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 α=10^-1--10^-4, a scalar mediator mass m_φ=1--500 MeV, and dark matter mass mX=0.1--10⁷ GeV, we show that fermion dark matter is unconstrained if it self-annihilates at a rate greater than 10^-40 cm³/s or coannihilates with baryons at a rate greater than 10^-50 cm³/s.