Primordial black holes captured by neutron stars: simulations in general relativity

Abstract: We present self-consistent numerical simulations in general relativity of putative primordial black holes inside neutron stars. Complementing a companion paper in which we assumed the black hole mass $m$ to be much smaller than the mass $M_$ of the neutron star, thereby justifying a point-mass treatment, we here consider black holes with masses large enough so that their effect on the neutron star cannot be neglected. We develop and employ several new numerical techniques, including initial data describing boosted black holes in neutron-star spacetimes, a relativistic determination of the escape speed, and a gauge condition that keeps the black hole hole at a fixed coordinate location. We then perform numerical simulations that highlight different aspects of the capture of primordial black holes by neutron stars. In particular, we simulate the initial passage of the black hole through the star, demonstrating that the neutron star remains dynamically stable provided the black-hole mass is sufficiently small, $m \lesssim 0.05 M_$. We also model the late evolution of a black hole oscillating about the center of an initially stable neutron star while accreting stellar mass and ultimately triggering gravitational collapse.

• The paper applies numerical general relativity to simulate the capture of primordial black holes by neutron stars.
• The paper introduces innovative numerical techniques, including gauge conditions to fix the black hole’s coordinate location for improved simulation stability.
• The paper finds that neutron stars remain stable when hosting low-mass black holes but may collapse with higher mass accretion, emitting potential gravitational wave signals.

Analysis of "Primordial black holes captured by neutron stars: simulations in general relativity"

The paper "Primordial black holes captured by neutron stars: simulations in general relativity" explores the theoretical implications of primordial black holes (PBHs) interacting with neutron stars. The study is motivated by the hypothesized presence of PBHs in the early Universe, which, despite the absence of direct evidence for their existence, remain plausible candidates for partially or completely explaining dark matter.

Key Contributions

1. Integration of General Relativity: The authors employ numerical simulations within the framework of general relativity to model the capture process of PBHs by neutron stars. This approach is particularly pertinent given the significant gravitational fields involved, which necessitate a fully relativistic treatment, especially for black holes with non-negligible masses relative to those of neutron stars.
2. Novel Numerical Techniques: The research incorporates several innovative numerical techniques. These include initial data sets that describe boosted black holes within neutron-star spacetimes, offering a more accurate description of their dynamic interactions. The implementation of a gauge condition is particularly noteworthy as it allows the black hole to remain at a fixed coordinate location, facilitating more stable and accurate numerical simulations.
3. Simulations and Results: The paper contrasts two scenarios—PBHs of varying masses traversing neutron stars and their subsequent evolutions. A primary result is that neutron stars remain dynamically stable after being pierced by a black hole if the black hole's mass is relatively small, approximately 5% of the neutron star's mass. Larger masses can result in the black hole accreting enough stellar material to trigger the collapse of the neutron star.
4. Late Evolution and Gravitational Collapse: For later stages of evolution where the PBH oscillates about the neutron star's center while accreting mass, the simulations provide insights into the conditions leading to eventual collapse. The black hole emits gravitational waves during its oscillatory inspiral phase, which, if detectable, may provide empirical evidence for PBHs and insights into the equation of state of nuclear matter within neutron stars.

Implications

• Theoretical Insights: The study enhances the theoretical understanding of PBHs' interactions with stellar bodies. This includes refinements to existing models, offering improvements over simpler point-source approximations used in literature for black hole capture processes.
• Impact on Astronomy and Astrophysics: Practically, the findings suggest potential observable phenomena, such as gravitational waves and other signatures from neutron stars interacting with PBHs. Future detections could help constrain the properties and distribution of PBHs, contributing to broader efforts in identifying dark matter components.

Future Directions

The research indicates several avenues for future exploration. One direction involves refining simulations to consider more complex environments or initial conditions, such as rotational dynamics or magnetic fields within neutron stars. Another potentially fertile area is the collaborative utilization of upcoming observational data from advancements in gravitational wave astronomy to search for empirical signatures predicted by the simulations.

Overall, the work by Baumgarte and Shapiro offers a detailed and methodologically sound exploration of PBHs within a complex astrophysical context, contributing significantly to the field's understanding of these elusive objects and their potential role in cosmic structure and dark matter research.