📝 SummarXiv

Abstract

Millisecond pulsars (MSPs) are prolific GeV $\gamma\text{-ray}$ emitters, and nearly 80\% of Fermi-LAT MSPs reside in compact binaries. We demonstrate that the companions in these compact MSPs binaries are also 511 keV annihilation line emitters using {\tt MEGAlib} simulations (a high energy radiation transport software built with {\tt Geant4}) to compute the particle showers and resulting back splash emission from the pulsar irradiation. The 511 keV signal exhibits strong flux modulation and red/blueshifts associated with a binary orbit, enabling powerful coherent searches. Measuring the 511 keV emission would enable direct $\gamma\text{-ray}$ 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 ultra-compact systems, against which pulsar surveys are systematically biased. These ultra-compact systems are also possibly prime LISA galactic sources. This necessitates future joint LISA-MeV $\gamma\text{-ray}$ 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.