MUSSES2020J: The Earliest Discovery of a Fast Blue Ultra-luminous Transient at Redshift 1.063

Kavli Affiliate: Naoki Yasuda

| First 5 Authors: Ji-an Jiang, Naoki Yasuda, Keiichi Maeda, Nozomu Tominaga, Mamoru Doi

| Summary:

In this letter we report a discovery of an ultra-luminous fast evolving
transient in rest-frame UV wavelengths, MUSSES2020J, soon after its occurrence
by using the Hyper Suprime-Cam (HSC) mounted on the 8.2m Subaru telescope. The
rise time of about 5 days with an extremely high UV peak luminosity share
similarities with 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 ultra-luminous
transient," FBUT). In addition, MUSSES2020J is located near the center of a
normal low-mass galaxy at 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 of 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, while whether the scenarios can
quantitatively explain the early photometric behavior of MUSSES2020J requires
systematical theoretical investigations. Thanks to the ultra-high luminosity in
UV and blue optical wavelengths of these extreme transients, a promising number
of FBUTs from local to high-z universe can be discovered through deep
wide-field optical surveys in the near future.

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