AstroNote 2023-115

We report activity at the location of the transient ZTF20acaazkt that is young, rising, bright (g < ~ 16.9 mag) and blue (g-r ~ -0.35 mag). Based on the ZTF g-band, the last non-detection was 17 days ago and the first detection 9 days ago (g=18 mag). The last observation indicates a ~ 3 mag rise from the last non-detection (16 days) and a  ~ 1.1 mag rise from the first detection (8 days).

Furthermore, it appears to be spatially coincident with the location of the blue transient AT2020vdq, picked up by both ZTF and Gaia around 3 years ago. The 2020 transient was detected around peak (MJD=59126), lasted for around 50 days and peaked at a g-band magnitude of 18.97. The decline was characterised by blue colours as well.

The photometric redshift of the host galaxy (SDSS J100853.43+424300.2; DR12) is z=0.08+-0.03. At z=0.08 the transient is superluminous (M ~ -20.92 mag in g-band). The host is not a known AGN / X-ray source.

We obtained a spectrum with the ALFOSC instrument mounted on the NOT telescope on the night of 2023-05-16 (MJD=60080.89) that shows an almost featureless very blue continuum (T > 20,000 K) with some possible faint broad features.  

The features of this transient (blue colours and featureless) make it a young core-collapse SN candidate but the nuclear location and the precursor activity makes its nature uncertain (a nuclear transient such as a TDE cannot yet be excluded). We will acquire another ALFOSC spectrum within a week, based on which the nature of the tranient can be potentially revealed (spectroscopic lines and blackbody temperature evolution). Swift telescope observations have been triggered. Further observations are encouraged.