Inherent self-consistency of the electron fraction between neutrino-dominated accretion flows and their progenitors

TL;DR

This study examines the electron fraction in neutrino-dominated accretion flows around stellar-mass black holes, showing consistency with GRB progenitors and supporting NDAFs as central engines.

Contribution

It provides a self-consistent analysis of electron fraction distributions in NDAFs, linking them to progenitor properties across different accretion regimes and black hole spins.

Findings

Outer boundary electron fractions align with massive collapsar progenitors.

High accretion rate disks exhibit mildly degenerate electron fractions consistent with merger scenarios.

Results are robust across varying black hole spins.

Abstract

Stellar-mass black holes (BHs) surrounded by neutrino-dominated accretion flows (NDAFs) are a leading central engine of gamma-ray bursts (GRBs). In this work, we investigate the electron fraction distribution in NDAFs with or without disk outflows for different accretion rates, BH spins, and outflow rates. As the results, for the cases of the massive disks at relatively low accretion rates, the outer boundary of the disks are predominantly advection-cooled, yielding electron fractions of \(Y_{\rm e} \sim 0.5\), as expected for massive collapsar progenitors. By contrast, in the cases of lower-mass disk at high accretion rates, neutrino cooling becomes highly efficient and mildly electron-degenerate disks emerge, characterized by \(Y_{\rm e} \lesssim 0.38\) at the outer boundary of the disk, even for the strong outflows, which is consistent with materials from compact object merger scenarios. Moreover, we find that these trends remain robust across different BH spins. Consequently, the self-consistent agreement between the electron fraction properties at the outer boundaries of NDAFs and those expected from GRB progenitors provides effectively support for NDAFs serving as the GRB central engines.