Luminous Fast Blue Optical Transients as "Failed" Gravitational Wave Sources: Helium Core$-$Black Hole Mergers Following Delayed Dynamical Instability

TL;DR

This work links binary-star evolution to luminous fast blue optical transients (LFBOTs) by proposing that delayed dynamical instabilities in BH+massive-star binaries drive BH–He-core mergers. Using MESA binary grids, analytic transient modeling, and rapid population synthesis across metallicities, the authors predict dense extended circumstellar media from long-lived stable mass transfer and a compact nearby CSM from the dynamical plunge, yielding UV/optical, X-ray, and radio signatures consistent with LFBOTs. They compute LFBOT rates of roughly 5–300 Gpc$^{-3}$ yr$^{-1}$ (depending on model assumptions) and find a preference for subsolar metallicities, aligning with observed LFBOT host properties. The results support interpreting LFBOTs as luminous indicators of “failed” gravitational-wave sources, while also forecasting a related population of longer-duration transients from MS+BH mergers. Overall, the paper provides a coherent, testable framework tying binary evolution, CSM structure, and multi-wavelength transient emission to the LFBOT phenomenon and potential GW-source failures.

Abstract

Binaries in which a massive donor star undergoes an extended ($\gtrsim$ kyr) phase of stable mass transfer onto a black hole (BH) accretor offer a promising channel for creating LIGO gravitational wave sources. However, in many systems the mass transfer terminates prematurely in a dynamical instability at orbital periods of a few days, culminating in the BH plunging into the donor and potentially disrupting and accreting its helium core at highly super-Eddington rates. Combining a suite of binary evolution models with analytic estimates and population synthesis, we predict the population of luminous transients from delayed dynamical instability (DDI) and attribute them to the "luminous" class of fast blue optical transients (LFBOTs). The initial plunge of the BH into the partially stripped envelope typically ejects $\sim 10M_{\odot}$ of H/He-enriched material at speeds $\sim 10^{2}-10^{3}$ km s$^{-1}$, generating a compact circumstellar medium (CSM) of radius $\lesssim 1000R_{\odot}$ by the time the BH meets and tidally disrupts the HeC. Rapid BH accretion generates a highly aspherical wind-driven explosion into the environment, powering UV/optical emission via CSM interaction and X-ray reprocessing that rises over a few days to a luminosity $\sim 10^{44}-10^{45}$ erg s$^{-1}$ before fading as the disk spreads outwards and accretion rate drops. Luminous radio/sub-mm emission is generated over several months as the jet collides with the slow quasi-spherical binary outflow, generated by the stable mass transfer preceding DDI, extending to radii $\sim 10^{17}$ cm, in agreement with the inferred CSM environments of LFBOTs. We estimate local rates of DDI merger transients $5-300$ Gpc$^{-3}$ yr$^{-1}$, with a preference for low-metallicities, in agreement with LFBOT demographics. Taken together, our results support LFBOTs as being luminous signposts of "failed" gravitational wave sources.

Figures (15)

