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

TL;DR

This paper reports deep radio and X-ray observations of the luminous FBOT AT2020xnd, revealing high-velocity outflows, dense circumstellar interaction, and a possible accretion-powered X-ray component, establishing it as a high-redshift analog to AT2018cow.

Contribution

First detailed multi-wavelength study of AT2020xnd, confirming its similarities to AT2018cow and identifying its high-velocity outflows and potential accretion-powered X-ray emission.

Findings

Luminous radio emission indicating high-velocity outflows (~0.1-0.2c).

Luminous, rapidly fading soft X-ray emission with a different origin from radio.

AT2020xnd is a high-redshift analog of AT2018cow, expanding the class of luminous FBOTs.

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 $L_{\nu}\approx8\times10^{29}$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.