Emission properties of 5 pulsars in the death-valley and implications on death line models

Abstract

In the framework of the coherent curvature radiation model of pulsar radio emission, charged particles responsible for the radio emission are generated on the polar cap in the localized pair cascade processes called sparks. When a pulsar can no longer sustain the sparking process on its polar cap, the coherent radio emission from the pulsar stops, and the pulsar is called dead. In this work, we revisit the pulsar death phenomena under two popular voltage gap models: vacuum voltage gap and partially screened voltage gap. We notice that a dying pulsar resorts to a single spark on the polar cap to sustain the pair production under both voltage gap frameworks. The presence of only one spark on the polar cap has important implications for the radio emission properties of the pulsar. We study the emission properties of five pulsars close to the lower boundary on the $P-\dot{P}$ plane of the current pulsar population, as these pulsars are expected to be dying. We find that the dying pulsars in our sample and the normal pulsar population have very similar emission properties. We show that the majority of pulsars in the pulsar death valley, including five pulsars in our sample, show evidence of multiple sparks as opposed to what is expected from single spark death line models.