Accretion-Induced Collapse of White Dwarfs

• Accretion-induced collapse is the process by which a massive white dwarf near the Chandrasekhar limit undergoes catastrophic collapse via electron captures, forming a neutron star.
• The collapse involves rapid rotation, disk formation, and magnetic field amplification through MRI, which drive jet formation and produce observable gravitational waves, neutrinos, and electromagnetic transients.
• Ejecta from AIC, marked by unique nucleosynthesis signatures such as first r-process peak elements and low 56Ni yields, provide key insights into binary evolution and compact object formation.

Accretion-induced collapse (AIC) is a critical evolutionary process wherein a compact white dwarf, typically of oxygen–neon–magnesium (ONeMg) composition, accumulates mass until it approaches the Chandrasekhar limit. Rather than detonating in a thermonuclear supernova, the white dwarf undergoes catastrophic collapse triggered by electron captures, producing a neutron star (NS) and, under certain conditions, distinctive transient signals. AIC is a theoretically expected channel for NS formation, with implications for nucleosynthesis, multimessenger transients, the formation of millisecond pulsars and magnetars, and the potential genesis of phenomena such as gamma-ray bursts.

1. Physical Mechanism and Collapse Dynamics

AIC is initiated when a massive WD accretes material (hydrogen, helium, or heavier elements) from a companion through stable mass transfer or merger. For ONeMg WDs, the accreting star grows in mass until it nears a critical value, typically $M_\mathrm{Ch} \approx 1.38$–$1.44\,M_\odot$. At central densities exceeding $\sim 10^{10}\,\mathrm{g\,cm}^{-3}$, electron captures on isotopes such as $^{24}$Mg and $^{20}$Ne reduce the electron pressure. This triggers dynamic collapse, on a timescale $t_\mathrm{ff} \sim 0.1\,\mathrm{s}$, until the stellar core reaches nuclear density ($$\rho_\mathrm{nuc} \sim 10^{14}\,\mathrm{g\,cm}^{-3}$$), at which point a proto-neutron star forms and the collapse halts in a “core bounce” (Sharon et al., 2019, Brooks et al., 2017, Yip et al., 8 Jan 2024).

Initial post-collapse conditions typically result in the formation of a rapidly rotating NS, with angular momentum inherited from the accretion history. The collapse is accompanied by nuclear burning, a shock breakout, emission of a neutrino burst (with peak luminosity $\sim 10^{53}\,\mathrm{erg\,s}^{-1}$), and the expulsion of a modest ejecta mass ($\sim 10^{-3}$–$10^{-2}\,M_\odot$), most apparent for rapid rotators (Micchi et al., 2023).

2. Disk Formation, Magnetic Field Amplification, and Jet Dynamics

If the progenitor white dwarf is rotating rapidly, the collapse does not proceed spherically: a centrifugally supported disk with mass $0.01$–$0.1\,M_\odot$ forms around the proto-neutron star (Darbha et al., 2010, Combi et al., 24 Sep 2025). This disk is initially hot, composed of free nucleons, and subject to weak interactions (notably, the balance of $e^+$ and $e^-$ captures) which help set the electron fraction $Y_e$.

During and after collapse, weak seed magnetic fields in the WD are exponentially amplified by a turbulent and mean-field dynamo, primarily through the magnetorotational instability (MRI). The characteristic MRI wavelength is

$\lambda_\mathrm{MRI} = \frac{2\pi v_A}{\Omega},$

where $v_A = B/\sqrt{4\pi\rho}$ is the Alfvén velocity and $\Omega$ the angular velocity (Combi et al., 24 Sep 2025). The field grows as $B(t) \approx B_0 \exp(t/\tau)$, with $\tau \sim 1$–$2$ ms near bounce, rapidly generating large-scale toroidal structures.

These amplified fields become buoyant and are advected outwards, generating a magnetic tower that drives a mildly relativistic, striped jet ($u^T \sim 0.3c$). The jet clears a polar channel through the degenerate envelope and accretion disk, enabling a subsequent, powerful, magnetized neutron-rich wind. These conditions are conducive to the formation of a millisecond pulsar, magnetar, or even the launch of a gamma-ray burst powered by magnetar spindown (Combi et al., 24 Sep 2025).

