On Disk Formation around Isolated Black Holes via Stream Accretion

Abstract: We investigate accretion onto an isolated black hole from uniform winds. If the winds are directed towards the black hole, then the accretion process can be well described by the classical Bondi-Hoyle Lyttleton or BHL accretion. If the wind is not directed towards the black hole and flows past it, then a smaller fraction of the flow can be attracted by the black hole, and this type of accretion cannot be described by the classical BHL, and we coin the second kind as the lateral BHL. We show that the classical BHL cannot form an accretion disk, while lateral BHL can form transient accretion disks. To describe the thermodynamics of the flow, we have used a variable adiabatic index equation of state which depends on the temperature of the flow as well as the composition of the gas. We show that the electron-proton gas forms an accretion disk, which disappears and forms a shock cone, only to form the disk again at a later time, while for flows with less protons, the accretion disk, once lost, does not reappear again. Only when the flow is pair-dominated does it form a persistent accretion disk. We also show that a shock cone is less luminous than the accretion disk.

• The paper presents high-resolution 2D simulations of classical and lateral BHL accretion, demonstrating that lateral accretion can induce disk formation.
• The paper shows that disk formation is sensitive to gas composition and wind Mach number, with electron-positron plasmas more likely to yield persistent disks.
• The paper discusses the role of shock cone formation and its lower luminosity emission compared to disk states, highlighting implications for detecting isolated black holes.

On Disk Formation around Isolated Black Holes via Stream Accretion: An Analytical Perspective

The study conducted by Tripathi et al. investigates the paradigms of accretion onto isolated black holes (BHs) using variations of the Bondi-Hoyle-Lyttleton (BHL) accretion model. This research adopts a dual approach in examining the accretion processes: classical BHL with a direct wind inflow towards the BH and a newly coined lateral BHL where the wind flows past the BH. Through a series of two-dimensional, high-resolution numerical simulations, the study provides significant insights into the conditions under which transient or persistent accretion disks might form around isolated BHs subjected to uniform wind streams.

Key Research Insights

1. Simulation Framework: Two-dimensional hydrodynamic simulations are utilized, where the classical BHL problem is solved on an axisymmetric plane, and the lateral BHL problem is addressed on an equatorial plane. The Paczyński–Wiita pseudo-potential mimics the relativistic gravitational influence of the BH, while a relativistic equation of state (EoS) with a variable adiabatic index captures the thermodynamics of the in-flowing gas.
2. Disk Formation Dynamics: A crucial finding is that while classical BHL does not naturally lead to disk formation due to symmetric accretion, the lateral BHL scenario can result in transient or even persistent disk structures. This is attributable to the asymmetry in the gravitational capture process when the wind flows sideways past the BH.
3. Influence of Composition and Mach Number: The study extensively explores variations in gas composition, focusing on electron-proton and electron-positron plasmas. Results suggest that electron-positron dominated flows more readily form persistent disks compared to protons. Variations in the Mach number of the inflowing winds also significantly affect the morphology of the resulting accretion structures.
4. Implications of Shock Cone Formation: When the wind is forced into accretion by the BH's gravity, a shock cone can form. The dynamics within this cone, including its luminosity, is less intense than in the accretion disk states, as confirmed by computed bremsstrahlung and synchrotron emissions. The transition between disk and cone states can result in fluctuating accretion rates and bolometric luminosity.

Implications and Future Directions

The exploration of lateral BHL offers a framework for understanding intermittent accretion phenomena in isolated BHs, which could be prevalent in regions with sparse gas distributions. Notably, the variable outcomes based on the gas composition and initial conditions highlight the need for tailored models in astrophysical simulations of BH environments.

From a theoretical perspective, this work sets the stage for incorporating more detailed physics, such as magnetic fields and radiative feedback, into accretion models. Practically, understanding disk formation could enhance the detection strategies for isolated BHs in the galaxy through their transient electromagnetic signatures. Future developments could include extending the simulations to fully three-dimensional settings to capture more complex interactions in diverse astrophysical contexts.

In conclusion, the study provides a detailed computational and theoretical examination of accretion processes in isolated BHs, advancing our understanding of how different wind inflow conditions may lead to accretion disk formation and, consequently, observable astrophysical phenomena.