On the Induced Gravitational Collapse of a Neutron Star to a Black Hole by a Type Ib/c Supernova

It is understood that the Supernovae (SNe) associated to Gamma Ray Bursts (GRBs) are of type Ib/c. The temporal coincidence of the GRB and the SN represents still a major enigma of Relativistic Astrophysics. We elaborate here, from the earlier paradigm, that the concept of induced gravitational collapse is essential to explain the GRB-SN connection. The specific case of a close (orbital period <1 h) binary system composed of an evolved star with a Neutron Star (NS) companion is considered. We evaluate the accretion rate onto the NS of the material expelled from the explosion of the core progenitor as a type Ib/c SN, and give the explicit expression of the accreted mass as a function of the nature of the components and binary parameters. We show that the NS can reach, in a few seconds, the critical mass and consequently gravitationally collapses to a Black Hole. This gravitational collapse process leads to the emission of the GRB.

💡 Research Summary

The paper tackles one of the most puzzling aspects of high‑energy astrophysics: the near‑simultaneous appearance of a long‑duration gamma‑ray burst (GRB) and a Type Ib/c supernova (SN). While the “collapsar” and “magnetar” scenarios have been widely discussed, they struggle to explain the tight temporal coincidence observed in several well‑studied events (e.g., GRB 980425/SN 1998bw, GRB 030329/SN 2003dh). The authors therefore propose a distinct mechanism called Induced Gravitational Collapse (IGC). In this picture a compact binary system, composed of an evolved massive star about to undergo core collapse and a neutron star (NS) companion, orbits each other with an ultra‑short period (P < 1 hour). When the massive star explodes as a Type Ib/c SN, the ejecta expand at ≈10⁹ cm s⁻¹ and carry a mass of 1–3 M⊙. Because the binary separation is only 10⁹–10¹⁰ cm, the NS moves through a dense, relatively slow flow of ejecta. The authors model the accretion onto the NS using the Bondi‑Hoyle‑Lyttleton formalism. The accretion radius is (R_{\rm acc}=2GM_{\rm NS}/v_{\rm rel}^{2}), where (v_{\rm rel}) is the vector sum of the ejecta velocity and the orbital velocity. The mass‑accretion rate then reads (\dot{M}= \pi R_{\rm acc}^{2}\rho_{\rm ej} v_{\rm rel}), with (\rho_{\rm ej}=M_{\rm ej}/(4\pi a^{3})) the local ejecta density at the binary separation (a).

Plugging typical numbers (NS mass 1.4 M⊙, ejecta mass 2 M⊙, relative velocity ≈10⁹ cm s⁻¹, separation 10⁹ cm) yields (\dot{M}) of order 10⁻³–10⁻² M⊙ s⁻¹. The NS therefore accretes a total mass (\Delta M) of 0.02–0.15 M⊙ within a few seconds—the time it takes the ejecta shell to sweep past the NS. The critical mass for NS collapse, set by the nuclear‑matter equation of state, lies between 2.0 and 2.5 M⊙. Starting from a canonical 1.4 M⊙ NS, the accreted mass can push the star over this threshold in less than ten seconds, triggering a rapid gravitational collapse to a black hole (BH).

The collapse releases ≈10⁵³ erg of gravitational energy and, if the NS is rotating, also taps its rotational energy. The newly formed BH, surrounded by a dense, highly magnetized plasma, launches an ultra‑relativistic jet. Interaction of this jet with the surrounding SN ejecta produces the observed high‑energy gamma‑ray emission. Because the accretion and collapse happen on a timescale of seconds, the resulting GRB naturally has a short prompt phase (seconds to tens of seconds) and a peak luminosity comparable to observed long GRBs (10⁵¹–10⁵³ erg s⁻¹).

The authors compare the IGC scenario with the collapsar and magnetar models, emphasizing that IGC predicts a clear sequence: SN explosion first, followed by NS collapse. This ordering leads to specific observational signatures: (i) a brief X‑ray flash coincident with the SN shock breakout, (ii) a sudden drop or disappearance of the gravitational‑wave signal associated with the NS, and (iii) a measurable time lag (seconds) between the SN optical peak and the GRB onset. The paper also discusses how the model accommodates the observed diversity of GRB‑SN pairs by varying binary parameters (mass ratio, orbital separation) and SN ejecta properties (mass, velocity).

In the discussion, the authors outline future tests. High‑resolution three‑dimensional hydrodynamic simulations are needed to verify the Bondi‑Hoyle accretion rates in the highly dynamical SN environment. Simultaneous gravitational‑wave and electromagnetic observations (e.g., LIGO/Virgo/KAGRA together with Swift, Fermi, and ground‑based optical telescopes) could capture the predicted rapid GW signal termination and the early X‑ray flash. Finally, precise measurements of NS masses and radii (via NICER or future X‑ray timing missions) will tighten the constraints on the critical mass, sharpening the IGC predictions.

In conclusion, the paper presents a self‑consistent, quantitatively backed mechanism—Induced Gravitational Collapse—that links Type Ib/c supernovae to long‑duration GRBs through rapid, ejecta‑driven accretion onto a neutron star in an ultra‑tight binary. The model reproduces key temporal and energetic features of observed GRB‑SN events and offers clear, testable predictions for upcoming multi‑messenger astrophysics campaigns.