Neutron Star Glitches Explained: Turbulence and Pressure in Superfluid Cores.

The interiors of neutron stars, remnants of collapsed massive stars, represent some of the most extreme environments in the universe, possessing densities exceeding that of atomic nuclei. Understanding the dynamics within these stellar objects is crucial for explaining observed phenomena, such as glitches and sudden increases in their rotational speed. These glitches are believed to originate from interactions within the superfluid core, a state of matter exhibiting zero viscosity. Recent research, published under the title ‘Anomalous Energy Injection in Turbulent Neutron Star Cores’, details a novel investigation into the mechanisms driving these events.

Anirudh Sivakumar, from Bharathidasan University, collaborates with Pankaj Kumar Mishra of the Indian Institute of Technology Guwahati, and Ahmad A. Hujeirat from the University of Heidelberg, alongside Paulsamy Muruganandam, affiliated with both the Department of Physics and the Department of Medical Physics at Bharathidasan University, to present a compelling analogue utilising a rotating atomic Bose-Einstein condensate. Their work reveals a previously unrecognised role for pressure in sustaining turbulence during glitches, offering a refined understanding of energy transfer within these dense stellar cores. The team demonstrate a transition from Kolmogorov-like turbulence (characterised by energy cascading to smaller scales) to Vinen-like scaling, a regime specific to rotating superfluids, and identifies an optimal damping coefficient for efficient energy injection.

Recent investigations establish a connection between quantum fluid dynamics and observed rotational glitches in neutron stars, linking terrestrial laboratory experiments with extreme astrophysical environments. These glitches, sudden increases in rotational frequency, originate from the rearrangement of quantised vortices within the superfluid core of these incredibly dense stellar objects. Consequently, researchers explore analogous systems on Earth to model these complex processes. They employ Bose-Einstein condensates (BECs), a state of matter exhibiting superfluidity – the ability to flow without any viscosity – as a terrestrial analogue.

The study utilises a two-dimensional rotating BEC, incorporating parameters that simulate the pinning potential and damping forces present in a neutron star’s crust. The pinning potential represents the forces that impede the movement of vortices, while damping forces represent the dissipation of energy. This allows for detailed examination of energy transfer mechanisms. Through careful manipulation of these parameters, scientists observe a dynamic transition in the turbulent cascade governing energy transfer, revealing crucial insights into how energy dissipates and redistributes during a glitch event.

Researchers initially observe Kolmogorov-like turbulence, characterised by a broad range of eddy sizes and energy dissipation, before transitioning to Vinen-like scaling. Vinen scaling describes a regime where energy cascades primarily through interactions between vortices and the surrounding fluid, differing from the broader range of scales in Kolmogorov turbulence. This transition provides insight into how energy is dissipated and redistributed during a glitch event, demonstrating a parallel between the behaviour of quantum fluids and the dynamics of neutron stars.

A key finding centres on the identification of an anomalous secondary injection mechanism, driven primarily by pressure fluctuations within the fluid, which sustains turbulent fluctuations and contributes significantly to the observed glitch dynamics. This pressure-driven injection plays a vital role in maintaining turbulence, extending the duration of the glitch and influencing its magnitude, suggesting that turbulence in neutron star glitches is not solely reliant on vortex reconnections, where vortices break and rejoin, releasing energy.

By systematically varying the damping coefficient, researchers pinpoint an optimal regime for efficient energy transfer, establishing a crucial link between internal dissipation and glitch characteristics. This optimisation highlights the sensitivity of glitch events to internal dissipation. It provides constraints on the physical properties of the neutron star’s crust and core, allowing for more accurate modelling of these complex systems. The identified regime offers a valuable benchmark for future modelling efforts, guiding the development of more realistic simulations.

The use of a rotating BEC, incorporating both damping and a pinning potential to simulate the neutron star crust, represents a robust analogue system, allowing for controlled experimentation and detailed analysis of the underlying physics. This approach enables researchers to isolate and study the key processes driving glitch dynamics, which would be impossible to achieve directly with astronomical observations.

Future work should focus on extending the two-dimensional model to three dimensions to more accurately represent the complex geometry of neutron stars, thereby enhancing the realism of the simulations. Investigating the effects of rotation and magnetic fields on the turbulent cascade will further refine the model and provide a more complete understanding of the dynamics of neutron star interiors. Exploring the role of different types of vortices and their interactions will also be crucial for advancing our knowledge of glitch dynamics.

Researchers should also investigate the potential for using machine learning techniques to analyse the vast amounts of data generated by these simulations, accelerating the discovery of new insights and improving the accuracy of the models. Developing more sophisticated numerical methods will also be crucial for capturing the complex physics of neutron star interiors.

The ongoing research promises to unlock new insights into the fundamental physics of neutron stars, shedding light on the behaviour of matter under extreme conditions and advancing our understanding of the universe. By combining theoretical modelling, laboratory experiments, and observational data, scientists are making significant progress in unravelling the mysteries of these enigmatic objects.