Magnetar as the Central Engine of AT2018cow: Optical, Soft X-Ray, and Hard X-Ray Emission

Abstract

AT2018cow is the most extensively observed and widely studied fast blue optical transient to date; its unique observational properties challenge all existing standard models. In this paper, we model the luminosity evolution of the optical, soft X-ray, and hard X-ray emission, as well as the X-ray spectrum of AT2018cow with a magnetar-centered engine model. We consider a two-zone model with a striped magnetar wind in the interior and an expanding ejecta outside. The soft and hard X-ray emission of AT2018cow can be explained by the leakage of high-energy photons produced by internal gradual magnetic dissipation in the striped magnetar wind, while the luminous thermal UV/optical emission results from the thermalization of the ejecta by the captured photons. The two-component energy spectrum yielded by our model with a quasi-thermal component from the optically thick region of the wind superimposed on an optically thin synchrotron component well reproduces the X-ray spectral shape of AT2018cow. The Markov Chain Monte Carlo fitting results suggest that in order to explain the very short rise time to peak of the thermal optical emission, a low ejecta mass M ej ≈ 0.1 M ⊙ and high ejecta velocity vSN≈0.17c are required. A millisecond magnetar with P 0 ≈ 3.7 ms and B p ≈ 2.4 × 1014 G is needed to serve as the central engine of AT2018cow.