• Figure 1: Stages of DDI transients and associated multi-wavelength light-curve: (1) a massive star transfer mass stably onto a BH companion, for thousands of years. A slow outflow from the $L2$ point tightens the binary, creating a dense, dusty, and opaque CSM extending to large radii $R_{\rm CSM} \sim 10^{16}-10^{17}$ cm (Fig. \ref{['fig:extendedCSM']}); (2) once enough of the donor envelope has been removed, the mass-transfer becomes dynamically unstable; the BH plunges into the star over just a few orbital periods (typically days), ejecting the envelope and creating "nearby" He-rich CSM extending to radii $R_{\rm nCSM} \sim 10^{13}-10^{14}$ cm; (3) the BH tidally disrupts the star's helium ("Wolf-Rayet") core, creating a massive disk around the black hole that feeds it at highly super-Eddington rates. Accretion peaks overs hours to a day, thereafter decaying as a power-law $\dot{M}_{\bullet} \propto t^{-\beta}$ with $\beta \sim 2-3$. The disk produces both fast bipolar outflows ("jet") as well as wider-angle slow winds, which initially collide with the nearby CSM. If the BH is rapidly spinning, it may power a GRB-like ultra-relativistic jet visible for on-axis viewers (Sec. \ref{['sec:GRB']}). The jet breaks out of the nearby CSM within a few hours, but the bulk of the UV emission occurs over the longer peak diffusion time of $t_{\rm d} \sim$ days (Eq. \ref{['eq:tdiff']}). A portion of the early UV emission is absorbed and reprocessed by dust in the extended CSM before the dust is sublimated prior to peak light; (4) the growing BH accretion funnel allows X-rays from the jet base to reach the distant observer. Time-variability in the jet or its orientation may result in late flares and associated non-thermal afterglow (Ho+23b). Shock interaction of the fast jet with the extended CSM produces synchrotron radio/sub-mm emission. The reprocessed dust emission from stage (3) reaches the observer as an "echo" over the several weeks light travel-time across the extended CSM.
• Figure 2: An overview of MESA binary evolution models and their final outcomes. Left: one of the nineteen binary model grids of BH+OB star systems employed in this study. The grid covers BH-OB systems with varying initial mass ratios and orbital periods for a fixed initial BH mass $M_{\bullet} = 10 M_{\odot}$ and metallicity $Z = 0.1 Z_{\odot}$, assuming fiducial core-overshooting ($\sigma_{\rm ov} = 0.35$) and L2 outflows of $f_{\rm L2} = 0.25$. See Fig. \ref{['fig:outcome_maps_1']} and Fig. \ref{['fig:outcome_maps_2']} for the other grids. Each binary model (circle) is color-coded according to its final outcome (see text). Solid lines mark approximate boundaries between different final outcomes. The LFBOT progenitors (marked in purple as BH+HeC mergers) are systems in which the mass transfer from a radiative post-MS donor encounters delayed dynamical instability. Star symbols mark models shown in the right panels. Top right: mass transfer rate as a function of the total mass transferred since the onset of RLOF, shown for a few example binaries of different final outcomes. Bottom right: mass transfer rate in the final 1750 years before dynamical instability for an LFBOT progenitor (purple) and a system with a convective donor, leading to traditional CE (red).
• Figure 3: Top: Mass-loss preceding dynamical instability ($\Delta M$) as a function of initial semi-major axis of the BH+OB orbit ($a_0$), shown separately for binary models that terminate in unstable mass transfer by a convective donor (orange, leading to traditional CE) or with a radiative post-MS donor (blue, leading to LFBOTs, as explored in this work). Middle: Properties of donor stars at the onset of dynamical instability. In color: the mass and radius of the remaining envelope, following the same convection as the top panel. In gray: the mass and radius of the core Bottom: Fractional mass-loss $\Delta M$/$M_{\rm env}$ and core mass $M_{\rm He}$/$M_{\rm env}$ for all the binary models that lead to LFBOTs (i.e., radiative donors from the other panels).
• Figure 4: Radial CSM density profiles $n_{\rm CSM}(r)$ (Eq. \ref{['eq:nCSM']}) at the time of dynamical instability, due to the combination of stellar winds and $L2$ mass-loss from the preceding stable mass-transfer phase, calculated from our binary evolution models. For the $L2$ mass-loss we assume an outflow velocity $v_{\rm w} = v_{\rm L2}$ (Eq. \ref{['eq:vL2']}) for $\chi = 0.1$ and mass-loss rate $\dot{M}$ following Lu+23 capped at 50% $\dot{M}_{\rm L1}$. Gray lines shown the density profiles of individual binary evolution models, the thick blue line shows the mean density profile, and the blue shading indicates the 10-90 percentiles. For comparison, symbols show measured CSM density profiles of individual LFBOTs obtained by modeling the radio/sub-mm synchrotron emission Bright+22Chrimes+24b. The shock microphysical parameters are uncertain and so error bars on the range of values are shown for $\epsilon_{e} = 10^{-2}(0.1), \epsilon_{B} = 10^{-3}(10^{-2})$, following the scaling $n_{\rm CSM} \propto \epsilon_{e}^{-6/19}\epsilon_{B}^{-13/19}$Chevalier98. For comparison, diagonal dashed lines show the density profiles for steady wind mass-loss at $\dot{M} = 0.1, 10^{-3}, 10^{-5}, 10^{-7} M_{\odot}$ yr$^{-1}$ for an assumed outflow speed $v_{\rm w} = 10$ km s$^{-1}$. To the extent that $L2$ mass-loss is preferentially concentrated in the binary plane, while the polar jets in LFBOTs interact with gas along the rotational axis, our predicted spherically-averaged CSM profiles may somewhat over-predict the afterglow-inferred densities at $r \lesssim 10^{17}$cm.
• Figure 5: Properties of the "nearby CSM" ejected when the BH dynamically plunges into the envelope of the stripped donor star. Histograms compiled from on our suite of binary simulations show estimates of the total mass of the nCSM (envelope mass) as well as its average radial velocity $v_{\rm nCSM}$ (Eq. \ref{['eq:nCSM']}), radial extent at the time of the HeC-BH merger, $R_{\rm nCSM}$ (Eq. \ref{['eq:RnCSM']}), and mean He abundance. Red arrows indicate that the estimated $v_{\rm nCSM}$ and $R_{\rm nCSM}$ values are likely upper limits (see text and Appendix \ref{['sec:nearbycsm_appendix']}).