Electromagnetic emission of white dwarf binary mergers

TL;DR

This paper models electromagnetic emissions from white dwarf binary mergers, predicting optical, infrared, gamma-ray, and X-ray signals, and compares these with neutron star merger kilonovae to identify observational signatures.

Contribution

It introduces detailed simulations of WD-WD mergers with magneto-hydrodynamics, predicting electromagnetic signals and their evolution, a novel approach for understanding these events.

Findings

Prompt gamma-ray emission up to 10^{46} erg within 0.1-1 s

Optical transients peaking around 7 days with luminosity 10^{41}-10^{42} erg/s

X-ray emission detectable around 150-200 days post-merger with luminosity 10^{39} erg/s

Abstract

It has been recently proposed that the ejected matter from white dwarf (WD) binary mergers can produce transient, optical and infrared emission similar to the "kilonovae" of neutron star (NS) binary mergers. To confirm this we calculate the electromagnetic emission from WD-WD mergers and compare with kilonova observations. We simulate WD-WD mergers leading to a massive, fast rotating, highly magnetized WD with an adapted version of the smoothed-particle-hydrodynamics (SPH) code Phantom. We thus obtain initial conditions for the ejecta such as escape velocity, mass and initial position and distribution. The subsequent thermal and dynamical evolution of the ejecta is obtained by integrating the energy-conservation equation accounting for expansion cooling and a heating source given by the fallback accretion onto the newly-formed WD and its magneto-dipole radiation. We show that magnetospheric processes in the merger can lead to a prompt, short gamma-ray emission of up to $\approx 10^{46}$ erg in a timescale of $0.1$-$1$ s. The bulk of the ejecta initially expands non-relativistically with velocity $0.01 c$ and then it accelerates to $0.1 c$ due to the injection of fallback accretion energy. The ejecta become transparent at optical wavelengths around $\sim 7$ days post-merger with a luminosity $10^{41}$-$10^{42}$ erg s$^{-1}$. The X-ray emission from the fallback accretion becomes visible around $\sim 150$-$200$ day post-merger with a luminosity of $10^{39}$ erg s$^{-1}$. We also predict the post-merger time at which the central WD should appear as a pulsar depending on the value of the magnetic field and rotation period.