3. Ejecta Properties, Nucleosynthesis, and Transient Signatures

The recombination of nucleons into helium (releasing $\sim7$ MeV per nucleon) unbinds the disk, resulting in ejecta masses of $M_\mathrm{ej} \sim 10^{-2}$–$10^{-1}\,M_\odot$ at velocities up to $0.1c$ (Darbha et al., 2010, Sharon et al., 2019). The disk and wind composition, governed by the electron fraction set by weak interactions and neutrino irradiation, is dominated by $^{56}$Ni and other iron-group elements, with small mass fractions of lighter elements.

A central result is the efficient synthesis of first neutron-capture peak elements (Sr, Y, Zr) for low-$Y_e$ ($\ll0.5$) ejecta, with overproduction factors up to $10^6$ relative to solar values. The $^{56}$Ni yield, typically $\lesssim10^{-3}\,M_\odot$, is two orders of magnitude lower than in Type Ia SNe (Yip et al., 8 Jan 2024), resulting in intrinsic faintness.

The electromagnetic transient powered by radioactive decay (notably $^{56}$Ni$\rightarrow$Co$\rightarrow$Fe) and recombination peaks at $\sim1$ day post-AIC with bolometric luminosity $\sim2\times10^{41}\,\mathrm{erg\,s}^{-1}$ and declines over several days. The optical signature is a rapid, faint “kilonova”-like event with broad, Doppler-smeared spectra due to $v\sim0.1c$ ejecta, with late-time infrared features from the Ca II triplet, and an overall duration much shorter and fainter than SNe Ia (Darbha et al., 2010).

Magnetized outflows enable additional observable signatures: reverse and termination shocks may produce hard and soft X-ray components, while strong radio transients may arise from synchrotron emission as the outflow interacts with dense circumstellar matter or forms a pulsar wind nebula (Piro et al., 2012, Moriya, 2016, Yu et al., 2019).

4. Binary Evolution Pathways and Progenitor Channels

AIC is realized in a variety of binary settings, with significant population synthesis implications:

• Single-degenerate pathways: ONeMg WDs accrete from non-degenerate companions (He stars, main-sequence, or red giants). Accretion rates, governed by the donor mass and orbital separation, must be high enough ($$\dot{M}_\mathrm{acc}\gtrsim2.05\times10^{-6} M_\odot\,\mathrm{yr}^{-1}$$ for CO+He channels) to prevent surface nuclear flashes that would otherwise expel mass (Wang, 2018). Magnetically confined accretion can enhance WD mass accumulation at low transfer rates, modestly expanding the progenitor parameter space and favoring the formation of highly magnetized NSs (magnetars) (Ablimit, 2021).
• Double-degenerate and merger pathways: Binary mergers involving ONeMg or CO WDs can produce conditions suitable for AIC, with the caveat that off-center carbon ignition must be avoided or converted to an ONeMg composition via carbon shell flashes and inward deflagrations (Brooks et al., 2017).
• Wide binary channels: AIC of ONeMg WDs accreting from red giants with long orbital periods ($50$–$1200$ d) can naturally produce systems matching observed wide-orbit millisecond pulsars with WD companions (Wang et al., 2022, Taani, 2022).

Across these channels, post-AIC mass transfer and recycling of the newly formed NS explain the observed diversity in MSP binary properties, with predicted orbital period distributions spanning $\sim0.1$–$1200$ days. When WD companions survive, the final system is typically a millisecond pulsar with a low-mass ($\sim0.3$–$0.55\,M_\odot$) He WD (Ablimit et al., 2014, Wang et al., 2022).

