Imprints of Different Types of Low-Angular-Momentum Accretion Flow Solutions in General Relativistic Hydrodynamic Simulations

TL;DR

This study uses 2D general relativistic hydrodynamic simulations to explore how different low-angular-momentum accretion flow solutions onto black holes influence jet formation and X-ray emission, aiding interpretation of astrophysical observations.

Contribution

It systematically classifies low-angular-momentum accretion flows and links flow types to jet production and radiative signatures without magnetic fields.

Findings

Solutions with two sonic points produce relativistic jets.

Inner sonic point solutions yield brighter, more extended X-ray emission.

Flow type significantly affects radiative properties and jet formation.

Abstract

Depending on the astrophysical source and its environment, the accretion flows can exhibit a variety of behaviors and characteristics in accordance with the type of solutions. We study low-angular-momentum accretion flows onto black holes using two-dimensional general relativistic hydrodynamic (GRHD) simulations to find imprints of different types of accretion solutions. Such flows, relevant to X-ray binaries and wind-fed low-luminosity active galactic nuclei, often lack sufficient angular momentum to form standard accretion disks. We initialize simulations with semi-analytical transonic solutions defined by specific energy (${\cal E}_0$) and angular momentum ($\lambda_0$), allowing a systematic classification of flow types with: (i) an outer sonic point, (ii) an inner sonic point, and (iii) both, exhibiting shock transitions. Only solutions with two sonic points produce hot, thermally driven bipolar jets/outflows with Lorentz factors up to $\gamma\sim2$, despite the absence of magnetic fields. Using a general relativistic radiation transfer calculation, we compute broadband spectra and images at X-ray ($1 \, \rm keV$) from bremsstrahlung emission. Radiative properties depend strongly on the type of accretion solution. Solutions with inner sonic points produce the brightest and most extended X-ray emission, while outer-point solutions produce compact, fainter signals. These multidimensional models are thus essential for predicting radiative signatures and will enable the development of semi-analytical tools for interpreting X-ray binaries and possibly Sgr~A$^*$ in weak magnetic field regimes.