Stellar-Mass Black Holes and Their Progenitors

If a black hole has a low spin value, it must double its mass to reach a high spin parameter. Although this is easily accomplished through mergers or accretion in the case of supermassive black holes in galactic centers, it is impossible for stellar-mass black holes in X-ray binaries. Thus, the spin distribution of stellar-mass black holes is almost pristine, largely reflective of the angular momentum imparted at the time of their creation. This fact can help provide insights on two fundamental questions: What is the nature of the central engine in supernovae and gamma-ray bursts? and What was the spin distribution of the first black holes in the universe?

💡 Research Summary

The paper investigates why the spin distribution of stellar‑mass black holes (typically 5–20 M☉) remains essentially unchanged from the moment of their birth, in contrast to the easily spun‑up supermassive black holes at galactic centers. Using the Kerr solution, the author shows that increasing the dimensionless spin parameter a* from a low value (≈0.2) to a high value (≈0.9) requires at least a doubling of the black‑hole mass because the angular momentum L scales as a* GM²/c. Supermassive black holes can achieve such mass growth through copious gas accretion and multiple mergers, but stellar‑mass black holes in X‑ray binaries have very limited mass supply: typical accretion rates are 10⁻⁸–10⁻⁶ M☉ yr⁻¹, amounting to only a few percent of their total mass over their lifetimes. Binary mergers are also exceedingly rare. Consequently, the present‑day spins measured for these objects largely reflect the angular momentum imparted during the core‑collapse supernova that formed them.

The paper then connects this “pristine” spin signature to two fundamental astrophysical questions. First, the collapsar model for long gamma‑ray bursts (GRBs) assumes a rapidly rotating black hole (a* ≳ 0.9) to power relativistic jets. Observationally, many stellar‑mass black holes exhibit modest spins (a* ≈ 0.1–0.5), suggesting that either GRB central engines do not always require such extreme rotation, or that alternative mechanisms—strong magnetic fields (magnetar‑driven jets) or highly asymmetric mass ejection—play a dominant role. Second, the spin distribution of the first black holes (Pop III remnants) can be inferred by comparing the primordial spin expectations (high a* due to low metallicity and rapid stellar rotation) with the observed low‑spin distribution of present‑day stellar‑mass black holes. A discrepancy would imply that early star‑formation environments differed markedly from those in the local universe.

Methodologically, the author reviews the three principal spin‑measurement techniques—relativistic Fe Kα line fitting, continuum‑fitting of the thermal disk spectrum, and high‑frequency quasi‑periodic oscillations—highlighting systematic uncertainties such as disk ionization, inclination, and model degeneracies. The paper emphasizes that future gravitational‑wave observations of binary black‑hole mergers, combined with next‑generation X‑ray missions (e.g., Athena, Lynx), will enable direct tracking of spin evolution before and after mass accretion events, providing a decisive test of the hypothesis.

In summary, the inability of stellar‑mass black holes to substantially increase their mass after formation preserves their natal spin, making them valuable probes of core‑collapse physics, GRB engine models, and the rotational properties of the earliest black holes in the universe.