Gamma-Ray Bursts Black hole accretion disks as a site for the vp-process

We study proton rich nucleosynthesis in windlike outflows from gamma-ray bursts accretion disks with the aim to determine if such outflows are a site of the vp-process. The efficacy of this vp-process depends on thermodynamic and hydrodynamic factors. We discuss the importance of the entropy of the material, the outflow rate, the initial ejection point and accretion rate of the disk. In some cases the vp-process pushes the nucleosynthesis out to A~100 and produces light p-nuclei. However, even when these nuclei are not produced, neutrino induced interactions can significantly alter the abundance pattern and cannot be neglected.

💡 Research Summary

The paper investigates whether the outflows from black‑hole accretion disks formed during gamma‑ray bursts (GRBs) can host the νp‑process, a proton‑rich nucleosynthesis pathway that proceeds via rapid (p,γ) captures interspersed with neutron production through neutrino‑induced reactions. Using a set of steady‑state disk models with mass‑accretion rates (ṁdisk) of 0.01, 0.1, and 1 M⊙ s⁻¹, the authors compute temperature, density, and neutrino‑flux profiles as functions of radius. They then parametrize the wind as a steady, spherically expanding outflow characterized by entropy per baryon (S), mass‑loss rate (ṁout), and launch radius (R0). A large nuclear reaction network (≈ 400 isotopes) is coupled to the hydrodynamic evolution, allowing the authors to follow the full nucleosynthetic trajectory from free nucleons to heavy nuclei.

Key findings are as follows. High entropy (S ≈ 80–120 kB per baryon) combined with a strong νe flux—conditions realized when the disk accretion rate is near 1 M⊙ s⁻¹—produces a sustained supply of free protons via νe + n → p + e⁻. This proton reservoir fuels rapid (p,γ) captures while occasional (n,p) reactions, supplied by antineutrino captures on protons, recycle neutrons back into the flow. Under these circumstances the nucleosynthetic flow can climb to mass numbers A ≈ 100, synthesizing the light p‑nuclei 94Mo, 96Ru, and 98Ru in appreciable amounts. The production is most efficient when the wind is launched close to the black hole (R0 ≈ 30–50 km), where the neutrino luminosity is highest, and when the outflow velocity is moderate (Ṁout ≈ 10⁻³–10⁻² M⊙ s⁻¹), allowing sufficient time for the νp‑process to operate before the temperature drops below ≈ 2 GK.

Conversely, low‑entropy winds, very rapid outflows, or launches at larger radii experience a weaker neutrino field; the νp‑process stalls near the iron peak, and the final composition is dominated by Fe‑group nuclei. Even in these “failed” cases, neutrino‑induced reactions still modify the abundance pattern: νe captures shift the neutron‑to‑proton ratio, subtly enhancing isotopes such as 58Fe at the expense of 56Ni. The authors also show that disks with low accretion rates (ṁdisk ≈ 0.01 M⊙ s⁻¹) are too cool to emit energetic neutrinos (average energies < 10 MeV), rendering the νp‑process ineffective.

The study concludes that GRB accretion‑disk winds provide a viable astrophysical site for the νp‑process, especially during the early, high‑accretion phase of the burst. This scenario offers a natural explanation for the observed over‑abundance of light p‑nuclei, which is difficult to reconcile with traditional core‑collapse supernova models alone. The authors recommend future multidimensional simulations and observational searches for isotopic signatures in GRB remnants to further test the viability of this site.