AT2018cow: A Luminous Millimeter Transient

Ho, AYQ, Phinney, ES, Ravi, V, Kulkarni, SR, Petitpas, G, Emonts, B, Bhalerao, V, Blundell, R, Cenko, SB, Dobie, D, Howie, R, Kamraj, N, Kasliwal, MM, Murphy, T, Perley, DA, Sridharan, TK and Yoon, I (2019) AT2018cow: A Luminous Millimeter Transient. Astrophysical Journal, 871 (1). ISSN 0004-637X

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AT2018cow A Luminous Millimeter Transient.pdf - Accepted Version

We present detailed submillimeter- through centimeter-wave observations of the extraordinary extragalactic transient AT2018cow. The apparent characteristics—the high radio luminosity, the rise and long-lived emission plateau at millimeter bands, and the sub-relativistic velocity—have no precedent. A basic interpretation of the data suggests ${E}_{k}\gtrsim 4\,\times \,{10}^{48}\,\mathrm{erg}$ coupled to a fast but sub-relativistic ($v\approx 0.13c$) shock in a dense (${n}_{e}\approx 3\,\times \,{10}^{5}\,{\mathrm{cm}}^{-3}$) 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 UV/optical/IR photons by energetic electrons within the forward shock. By ${\rm{\Delta }}t\approx 20\,\mathrm{days}$, 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.