MUSSES2020J: The Earliest Discovery of a Fast Blue Ultraluminous Transient at Redshift 1.063

TL;DR

This paper reports the discovery of MUSSES2020J, an ultraluminous, fast-evolving UV transient at redshift 1.063, highlighting its unique properties and potential physical mechanisms, and emphasizing its significance for future high-redshift transient surveys.

Contribution

First discovery of a fast blue ultraluminous transient at high redshift, with detailed early-phase light curve analysis and discussion of possible powering mechanisms.

Findings

MUSSES2020J has a rise time of about 5 days and extremely high UV luminosity.

Located near the galaxy center at redshift 1.063, suggesting a possible link to the host galaxy's core.

Potential physical mechanisms include wind-driven tidal disruption or supernova circumstellar interaction.

Abstract

In this Letter, we report the discovery of an ultraluminous fast-evolving transient in rest-frame UV wavelengths, MUSSES2020J, soon after its occurrence by using the Hyper Suprime-Cam (HSC) mounted on the 8.2 m Subaru telescope. The rise time of about 5 days with an extremely high UV peak luminosity shares similarities to a handful of fast blue optical transients whose peak luminosities are comparable with the most luminous supernovae while their timescales are significantly shorter (hereafter "fast blue ultraluminous transient," FBUT). In addition, MUSSES2020J is located near the center of a normal low-mass galaxy at a redshift of 1.063, suggesting a possible connection between the energy source of MUSSES2020J and the central part of the host galaxy. Possible physical mechanisms powering this extreme transient such as a wind-driven tidal disruption event and an interaction between supernova and circumstellar material are qualitatively discussed based on the first multiband early-phase light curve of FBUTs, although whether the scenarios can quantitatively explain the early photometric behavior of MUSSES2020J requires systematical theoretical investigations. Thanks to the ultrahigh luminosity in UV and blue optical wavelengths of these extreme transients, a promising number of FBUTs from the local to the high-z universe can be discovered through deep wide-field optical surveys in the near future.