Pulsars as a tool to detect event horizons

Spinning collapsed objects like Kerr black holes or naked singularities cause the spin axes of pulsars in their vicinity to precess significantly due to frame-dragging. Here we report that the spin-precession rate, and hence the observed pulse rate, of such a pulsar moving in a circular orbit near an event horizon rises sharply as the orbit shrinks. This provides a new way to detect such a horizon, not only from an observed high electromagnetic pulse rate, but also from a plausible detection of gravitational waves due to the increased pulsar spin. The pulsar, approaching the event horizon, should eventually be disrupted causing a new astronomical phenomenon, which we call a spin-precessional disruption event (SPDE). We argue that a significant difference between the radioactive glow of an SPDE and that of a tidal disruption event of a pulsar near a naked singularity could distinguish such a singularity from a black hole.