Bounds on self-interacting fermion dark matter from observations of old neutron stars

The existence of old neutron stars deeply constrains self-interacting fermion dark matter, which can form star-killing black holes. We quantify this constraint on dark matter-nucleon scattering, considering collapse scenarios that broaden bounds over intermediate masses. We then find the self-annihilation and coannihilation rates necessary to lift these dark matter-nucleon scattering bounds. For Yukawa-coupled dark matter that fits dwarf galaxy halo profiles with a coupling alpha = 10(-1)-10(-4), a scalar mediator mass m(phi) = 1-500 MeV, and dark matter mass m(X) = 0.1-10(7) GeV, we show that fermion dark matter is unconstrained if it self-annihilates at a rate greater than 10(-40) cm(3)/s or coannihilates with baryons at a rate greater than 10(-50) cm(3)/s.