On the optical transients from double white-dwarf mergers

TL;DR

This paper models the optical transients from double white-dwarf mergers, predicting their brightness, spectra, and detection rates, and suggests LSST will observe hundreds annually, providing new insights into compact object physics.

Contribution

It provides the first detailed simulations of optical transients from DWD mergers with stable remnants, predicting their observational signatures and detection prospects.

Findings

Transient peaks at 1-10 days post-merger with luminosities of 10^40-10^41 erg/s.

LSST is expected to detect hundreds of these transients annually.

The properties are consistent with current non-detections by ZTF.

Abstract

Double white-dwarf (DWD) mergers are relevant astrophysical sources expected to produce massive, highly-magnetized WDs, supernovae (SNe) Ia, and neutron stars (NSs). Although they are expected to be numerous sources in the sky, their detection has evaded the most advanced transient surveys. This article characterizes the optical transient expected from DWD mergers in which the central remnant is a stable (sub-Chandrasekhar) WD. We show that the expansion and cooling of the merger's dynamical ejecta lead to an optical emission peaking at $1$-$10$ d post-merger, with luminosities of $10^{40}$-$10^{41}$ erg s$^{-1}$. We present simulations of the light-curves, spectra, and the color evolution of the transient. We show that these properties, together with the estimated rate of mergers, are consistent with the absence of detection, e.g., by The Zwicky Transient Facility (ZTF). More importantly, we show that the Legacy Survey of Space and Time (LSST) of the Vera C. Rubin Observatory will likely detect a few/several hundred per year, opening a new window to the physics of WDs, NSs, and SN Ia.