  Scattering interactions between dark matter and Standard Model states
mediated by pseudoscalars are generically challenging to uncover at direct
detection experiments due to rates suppressed by powers of the local dark
matter velocity v ~ 0.001 c. However, they may be observed in the dark
matter-induced heating of neutron stars, whose steep gravitational potentials
prevent such suppression by accelerating infalling particles to
semi-relativistic speeds. We investigate this phenomenon in the context of two
specific, self-consistent scenarios for pseudoscalars coupled to dark matter,
and compare the sensitivity of neutron star heating to bounds from direct
searches for the mediators and dark matter. The first "lighter" scenario
consists of sub-10 GeV mass dark matter mediated by an axion-like particle
(ALP), while the second "heavier" scenario has dark matter above 10 GeV
mediated by a dark pseudoscalar that mixes with a pseudoscalar from a two-Higgs
doublet (the so-called 2HDM+a model). In both frameworks, we show that imminent
measurements of neutron stars will be able to test pseudoscalar-mediated dark
matter beyond the reach of direct dark matter searches as well as bounds on the
mediators from flavor observables, beam dump experiments, and high-energy
colliders.
