AT2018cow: a luminous millimeter transient

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 $\Delta t\approx 20$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.

Accepted to the Astrophysical Journal on 26 Nov 2018. 41 pages, 9 figures. In this version we replaced the figures with higher-quality versions, and fixed several typos