Extending the Pulsar radio population through Quantum Vacuum Cherenkov Emission
The presence of some radio pulsars within and below the so-called ``death valley'' in the $P$--$\dot{P}$ diagram challenges the standard pair-cascade models based on curvature radiation (CR) and resonant inverse Compton scattering (RICS) in neutron star polar caps. We show that Cherenkov radiation from the quantum vacuum, a strong-field quantum electrodynamics (QED) phenomenon enabled by vacuum polarization, constitutes a distinct pair-creation channel within the vacuum gap model. Within this model, the acceleration length scale for Cherenkov photon emission and the subsequent photon mean free path are sufficiently short to ensure rapid conversion into $e^+e^-$ pairs. This process acts as a complementary pair-creation channel, remaining viable even in regimes where classical mechanisms become heavily suppressed. This leads to a new set of death lines in the $P$--$\dot{P}$ diagram that partially overlap with and extend beyond the CR and RICS death valleys, enlarging the viable parameter space for radio-emitting neutron stars. The result provides a physical basis for coherent radio emission from long-period neutron stars previously considered inactive, and offers a new window for testing non-linear QED with next-generation radio facilities such as the Square Kilometre Array.