Collapsar Disk: Solving the Puzzle of the Anomalous Merger GW190814 ✦
Context
Gravitational-wave observations, made possible by the pioneering work of Einstein and Weiss, have opened a new window into the compact object population. However, the event GW190814 posed a challenge to standard binary formation models. With a primary black hole mass of about 23 solar masses and a secondary component of only 2.6 solar masses, it lies between neutron star and black hole mergers. Traditional channels—isolated binary evolution, dynamical interactions in globular clusters or active galactic nuclei—cannot simultaneously explain the extreme mass ratio, the secondary's mass in the "mass gap", and the merger rate. This suggests an alternative birth channel linked to the death of massive stars.
Methods
To test the hypothesis, the authors simulated the dynamics of the accretion disk of a collapsar after the collapse of a rapidly rotating star. According to previous work, such a disk cools via neutrinos and becomes gravitationally unstable according to the Jeans criterion, fragmenting into clumps that form neutron stars or low-mass black holes. The study performed numerical simulations of a system consisting of a central black hole and two fragments on nearly coplanar orbits, including post-Newtonian corrections up to 3.5 PN beyond the general theory of relativity. The AR-CHAIN code was used, allowing accurate calculation of close encounters and gravitational-wave losses. Two main outcomes were analyzed: merger of the fragments with each other followed by infall of the product onto the central hole, or scattering where one fragment quickly falls in while the second is ejected onto a wide orbit and later merges due to gravitational wave emission.
Results
The simulation results showed that the delay between the core collapse of the star (accompanied by a supernova) and the gravitational-wave merger can be weeks or months, consistent with the ~60-day interval between SN2019npv and GW190814. The delay distribution is bimodal: for fragment merger products, minutes to days; for the surviving outer companion, days to a year with a peak at ~30 days. The orbit of the surviving fragment has high eccentricity upon leaving the scattering zone, but then quickly circularizes under gravitational-wave radiation, so the merger in the detector sensitivity band occurs on a nearly circular orbit. The numerical experiments also reproduce the observed mass ratio: for typical disk parameters, with disk thickness H~0.1–0.5, fragments have masses of order H^3 * M_central, giving q~0.001–0.125. GW190814 with q=0.11 falls within this range, and the secondary mass is close to the Chandrasekhar limit. Furthermore, if SN2019npv is indeed the host galaxy, the event becomes a "bright standard siren": the distance measured from gravitational waves and the redshift from optical observations allow independent determination of the Hubble constant, using Hubble's law, yielding H0 = 70.5^{+9.2}_{-6.4} km/s/Mpc, which is in agreement with both CMB and Cepheid measurements.
Implications
The proposed scenario links gravitational-wave events to the collapse of massive stars, with several important implications. First, it naturally explains not only the extreme mass ratio but also the low spin of the primary black hole: the collapsar jet can efficiently remove angular momentum. Second, a tight correlation with certain supernova types (Ib, Ic, Ic-BL) is predicted, changing the strategy for electromagnetic follow-up: the most informative signal may precede the merger. Third, this channel could be responsible for a population of mergers with sub-solar masses, and also predicts a new class of kilonovae embedded in the supernova envelope, opening up opportunities to detect unique transients.
Future development
In the future, as LIGO–Virgo–KAGRA accumulates data and next-generation telescopes come online, we can expect to detect new events with extreme mass ratios associated with supernovae. Advances in numerical modeling will allow detailed tracking of the fragmenting disk's evolution, including magnetic fields and radiation interaction. It also remains to be understood how often collapsars produce disks capable of fragmenting, depending on metallicity—this requires further studies of stellar evolution and nucleosynthesis in extreme conditions.
Impact
This work connects gravitational-wave astronomy, supernova physics, and cosmology, opening the possibility of measuring the expansion of the Universe with bright sirens without the need for an electromagnetic counterpart at the time of merger. The predicted "embedded" transients motivate joint campaigns of gravitational-wave and electromagnetic observatories.
Next steps
Near-term steps include more complete modeling of multi-fragment system dynamics to predict gravitational-wave signal shapes, searching archival data for supernovae preceding registered events, and developing rapid alert strategies for telescopes like ZTF and the future Roman Space Telescope to catch electromagnetic precursors.
Key open problems
The presented mechanism directly addresses several fundamental problems: the nature of objects in the "mass gap" between neutron stars and black holes, the origin of long gamma-ray bursts, and the Hubble tension, offering a new class of standard sirens.
🎯 The probability that a Type Ib supernova randomly fell within the 90% localization volume of GW190814 within 60 days of the event is only about 1.3–1.5% (about 2.2σ), making SN2019npv a plausible—though not incontrovertible—precursor.
Key numbers
- primary black hole mass: 23 M_⊙
- secondary object mass: 2.6 M_⊙
- mass ratio: 0.11
- delay between supernova and merger: ~60 days
- measured Hubble constant: 70.5^{+9.2}_{-6.4} km/s/Mpc