White dwarf-neutron star binaries: a plausible pathway for long-duration gamma-ray bursts from compact object mergers?

TL;DR

This work tests whether white dwarf–neutron star (WDNS) and white dwarf–black hole (WDBH) mergers can account for long-duration gamma-ray bursts lacking supernovae by combining binary population synthesis with detailed host-galaxy offset modeling. Using BPASS to predict formation channels, delay times, and merger rates across metallicities, the authors seed binaries in two GRB host galaxies and trace their trajectories within realistic potentials to obtain offset distributions, comparing them to the observed offsets of GRB 211211A and GRB 230307A. They find that WDNS mergers can yield offsets consistent with these events and predict WDNS merger rates similar to those of binary neutron star mergers and long GRBs, while WDBH mergers are roughly an order of magnitude rarer with substantial rate uncertainties. The study also highlights that offsets and rates alone cannot definitively distinguish compact binary progenitors for SN-less long GRBs, emphasizing the need for larger samples and improved rate constraints, though future GW non-detections or detections could provide decisive support for WD-involved channels. Overall, the paper demonstrates that WDNS mergers are a plausible channel for SN-less long GRBs and motivates a population-wide approach to interpreting offsets and event rates in the growing landscape of merger-driven transients.

Abstract

Two long-duration gamma-ray bursts were recently discovered with kilonovae, the signature of r-process element production in a compact binary merger, rather than supernovae. This has forced a re-evaluation of the long-established dichotomy between short bursts (< 2s, arising from compact binary mergers) and long bursts (> 2s, a class of massive star core-collapse event). We aim to determine whether white dwarf-neutron star (WDNS) and white dwarf-black hole (WDBH) mergers are plausible explanations for long-duration compact merger GRBs, in terms of their galactocentric merger offsets and cosmological rates. We model the host galaxies of GRBs 211211A and 230307A, and employ binary population synthesis, to predict the offset distributions of compact mergers. We compare with the observed offsets, investigate evolutionary pathways, predict their cosmological rates, and compare with volumetric GRB rates. We find that WDNS mergers occur at lower host offsets than binary NS mergers, but that in the specific cases of GRBs 211211A and 230307A, the observed offsets are consistent with either scenario. We predict that WDNS mergers occur at a similar rate to binary NS mergers and long GRBs, and that WDBH mergers are a factor of ten rarer, with the caveat that these rates currently carry uncertainties at the order of magnitude level. We have demonstrated, solely in terms of galactocentric offsets and event rates, that WDNS mergers are a plausible explanation for GRBs 211211A and 230307A, and long GRBs from compact object mergers more generally. WDNS binaries have lower systemic velocities than binary neutron stars, but longer delay times, and ultimately merge with an offset distribution that is not measurably different without large samples. Therefore, offsets and rates alone cannot currently distinguish between compact binary progenitor models for supernova-less long duration GRBs.

Figures (10)

• Figure 1: The number of compact binary models containing a white dwarf and a core-collapse remnant for each initial semi-major axis $a_{0}$ (immediately post second remnant formation) and delay time $T_{\rm delay}$. The shading corresponds to the number of compact binaries produced in each 2D bin per 10$^{6}$ M$_{\odot}$. The left-hand column shows results at one-fifth Solar metallicity, on the right at half-Solar. Top row: WDNS binaries in which the WD forms first. Eccentricity is retained from the supernova, which occurs second, producing a greater range of semi major axes with a tail extending to higher values at fixed delay time. Middle row: WDNS binaries in which the NS forms first. Bottom row: WDBH binaries in which the BH forms first. We find no WD1-BH2 models. In each case, a vertical dashed line indicates a Hubble time. The vast majority of such binaries merge on much longer timescales (see Table \ref{['tab:evolution']}). 2017AA...608A..57V natal kicks are adopted, equivalent plots with 2005MNRAS.360..974H and 2016MNRAS.461.3747B kicks are given in Appendix \ref{['app:A']}.
• Figure 2: Systemic velocities (at birth) for compact binaries (left), and their total delay time distributions (from ZAMS to merger, right), at 20% Solar metallicity (top, similar to the host galaxy of GRB211211A) and 50% Solar metallicity (bottom, matching the host of GRB230307A). These results adopt the 2017AA...608A..57V distribution for neutron star natal kicks (results for alternative natal kicks are given in appendix figures \ref{['fig:apx_hobbs']} and \ref{['fig:apx_bray']}). Only systems which merge within a Hubble time are shown. We split the WDNS mergers into NS1-WD2 and WD1-NS2 channels; WDBH mergers are however much rarer and exclusively have the BH forming first. These velocities do not include any contribution from the initial galactic orbits of their progenitor systems (these are introduced in Section \ref{['sec:offsets']}) for performing offset simulations.
• Figure 3: Star formation histories, determined with prospector, for the host galaxies of GRBs 211211A and 230307A 2025ApJ...982..144N. Errorbars are given on the SFR estimates at each time-step, while the red dashed lines are the constructed histrograms which we use to weight the model seeding in time. The populations of both galaxies are dominated by star-formation within the last 10 Gyr. This biases the merging WDNS binaries in these galaxies to the WD1-NS2 pathway (see Fig. \ref{['fig:vsystdelay']}).
• Figure 4: WDNS and BNS offsets for the host galaxies of GRB 211211A (top row) and GRB 230307A (bottom row). The offsets are projected assuming random, isotropic viewing angles. The observed projected offsets of GRB 211211A (7.92 kpc) and GRB 230307A (38.9 kpc) are marked on each panel by blue and orange vertical lines respectively.
• Figure 5: The total number of BNS, NSBH, WDNS and WDBH binaries merging within a Hubble time, per 10$^{6}$ M$_{\odot}$ of stars formed, as a function of metallicity.