AT2018cow Powered by a Shock in Aspherical Circumstellar Media

Abstract

We present a quantitative model for the luminous fast blue optical transient AT2018cow in which a shock propagating through an aspherical circumstellar medium (CSM) produces the X-ray and UV/optical/NIR emission. X-rays are emitted from hot post-shock electrons, and soft X-ray photons are reprocessed into optical/UV emission in the cool downstream. This naturally explains two previously puzzling features: (i) the coordinated evolution of the optical and soft X-ray after day 20, (ii) the hard X-ray hump above 10 keV that disappears around day 15 as the Thomson optical depth transitions from $\tau_T \gg1$ to $\tau_T \sim 1$. Our model is over-constrained, and it quantitatively reproduces the bolometric luminosity evolution, soft X-ray spectrum, and time-dependent soft/hard X-ray and soft X-ray/optical luminosity ratios. It also explains additional puzzles: X-ray fluctuations with $\sim4-10$ day timescales arise from a global radiative shock instability, while the NIR excess and the apparent receding blackbody radius result from reprocessed X-rays in matter far from thermodynamic equilibrium. The radio is naturally explained as originating from a shock driven by the same ejecta in the more dilute CSM. The light curve steepening after $\sim 40$ days likely indicates the shock reaches the edge of the dense CSM at $\sim {\rm few} \times 10^{15}$ cm. We infer explosion energy $\sim 1-5 \times 10^{50}$ erg, carried by an ejecta at $\sim 0.1c$ and a mass of $0.01-0.05 M_\odot$, in a dense asymmetric CSM with $\sim 0.3 M_\odot$, embedded in a more dilute CSM.