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We present deep X-ray and radio observations of the Fast Blue OpticalTransient (FBOT) AT2020xnd/ZTF20acigmel at $z=0.2433$ from $13$d to $269$dafter explosion. AT2020xnd belongs to the category of optically luminous FBOTswith similarities to the archetypal event AT2018cow. AT2020xnd shows luminousradio emission reaching $L_{\nu}\approx8\times10^{29}$ergs$^{-1}$Hz$^{-1}$ at20GHz and $75$d post explosion, accompanied by luminous and rapidly fading softX-ray emission peaking at $L_{X}\approx6\times10^{42}$ergs$^{-1}$. Interpretingthe radio emission in the context of synchrotron radiation from the explosion'sshock interaction with the environment we find that AT2020xnd launched ahigh-velocity outflow ($v\sim$0.1-0.2$c$) propagating into a densecircumstellar medium (effective $\dot M\approx10^{-3}M_{\rm{sol}}$yr$^{-1}$ foran assumed wind velocity of $v_w=1000$kms$^{-1}$). Similar to AT2018cow, thedetected X-ray emission is in excess compared to the extrapolated synchrotronspectrum and constitutes a different emission component, possibly powered byaccretion onto a newly formed black hole or neutron star. These properties makeAT2020xnd a high-redshift analog to AT2018cow, and establish AT2020xnd as thefourth member of the class of optically-luminous FBOTs with luminousmulti-wavelength counterparts.

Radio and X-ray observations of the luminous Fast Blue Optical Transient AT2020xnd