Neutron Stars as Dark Matter Probes (1004.0629v1)

Abstract: We examine whether the accretion of dark matter onto neutron stars could ever have any visible external effects. Captured dark matter which subsequently annihilates will heat the neutron stars, although it seems the effect will be too small to heat close neutron stars at an observable rate whilst those at the galactic centre are obscured by dust. Non-annihilating dark matter would accumulate at the centre of the neutron star. In a very dense region of dark matter such as that which may be found at the centre of the galaxy, a neutron star might accrete enough to cause it to collapse within a period of time less than the age of the Universe. We calculate what value of the stable dark matter-nucleon cross section would cause this to occur for a large range of masses.

Neutron Stars as Dark Matter Probes

The paper "Neutron Stars as Dark Matter Probes" by Arnaud de Lavallaz and Malcolm Fairbairn presents a meticulous analysis exploring how neutron stars can serve as potential tools for constraining and understanding the nature of dark matter. Focused on the interaction between dark matter (DM) and neutron stars, the work explores both the theoretical underpinnings and practical observational implications of such phenomena.

Overview of Dark Matter and Neutron Star Interaction

This paper examines two primary scenarios involving dark matter interactions with neutron stars. First, it considers the accretion of annihilating dark matter, which results in heating the neutron stars through self-annihilation energy release. Second, it investigates the consequences of non-annihilating dark matter accumulating in the core of neutron stars, potentially leading to catastrophic outcomes like induced gravitational collapse.

Methodology and Calculations

For the interaction with annihilating dark matter, the authors derive the capture rate of dark matter onto neutron stars. They consider factors such as the density profiles of dark matter in the Milky Way—using Einasto profiles calibrated to galactic observations—and utilize relativistic corrections for the escape velocity within neutron stars. The computation integrates dark matter interaction cross-sections and mass ranges to predict capture rates under various density scenarios.

In terms of non-annihilating dark matter, the researchers explore the accumulation limits and the conditions necessary to trigger a collapse of the neutron star itself. They analyze the Chandrasekhar limit for the accreted dark matter core, which for certain DM masses and cross-sections could reach criticality within the age of the universe.

Numerical Outcomes and Conclusions

• Annihilating Dark Matter: The paper reveals that while dark matter can theoretically significantly heat neutron stars, the maximum surface temperatures due to this heating reach around $10^6$ Kelvin. These temperatures would create radiation primarily in the extreme UV or soft X-ray spectrum. Thus, observational prospects are hampered both by absorption through interstellar mediums and existing X-ray backgrounds.
• Non-Annihilating Dark Matter: The paper finds a wider range of implications. If a neutron star were in a dense DM environment, it could accrete enough mass to reach the Chandrasekhar limit of the dark matter core, leading to collapse. Only at extremely high DM densities could this plausibly align with current direct detection constraints.

Implications and Future Prospects

This investigation suggests that while immediate observational technologies may struggle to leverage neutron stars as effective DM probes due to observational limitations or galaxy center obfuscation, the theoretical framework lays crucial groundwork. It highlights neutron stars as intriguing candidates for potential indirect DM detection, especially if denser DM reservoirs or improved detection technologies become available.

The paper also prompts further inquiry into gamma-ray burst phenomena possibly linked to neutron star collapse via DM accretion, presenting an exciting intersection for astrophysics research. It underscores the importance of continued cross-disciplinary effort, exploring both new observational strategies in astronomy and evolution in understanding the fundamental nature of dark matter in cosmology.