Does Nature use neutral beams for interstellar plasma heating around compact objects?

Abstract: A neutral beam injection technique is employed in all major TOKAMAK facilities for heating of magnetically confined plasma. The question then arises, whether a similar mechanism might work in astrophysical objects? For instance, a hyper-Eddington galactic binary SS433 possesses baryonic jets, moving at a quarter of the speed of light, and observations revealed signs of gas cooling and recombination on sub-pc scales and equally strong signs of powerful energy deposition on much larger scales $\sim$100 pc. Here we consider a model where neutral atoms transport this energy. A sub-relativistic beam of neutral atoms penetrates the interstellar medium, these atoms gradually get ionised and deposit their energy over a region, whose longitudinal dimension is set by the "ionisation length". The channel, where the energy is deposited, expands sideways and drives a shock in the lateral direction. Once the density in the channel drops, the heating rate by the beam drops accordingly, and the region of the energy release moves along the direction of the beam. We discuss distinct features associated with this scenario and speculate that such configuration might also boost shock acceleration of the "pick-up" protons that arise due to ionisation of neutral atoms both upstream and downstream of the shock.

• The paper proposes a novel mechanism where natural neutral beams heat interstellar plasma around compact objects through ionization, contrasting with traditional hydrodynamic jet models.
• This neutral beam mechanism predicts specific spatial structures and distinct observational signatures, like faster propagation and characteristic energy distributions, potentially guiding astronomical observations.
• The model could revise how astronomers interpret observations of compact objects like SS433 and opens new avenues for understanding energy transfer and particle acceleration in astrophysical plasmas.

Neutral Beam Heating of the Interstellar Medium: A Novel Approach to Astrophysical Plasma Interaction

The paper authored by E. Churazov et al. presents an innovative examination of interstellar plasma heating, specifically questioning whether naturally occurring neutral beams, similar to those used in controlled fusion experiments like TOKAMAK, can heat interstellar plasma around compact objects. This exploration is grounded in astrophysical phenomena, particularly using the case of SS433, a binary system known for its baryonic jets.

Summary of Key Concepts

The model proposed in the paper is built around the idea of neutral hydrogen beams traveling at sub-relativistic velocities penetrating the interstellar medium (ISM), a distinctly new approach compared to conventional hydrodynamic jet models. The novelty lies in the transformation of kinetic energy into thermal energy through ionization processes as neutral hydrogen atoms collide with ISM components. This process creates distinctive thermal effects and has implications for energy distribution at macroscopic scales.

Theoretical Implications

1. Energy Transfer Mechanisms: The authors provide a robust theoretical framework for understanding how energy from neutral beams is transferred to the ISM. Upon penetrating the medium, these neutral atoms become ionized, resulting in energy deposition and subsequent shock wave propagation. The paper mathematically models these interactions, estimating the ionization length and subsequent expansion dynamics.
2. Spatial Structure and Dynamics: The proposed mechanism suggests a dynamically evolving region where the neutral beam's energy is deposited, characterized by lateral expansion and longitudinal movement as the beam ionizes further ISM regions. The framework predicts specific structures resulting from this energy transfer, such as a "double-horn" geometry, which has observational parallels in real astrophysical environments.

Practical and Observational Implications

From a practical standpoint, the proposed neutral beam heating mechanism could potentially redefine how we interpret observations of interstellar regions influenced by compact objects. The paper argues for the potential observational signatures that could distinguish neutral beam interactions from traditional hydrodynamic jets, such as faster propagation velocities and characteristic energy distributions.

1. Shock Acceleration: The configuration described could enhance shock acceleration processes for protons created through ionization, providing a unique environment for studying particle acceleration in astrophysical plasmas.
2. Comparative Analysis with Hydrodynamic Jets: The paper further simulates and contrasts this neutral beam model with classical hydrodynamic jets, noting discrepancies in propagation speeds and resulting energy distribution. This may guide astronomers in revising energy transport models for observed systems like SS433 and similar objects.

Future Directions

As astrophysics continues to evolve, this research opens up numerous avenues for further exploration and testing. The authors speculate on the applicability of the model to other high-accretion scenarios, including ultraluminous X-ray sources and supermassive black hole growth processes. The simplifications in modeling also pave the way for more complex simulations, integrating three-dimensional dynamics, beam variability, and heterogeneous medium conditions.

Ultimately, while the model remains a theoretical construct, its implications suggest a need for re-evolving astrophysical paradigms, inviting experimental astronomers to search for confirmatory evidence in their observational data. This could facilitate greater understanding of energy transfer processes in the universe, particularly in the vicinity of compact, energetic astrophysical objects.