Fast Radio Bursts from White Dwarf Binary Mergers: Isolated and Triple-Induced Channels

Abstract

The detection of fast radio bursts (FRBs) in both young and old stellar populations suggests multiple formation pathways, beyond just young magnetars from core-collapse supernovae. A promising delayed channel involves the formation of FRB-emitting neutron stars through merger- or accretion-induced collapse of a massive white dwarf (WD). By simulating a realistic stellar population with both binaries and triples, we identify pathways to WD collapse that could produce FRB candidates. We find that (i) triple dynamics open new merger channels inaccessible to isolated binaries, significantly enhancing the overall merger rate; (ii) triple-induced mergers broaden the delay-time distribution, producing long-delay ($\gtrsim1$-8~Gyr) events largely independent of metallicity, alongside a shorter-delay population ($\lesssim100$~Myr) of rapid mergers; (iii) these long delays naturally yield FRBs in older environments such as quiescent host galaxies and galactic halos; (iv) when convolved with the cosmic star-formation history, binary channels track the star-formation rate ($z_{\rm peak} \sim 2$), while triple channels peak later ($z_{\rm peak} \sim 1$), giving a combined local source rate of $R_0 \approx 2\times10^4~{\rm Gpc^{-3}~yr^{-1}}$, consistent with observations; and (v) applying the same framework to Type~Ia supernovae, we find that triples extend the delay-time tail and roughly double the Ia efficiency relative to binaries, yielding rates and redshift evolution in good agreement with observations. If FRBs originate from the collapse of WDs, our results establish triples, alongside binaries, as a crucial and previously overlooked formation pathway whose predicted rates, host demographics, and redshift evolution offer clear tests for upcoming surveys.

Figures (4)

• Figure 1: Possible formation pathways of FRB candidates in triple star systems. Stages of mass transfer (MT), either dynamically stable or unstable, are denoted with green arrows. On the left, we first start with a triple star system containing three main-sequence stars. The first stage of MT in the inner binary can occur when it is an RG+MS, WD+MS/RG, or WD+WD. The evolution follows until the merger of two WDs ($35\%$ of FRB progenitors) or AIC of a white dwarf in a single degenerate scenario ($65\%$). Note that these pathways only represent systems that will become FRB candidates. Broader triple outcomes and their relative fractions are described in Shariat et al. (in prep).
• Figure 2: Predicted delay time distributions of binary mergers in isolated binaries (filled histograms) and triples (dashed histograms) for various metallicities. We show the results for the different FRB progenitor candidates, including all double WD mergers, double WD mergers including one massive WD, and O/Ne WD+any (including MS or RG) mergers, from left to right. The different rows show different metallicity models.
• Figure 3: Predicted redshift evolution of FRB source rates in both triples and isolated binaries. The top row shows the distinct contribution from triples (solid lines) and binaries (dashed lines) for different collapse scenarios, while the bottom row combines both channels. In each panel, we show three different FRB progenitor scenarios, including O/Ne WD + any secondary (red), CO WD+WD (black), and massive WD+WD (blue). The bottom panel also contains the cosmic star formation history in gray and various estimates of the local FRB event rate at $z\approx0$, derived by modeling observations. These include James22 (A), Shin23 (B), Meng25 (C), Chen24 (D), and Hashimoto22 (E). Note that the rates from our models include only unique FRB candidates, without accounting for repeaters.
• Figure 4: Predicted rates and redshift evolution of Type Ia supernova from a stellar population containing both binaries and triples. The top panel shows the individual rates of triples (solid lines) and isolated binaries (dashed lines), while the bottom panel shows their combined rates. We assume two different Type Ia progenitor models: one including all CO + CO WD mergers (black), and one that only includes massive CO WD mergers ($>0.9~{\rm M_\odot}$) with either a CO WD or a He WD (blue). The bottom panels also displays observed rates from Dilday10 (purple), Graur13 (orange), Perrett12 (green), and Graur11 (yellow).