Generating ultracompact neutron stars with bosonic dark matter
Sarah Louisa Pitz, Jürgen Schaffner–Bielich
Abstract
In this work we investigate the properties of neutron stars admixed with self-interacting scalar bosonic dark matter. The dark matter interaction is described by a generalized ${\ensuremath{\phi}}^{n}$ power-law potential. We perform a stability analysis of these two-fluid objects by studying the onset of the unstable radial modes. We find ultracompact neutron star-dark matter configurations where the neutron star matter is confined to a core radius of values below 7 km which is unreachable for pure neutron stars. The total gravitational maximum mass of these ultracompact configurations can have values of $3.4{M}_{\ensuremath{\bigodot}}$. With our general ansatz of the power-law potential we show that the compactness of these solutions can be extreme, i.e. the compactness is $C=1/3$ or even larger, making them compact enough to have a light-ring mimicking black holes. These ultracompact objects are stable and possess a dark matter halo while having a hadronic matter core. With the addition of dark matter to neutron stars recent unusual mass-radius measurements of compact stars can be explained. We conclude that apparently contradictory measurements of neutron star masses and radii could be not only an indication of the presence of dark matter around a the hadronic matter core which is stabilized by the gravitational potential of dark matter but could also serve to disentangle the self-interaction strength of dark matter. Our work points to a stiff equation of state for the dark matter fluid, rather than a soft one.