A Model for Fast Blue Optical Transient AT2018Cow: Circumstellar Interaction of a Pulsational Pair-instability Supernova

TL;DR

This paper models the rapid brightening and fading of the FBOT AT2018cow through circumstellar interaction of a pulsational pair-instability supernova, explaining its light curve with hydrodynamical simulations and radioactive decay.

Contribution

It introduces a detailed PPISN model that successfully reproduces AT2018cow's light curve, linking FBOTs and SLSNe to CSM mass variations in PPISNe.

Findings

The early light curve is explained by shock heating of circumstellar matter.

The later light curve is consistent with radioactive decay of $^{56}$Co.

Model parameters influence the shape of the light curve.

Abstract

The Fast Blue Optical Transient (FBOT) ATLAS18qqn (AT2018cow) has a light curve as bright as superluminous supernovae but rises and falls much faster. We model this light curve by circumstellar interaction of a pulsational pair-instability (PPI) supernova (SN) model based on our PPISN models studied in previous work. We focus on the 42 $M_\odot$ He star (core of a 80 $M_{\odot}$ star) which has circumstellar matter of mass 0.50 $M_\odot$. With the parameterized mass cut and the kinetic energy of explosion $E$, we perform hydrodynamical calculations of nucleosynthesis and optical light curves of PPISN models. The optical light curve of the first $\sim$ 20 days of AT2018cow is well-reproduced by the shock heating of circumstellar matter for the $42 ~M_{\odot}$ He star with $E = 5 \times 10^{51}$ erg. After day 20, the light curve is reproduced by the radioactive decay of 0.6 $M_\odot$ $^{56}$Co, which is a decay product of $^{56}$Ni in the explosion. We also examine how the light curve shape depends on the various model parameters, such as CSM structure and composition. We also discuss (1) other possible energy sources and their constraints, (2) origin of observed high-energy radiation, and (3) how our result depends on the radiative transfer codes. Based on our successful model for AT2018cow and the model for SLSN with the CSM mass as large as $20 ~M_\odot)$, we propose the working hypothesis that PPISN produces SLSNe if CSM is massive enough and FBOTs if CSM is less than $\sim 1 ~M_\odot$.