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

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.

Figures (6)

• Figure 1: Density and temperature profiles in (a)-(b) without outflows for $\dot{m}=0.1$ and $1$, and in (c)-(f) with outflows for $\dot{m}_{\rm outer}=0.1$ and $1$. Solid, dashed, and dot-dashed lines in (a) and (b) correspond to $a_*$ = 0.1, 0.5, and 0.9, respectively, while solid and dashed lines in (c)-(f) denote $p$ = 0.5 and 0.8, respectively.
• Figure 2: Profiles of pressure contributions and cooling fractions of NDAFs without outflows with $\dot{m}$ = 0.1 and 1. Solid, dashed, and dot-dashed lines correspond to $a_*$ = 0.1, 0.5, and 0.9, respectively.
• Figure 3: Profiles of pressure contributions and cooling fractions with outflows for $\dot{m}_{\rm outer}=0.1$. Solid and dashed lines denote $p$ = 0.5 and 0.8, respectively.
• Figure 4: Same as Figure \ref{['fig3']} except for $\dot{m}_{\rm outer}=1$.
• Figure 5: Electron fraction $Y_{\rm e}$ of NDAFs without outflows as a function of disk mass $M_{\rm disk}(R)$ inside of $R$ for three accretion rates $\dot m$ = 0.1, 1, and 5, and black, red, and blue lines denote BH spins $a_\ast$ = 0.1, 0.5, and 0.9, respectively.