Effect of dark matter halo on transonic accretion flow around a galactic black hole

TL;DR

This study explores how a dark matter halo influences transonic accretion flows around supermassive black holes, revealing significant deviations in accretion properties and observational signatures compared to isolated Schwarzschild black holes.

Contribution

It introduces a general relativistic model of accretion onto a black hole with an anisotropic dark matter halo, highlighting the impact of halo parameters on accretion dynamics and shock properties.

Findings

Accretion solutions are significantly affected by halo mass and concentration.

Spectral energy distribution and luminosity deviate from Schwarzschild predictions.

Shock properties and parameter space are altered by halo compactness.

Abstract

We investigate the transonic accretion flow in the spacetime of a supermassive black hole (BH) coupled to an anisotropic dark matter fluid, as proposed by Cardoso {\it et al}. We essentially compare the accretion properties of the Cardoso BH with those of an isolated Schwarzschild BH. The Cardoso BH is described by the halo mass ($M_{\rm H}$) and its characteristic length scale ($a_0$). Various classes of accretion solution topologies (e.g., A and W-types) are obtained by solving the dynamical equations of the flow in a fully general relativistic framework. We find that the global accretion solutions in the identified solution topologies are substantially influenced by the halo parameters ($M_{\rm H}, a_0$) when the halo mass is high or the dark matter distribution is concentrated near the black hole. In this high compactness regime, different observational signatures of the accretion disc, like the spectral energy distribution (SED) and bolometric disc luminosity, are found to exhibit considerable deviations from the known results in the Schwarzschild BH model. Furthermore, we obtain shock-induced accretion solutions, where different shock properties, such as the shock radius ($r_{\rm sh}$), flow mass density ($\rho$) compression, and electron temperature ($T_e$) compression across the shock front, are potentially altered from those in the Schwarzschild BH model when the halo compactness is high. Interestingly, the existing shock parameter space, defined by the flow specific angular momentum ($\lambda$) and energy ($E$), is largely reduced for higher halo compactness compared to that of the Schwarzschild BH. These unique features offer a possible valuable tool for characterizing the presence or absence of a dark matter halo around a galactic black hole.