5. Multi-Messenger Signatures

AIC events are predicted to emit gravitational waves (GWs), neutrinos, and electromagnetic (EM) radiation:

• Gravitational waves: Rapid rotation amplifies the GW signal significantly. Rotating AICs display non-axisymmetric structures (e.g., spiral arms, $m=1$ modes) leading to GW amplitudes $1$–$2$ orders of magnitude higher than non-rotating cases (Micchi et al., 2023). For maximally rotating WDs, LIGO-class detectors may detect AIC GWs to $\sim1$ Mpc, while third-generation detectors can reach up to $\sim10$ Mpc.
• Neutrino emission: The AIC neutrino signature is characterized by a prompt electron neutrino burst at bounce (peak $L_\nu\sim5\times10^{53}\,\mathrm{erg\,s}^{-1}$), followed by sustained multi-flavor luminosity ($\sim10^{52}\,\mathrm{erg\,s}^{-1}$). Neutrino energies and luminosities are quantitatively similar to those in core-collapse SNe, making discrimination challenging (Yip et al., 8 Jan 2024, Micchi et al., 2023).
• Electromagnetic counterpart: The peak optical emission is typically $\sim2$ orders of magnitude fainter and evolves on timescales $\sim1/10$ those of Type Ia SNe (Darbha et al., 2010, Micchi et al., 2023). UV/X-ray transients, powered by magnetar spin-down, are also expected if strong fields are present, and may manifest as luminous, fast-evolving UV-bright events (Zhu et al., 2021).

AIC in the presence of strong circumstellar media or in AGN disks can enhance EM signatures, and in special conditions, rapid spindown or collapse may yield short gamma-ray bursts or fast radio bursts (Piro et al., 2012, Moriya, 2016, Perna et al., 2021, Zhu et al., 2021).

6. Astrophysical Implications and Rates

AIC provides a formation route for several astronomical classes:

• Binary millisecond pulsars: The AIC channel explains the observed period distributions and companion properties of MSP/WD binaries, including wide and circular orbits, which are not well accounted for by standard core-collapse or canonical recycling alone (Ablimit et al., 2014, Taani, 2022, Wang et al., 2022).
• Magnetars: Accretion of sufficient mass onto a highly magnetized WD can result in a magnetized NS via magnetic flux conservation, with estimated Galactic formation rates of $\sim0.34\times10^{-4}\,\mathrm{yr}^{-1}$ (Ablimit, 2021).
• Production of r-process elements: The neutron-rich disk winds and ejecta from AIC are prime candidates for the galactic inventory of first r-process peak elements (e.g., Sr, Y, Zr), compatible with overproduction factors and low event rates ($\lesssim10\%$ of CCSNe or SNe Ia) constrained by nucleosynthetic yields (Yip et al., 8 Jan 2024).
• Embedded environments: In AGN disks, WDs may undergo AIC upon rapid accretion, producing luminous transients that, in certain regimes, outshine the AGN emission itself (Zhu et al., 2021).

The theoretical Galactic AIC rate, inferred from population synthesis and rare-element overproduction, is $\sim10^{-4}$–$10^{-3}\,\mathrm{yr}^{-1}$, i.e., at most $\sim1$–$10\%$ the Type Ia SN rate (Darbha et al., 2010, Wang, 2018, Piro et al., 2014, Yip et al., 8 Jan 2024). This is consistent with constraints obtained from solar abundance ratios and transient surveys.

7. Extensions: Dark Matter, Equation of State, and Future Directions

Inclusion of dark matter (DM) within the WD core alters the AIC outcome. The presence of a compact DM admixture slows collapse and yields lower proto-NS masses, enabling formation of low-mass NSs ($\sim1\,M_\odot$) not realizable in ordinary core-collapse (Leung et al., 2019, Chan et al., 2023). This DM-related modification broadens the mass spectrum of neutron star remnants and affects GW/EM observables. Detailed two-fluid simulations show that DM–admixed AIC events produce distinctive gravitational wave signatures and alter the universal I–Love–Q relations, potentially allowing indirect detection via multimessenger data (Chan et al., 2023).

Future work is focused on multi-dimensional simulations incorporating detailed neutrino transport, general relativity, and rotation, as well as systematic multi-wavelength and multimessenger searches for AIC in time-domain surveys and GW detectors (Micchi et al., 2023, Combi et al., 24 Sep 2025). Persistent discrepancies in nucleosynthetic outcomes, event rates, and electromagnetic signatures are anticipated to refine the understanding and identification of AIC as a distinct channel in stellar evolution and compact object formation.