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

Abstract

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 $L_{\hat{I}}½\approx 8\times 10^{29}$ergs${}^{-1}$Hz${}^{-1}$ at 20GHz and $75$d post explosion, accompanied by luminous and rapidly fading soft X-ray emission peaking at $L_X\approx 6\times 10^{42}$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 $\dot{M}\approx 10^{-3}{M}_{\mathrm{sol}}$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.

21 pages, 6 figures, 6 tables. Submitted to ApJ