Dark Matter in Compact Stars

Compact stellar objects are promising cosmic laboratories to test fundamental interactions, in particular they could shed light on the nature of dark matter (DM). DM captured by the strong gravitational field of these stellar remnants transfers kinetic energy to the star during the collision. This together with further DM annihilation in the stellar interior can have various observational consequences such as the anomalous heating of old compact stars and compact star destruction for non-annihilating DM.

We have improved former calculations of the capture and thermalization rates in both white dwarfs (WDs) and neutron stars (NSs), making little approximations about the physics of compact stars. While DM deposits its kinetic energy in the star quite quickly, for appreciable annihilation heating to be achieved, capture and annihilation processes should reach a state of equilibrium. We also revisit the calculation of the capture-annihilation equilibrium timescales in neutron stars. For NSs, we show that capture-annihilation equilibrium, and hence maximal annihilation heating, can be achieved without complete thermalization of the captured dark matter for all types of dark matter - baryon interactions.

This includes cases where the scattering or annihilation cross sections are momentum or velocity suppressed in the non-relativistic limit. For scattering cross sections that saturate the capture rate, we find that capture-annihilation equilibrium is typically reached on a timescale of less than a year for vector interactions and 10 thousand years for scalar interactions. For fermionic non-annihilating heavy DM, we also revisit black hole (BH) formation, accretion, and evaporation. We find that previous results on the DM-nucleon scattering cross section for a NS to be destructed by a BH from accumulated DM can be relaxed by a few orders of magnitude.