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

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.