# FBOTs and AT2018cow following electron-capture collapse of merged white dwarfs

## TL;DR

This paper proposes a model where FBOTs and AT2018cow are caused by electron-capture collapse of merged white dwarfs, leading to a neutron star and observable optical and high-energy emissions.

## Contribution

It introduces a novel scenario linking white dwarf mergers and electron-capture collapse to explain FBOTs and AT2018cow phenomena, including their spectral and temporal features.

## Key findings

- Explains the bright optical peak via light ejecta from collapse.
- Accounts for high-energy emission through pulsar wind nebula mechanisms.
- Describes late-time infrared features due to hydrogen in the wind.

## Abstract

We suggest that fast-rising blue optical transients (FBOTs) and the brightest event of the class AT2018cow result from an electron-capture collapse to a \NS\ following a merger of a massive ONeMg white dwarf (WD) with another WD. Two distinct evolutionary channels lead to the disruption of the less massive WD during the merger and the formation of a shell burning non-degenerate star incorporating the ONeMg core. During the shell burning stage a large fraction of the envelope is lost to the wind, while mass and angular momentum are added to the core. As a result, the electron-capture collapse occurs with a small envelope mass, after $\sim 10^2-10^4$ years. During the formation of a neutron star as little as $\sim 10^{-2} M_\odot $ of the material is ejected at the bounce-off with mildly relativistic velocities and total energy $\sim$ few $ 10^{50}$ ergs. This ejecta becomes optically thin on a time scale of days - this is the FBOT. During the collapse, the neutron star is spun up and magnetic field is amplified. The ensuing fast magnetically-dominated relativistic wind from the newly formed neutron star shocks against the ejecta, and later against the wind. The radiation-dominated forward shock produces the long-lasting optical afterglow, while the termination shock of the relativistic wind produces the high energy emission in a manner similar to Pulsar Wind Nebulae. If the secondary WD was of the DA type, the wind will likely have $\sim 10^{-4} M_\odot$ of hydrogen; this explains the appearance of hydrogen late in the afterglow spectrum. The model explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing a bright optical peak, afterglow high energy emission of similar luminosity to the optical, and late infra-red features.