arXiv2025
Radio pulsar timing array (PTA) experiments using millisecond pulsars (MSPs) are beginning to detect nHz gravitational waves (GWs). MSPs are bright GeV gamma-ray emitters, and all-sky monitoring of about 100 MSPs with the Fermi Large Area Telescope (LAT) has enabled a gamma-ray Pulsar Timing Array. The GPTA provides a complementary view of nHz GWs because its MSP sample is different, and because the gamma-ray data are immune to plasma propagation effects, have minimal data gaps, and rely on homogeneous instrumentation. To assess GPTA performance for future gamma-ray observatories, we simulated the population of Galactic MSPs and developed a high-fidelity method to predict their gamma-ray spectra. This combination reproduces the properties of the LAT MSP sample, validating it for future population studies. We determined the expected signal from the simulated gamma-ray MSPs for instrument concepts with a wide range of capabilities. We found that the optimal GPTA energy range runs about 0.1 to 5 GeV, but we also examined Compton/MeV instruments. With the caveat that the MSP spectra models are extrapolated beyond observational constraints, we found low signal-to-background ratios, yielding few MSP detections. GeV-band concepts would detect 10$^3$ to 10$^4$ MSPs and achieve GW sensitivity on par with and surpassing the current generation of radio PTAs, reaching the GW self-noise regime. When considering two possible scenarios for the formation of MSPs in the Galactic bulge, the collective signal from which is a potential source of an excess GeV signal observed towards the Galactic center, we find that most of the concepts can both detect this bulge population and distinguish the production channel. In summary, the high discovery potential, strong GW performance, and tremendous synergy with radio PTAs all argue for the pursuit of next-generation gamma-ray pulsar timing.