Death Line of Gamma-ray Pulsars with Outer Gaps

TL;DR

This paper analytically determines the conditions under which gamma-ray pulsars with outer gaps cease to emit gamma-rays, introducing new death lines that refine the understanding of pulsar activity limits.

Contribution

The study derives a new outer-gap death line considering photon-photon pair production, enhancing the traditional understanding of pulsar gamma-ray emission cessation.

Findings

Identified a modified outer-gap death line for gamma-ray pulsars.

Discovered a new death line relevant for millisecond pulsars.

Predicted gamma-ray detectability thresholds for Fermi and AGILE.

Abstract

We analytically investigate the condition for a particle accelerator to be active in the outer magnetosphere of a rotation-powered pulsar. Within the accelerator (or the gap), magnetic-field-aligned electric field accelerates electrons and positrons, which emit copious gamma-rays via curvature process. If one of the gamma-rays emitted by a single pair materializes as a new pair on average, the gap is self-sustained. However, if the neutron-star spin-down rate decreases below a certain limit, the gap becomes no longer self-sustained and the gamma-ray emission ceases. We explicitly compute the multiplicity of cascading pairs and find that the obtained limit corresponds to a modification of previously derived outer-gap death line. In addition to this traditional death line, we find another death line, which becomes important for millisecond pulsars, by separately considering the threshold of photon-photon pair production. Combining these traditional and new death lines, we give predictions on the detectability of gamma-ray pulsars with Fermi and AGILE. An implication on the X-ray observations of heated polar-cap emission is also discussed.