g - r = -0.3 mag, corrected for Milky Way extinction). The transient was reported to the Transient Name Server as AT2024hcb (Hinds et al. 2024). One night later, ZTF photometry suggested fading by 0.3 magnitudes, to g = 19.3 +/- 0.2 mag. 

The possible fast timescale and implied high peak luminosity from the photometric redshift of the nearby galaxy (SDSS z_ph = 0.289 ± 0.172, implying M < -19.5 mag) motivated us to trigger follow-up observations. We acquired spectroscopy of the candidate using GMOS-N at Gemini Observatory on 2024-04-25, under the target-of-opportunity program GN-2024A-Q-127 (PI Ho). The spectrum shows a blue continuum with a narrow emission line at 7003 Angstroms that, if H alpha, corresponds to a redshift of z=0.067. We do not detect any other features at high significance. In addition, aperture photometry on the GMOS-N acquisition image suggests that the light curve has flattened at r~19.4 mag. 

The absolute magnitude (-18.5 mag in g-band), spectrum, and flat light curve are consistent with a young Type II supernova. 

We thank the Gemini Observatory staff for assistance in rapidly obtaining these observations.

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.