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 τT≫1 to τT∼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 ∼4−10 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 ∼40 days likely indicates the shock reaches the edge of the dense CSM at ∼few×1015 cm. We infer explosion energy ∼1−5×1050 erg, carried by an ejecta at ∼0.1c and a mass of 0.01−0.05M⊙, in a dense asymmetric CSM with ∼0.3M⊙, embedded in a more dilute CSM.
@article{arxiv.2601.18887,
title = {AT2018cow Powered by a Shock in Aspherical Circumstellar Media},
author = {Taya Govreen-Segal and Ehud Nakar and Kenta Hotokezaka and Christopher M Irwin and Eliot Quataert},
journal= {arXiv preprint arXiv:2601.18887},
year = {2026}
}