On Disk Formation around Isolated Black Holes via Stream Accretion

TL;DR

This paper explores how isolated black holes can form transient or persistent accretion disks from wind accretion, highlighting the differences between classical and lateral BHL accretion and the effects of gas composition.

Contribution

It introduces the concept of lateral BHL accretion and demonstrates conditions under which accretion disks form or disappear around isolated black holes.

Findings

Classical BHL cannot form accretion disks.

Lateral BHL can produce transient disks.

Pair-dominated flows form persistent disks.

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.