Irradiated Pulsar Planets and Companions as 511 keV Positron Annihilation Line Sources

Millisecond pulsars (MSPs) are prolific GeV γ-ray emitters, and nearly 80% of Fermi Large Area Telescope MSPs reside in compact binaries. For the first time in the literature, we demonstrate that the companions in these compact MSPs binaries are also 511 keV annihilation line emitters using MEGAlib simulations (a high-energy radiation transport software built with Geant4) to compute the particle showers and resulting backsplash emission from the pulsar irradiation. The 511 keV signal exhibits strong flux modulation and red- and blueshifts associated with a binary orbit, enabling powerful coherent searches. Measuring the 511 keV emission would enable direct γ-ray characterization of unusual pulsar exoplanets and companions, and allow one to identify the unambiguous presence of active pulsars whose beams do not intercept Earth. Intriguingly, the 511 keV flux is brightest for ultracompact systems, against which pulsar surveys are systematically biased. These ultracompact systems are also possibly prime Laser Interferometer Space Antenna (LISA) Galactic sources. This necessitates future joint LISA–MeV γ-ray techniques to characterize MSP binaries. These MSP binaries may also contribute to the puzzling source of the excess 511 keV photons near the Galactic bulge and center.