Luminous Millimeter, Radio, and X-ray Emission from ZTF20acigmel (AT2020xnd)

We present millimeter (80-230 GHz), radio (6-45 GHz), and X-ray (0.2-10 keV) observations of ZTF20acigmel (AT2020xnd), a short-duration luminous optical transient at $z=0.2433$. The 100 GHz peak luminosity is similar to that of long-duration gamma-ray bursts ($2\times10^{30}$ erg sec$^{-1}$ Hz$^{-1}$) but the light curve rises on a much longer timescale (one month). In the standard framework of synchrotron self-absorption of electrons in a power-law energy distribution, the data imply a fast ($v\approx0.2c$) shock with large energy ($U\gtrsim 10^{49}$ erg) propagating in a medium with a steep ($n_e \propto r^{-3}$) density profile. The forward-shock properties are similar to those of the fast-luminous transient AT2018cow, and in both cases the model for the late-time ($\Delta t>70$ d) low-frequency ($\nu 70$ GHz) emission. Motivated by the observation of a steep spectral index ($f_\nu \propto \nu^{-2}$) across the millimeter bands, we favor a thermal electron population (relativistic Maxwellian) for the synchrotron emission, the first such inference for a cosmic explosion. We find that the X-ray luminosity of $L_X\approx10^{43}$erg sec$^{-1}$ exceeds simple predictions from the radio and UVOIR luminosity and likely has a separate physical origin, such as a central engine. Our work suggests that luminous millimeter, radio, and X-ray emission are a generic feature of transients with fast ($\approx3$ days) and luminous ($M\approx-21$ mag) optical light curves. We estimate the rate at which transients like AT2018cow and AT2020xnd will be detected by future wide-field millimeter transient surveys like CMB-S4, and conclude that energetic explosions in dense environments may represent a significant population of extragalactic transients in the 100 GHz sky.