AT2018cow: a luminous millimeter transient

TL;DR

This paper reports detailed multi-wavelength observations of AT2018cow, a unique extragalactic transient with unprecedented radio luminosity and emission characteristics, suggesting a new class of energetic events involving dense media and central engines.

Contribution

It provides the first detailed submillimeter to centimeter observations of AT2018cow, revealing its extraordinary properties and proposing a new class of dense-medium shocks with central engine activity.

Findings

High radio luminosity and long-lived millimeter emission plateau.

Sub-relativistic shock velocity of approximately 0.13c.

Evidence for an additional X-ray emission component from a central engine.

Abstract

We present detailed submillimeter- through centimeter-wave observations of the extraordinary extragalactic transient AT2018cow. The apparent characteristics -- the high radio luminosity, the long-lived emission plateau at millimeter bands, and the sub-relativistic velocity -- have no precedent. A basic interpretation of the data suggests $E_k \gtrsim 10^{48}$ erg coupled to a fast but sub-relativistic ($v \approx 0.13c$) shock in a dense ($n_e \approx 3 \times 10^{5}\,$cm$^{-3}$) medium. We find that the X-ray emission is not naturally explained by an extension of the radio-submm synchrotron spectrum, nor by inverse Compton scattering of the dominant blackbody UVOIR photons by energetic electrons within the forward shock. By $\Delta t \approx20\,$days, the X-ray emission shows spectral softening and erratic inter-day variability. Taken together, we are led to invoke an additional source of X-ray emission: the central engine of the event. Regardless of the nature of this central engine, this source heralds a new class of energetic transients shocking a dense medium, which at early times are most readily observed at millimeter wavelengths.