  We present deep X-ray and radio observations of the Fast Blue Optical
Transient (FBOT) AT2020xnd/ZTF20acigmel at $z=0.2433$ from $13$d to $269$d
after explosion. AT2020xnd belongs to the category of optically luminous FBOTs
with similarities to the archetypal event AT2018cow. AT2020xnd shows luminous
radio emission reaching $L_{\nu}\approx8\times10^{29}$ergs$^{-1}$Hz$^{-1}$ at
20GHz and $75$d post explosion, accompanied by luminous and rapidly fading soft
X-ray emission peaking at $L_{X}\approx6\times10^{42}$ergs$^{-1}$. Interpreting
the radio emission in the context of synchrotron radiation from the explosion's
shock interaction with the environment we find that AT2020xnd launched a
high-velocity outflow ($v\sim$0.1-0.2$c$) propagating into a dense
circumstellar medium (effective $\dot M\approx10^{-3}M_{\rm{sol}}$yr$^{-1}$ for
an assumed wind velocity of $v_w=1000$kms$^{-1}$). Similar to AT2018cow, the
detected X-ray emission is in excess compared to the extrapolated synchrotron
spectrum and constitutes a different emission component, possibly powered by
accretion onto a newly formed black hole or neutron star. These properties make
AT2020xnd a high-redshift analog to AT2018cow, and establish AT2020xnd as the
fourth member of the class of optically-luminous FBOTs with luminous
multi-wavelength counterparts.
