Future Space-basedγ-Ray Pulsar Timing Arrays

Radio pulsar timing arrays (PTAs) of millisecond pulsars (MSPs) are beginning to detect nHz gravitational waves (GWs). MSPs are bright γ-ray emitters, and all-sky monitoring of ∼100 MSPs with the Fermi Large Area Telescope enables the γ-ray PTA (GPTA). The GPTA complements radio PTAs: its MSP sample differs, and γ-ray data are immune to plasma propagation effects, have minimal data gaps, and rely on homogeneous instrumentation. To assess the GPTA performance of future γ-ray observatories, we simulated the population of Galactic MSPs and developed a method to predict their γ-ray spectra. The approach reproduces the properties of the observed MSP sample, motivating it for future population studies. We then computed the expected signal for new instrument concepts covering a wide range of capabilities. We found that the optimal GPTA band is 0.1–0.3 to 5 GeV and that future GeV-band instruments could detect 10³–10⁴ MSPs and achieve GW sensitivity surpassing current radio PTAs, reaching the GW self-noise regime. If a bulge population of MSPs is the source of the GeV signal observed toward the Galactic center, such instruments could detect the population and determine the production channel. We also examined MeV-band instruments, but—with the caveat that the spectral model extrapolation is uncertain—we found low sensitivity to MSPs. The high discovery potential, strong GW performance, and substantial synergy with radio PTAs all argue for the pursuit of next-generation γ-ray pulsar timing.