Oral Presentation
SMA Observations of AT2018cow: A Prototype for Millimeter Time-Domain Astronomy
Presenter: Anna Ho (Caltech)
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 > 10^48 erg coupled to a fast but sub-relativistic (v ≈ 0.13c) shock in a dense (n_e ≈ 3×10^5 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 Δt ≈ 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.
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