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

TL;DR

This paper studies the millimeter, radio, and X-ray emissions from the transient ZTF20acigmel, revealing a dense environment and a possible thermal electron population, and discusses the implications for future millimeter transient surveys.

Contribution

It presents detailed multi-wavelength observations and modeling of ZTF20acigmel, proposing a thermal-electron synchrotron model and estimating detection rates for future surveys.

Findings

Thermal-electron model suggests a sub-relativistic shock at 0.3c.

High ambient density around the transient ($n_e oughly 4 imes 10^3$ cm$^{-3}$).

Luminous millimeter emission is common in fast, luminous transients.

Abstract

Observations of the extragalactic ($z=0.0141$) transient AT2018cow established a new class of energetic explosions shocking a dense medium, which produce luminous emission at millimeter and sub-millimeter wavelengths. Here we present detailed millimeter- through centimeter-wave observations of a similar transient, ZTF20acigmel (AT2020xnd) at $z=0.2433$. Using observations from the NOrthern Extended Millimeter Array and the Very Large Array, we model the unusual millimeter and radio emission from AT2020xnd under several different assumptions, and ultimately favor synchrotron radiation from a thermal electron population (relativistic Maxwellian). The thermal-electron model implies a fast but sub-relativistic ($v\approx0.3c$) shock and a high ambient density ($n_e\approx4\times10^{3}$cm$^{-3}$ at $\Delta t\approx40$ days). 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. Using the fact that month-long luminous ($L_\nu\approx 2\times10^{30}$ erg sec$^{-1}$ Hz$^{-1}$ at 100 GHz) millimeter emission appears to be a generic feature of transients with fast ($t_{1/2}\approx3$ days) and luminous ($M_\mathrm{peak}\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 such as CMB-S4, and conclude that energetic explosions in dense environments may represent a significant population of extragalactic transients in the 100 GHz sky.