Future Space-based Gamma-ray Pulsar Timing Arrays

TL;DR

This work evaluates the potential of a next-generation gamma-ray pulsar timing array (GPTA) to detect nHz gravitational waves, by constructing realistic millisecond pulsar populations (disk and bulge) and a high-fidelity gamma-ray emission model, then forecasting performance for multiple instrument concepts. The authors show that GeV (pair-production) gamma-ray instruments, especially with LAT-like PSFs and large grasp, could discover $\sim$10^3–10^4 MSPs and reach GW sensitivities that rival or exceed current radio PTAs, potentially entering the GW self-noise regime. They also examine two bulge MSP formation scenarios and demonstrate that most concepts can either detect bulge populations or distinguish their production channels, offering a path to addressing the Galactic Center Excess in gamma rays. The study further discusses synergy with radio PTAs, the importance of angular resolution, and feasible implementation routes (including LAT-inspired, AMEGO-X-like, and GammaTPC-like designs) while noting the MeV gap remains an area for future exploration. Overall, the work argues for pursuing next-generation gamma-ray pulsar timing to complement and enhance low-frequency gravitational-wave astronomy and Galactic MSP science.

Abstract

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.

Figures (9)

• Figure 1: The distribution in $P$ and $\dot{E}$ for various populations of MSPs and young pulsars. The black points are pulsars tabulated in the ATNF pulsar catalog Manchester05 and the orange points are LAT-detected pulsars. The blue cloud of points is one realization of the disk population of MSPs (S1) discussed in the text, while the red cloud shows the distribution of a possible bulge population of MSPs formed via accretion-induced collapse (S2). The MSPs of another bulge population scenario, S3, follow the same distribution as S1. The period cutoffs of 30 ms (S1/S3) and 100 ms (S2) that were assumed in the synthesis are evident, but these cutoffs are irrelevant for PTAs, which prioritize rapid rotators.
• Figure 2: The predicted cutoff energy ($E_c$, left) and $\gamma$-ray efficiency (right) distributions for one realization of the disk population with the $\gamma$-ray spectrum derived using the fundamental plane relation outlined in the main text. The orange points indicate the direct application of the relation to the synthesized $P$ and $\dot{P}$, and the blue points indicate the introduction of scatter on both $E_c$ and $\dot{E}$. The former captures the observed observational spread in $E_c$, and the latter captures beaming and other neglected physical influences on the luminosity (e.g. magnetic inclination).
• Figure 3: The cumulative distribution of $\gamma$-ray fluxes observed at earth as predicted from the fundamental plane relation for 10 synthesized MSP populations. The blue trace indicates the mean over the realizations and includes additional scatter in $E_c$ and $\mathcal{L}_{\gamma}$, while the orange trace gives the results without this scatter. The envelope of the same color indicates the minimum and maximum values encountered over the 10 realizations. The green trace is the observed flux distribution of LAT MSPs.
• Figure 4: As in Figure \ref{['fig:logNlogS']}, the blue trace and envelope give the mean and range over MSP population realizations of the statistical significances (TS) achieved using the virtual LAT described in the main text. The TS values have been scaled to a data set length of 12 years. The green trace again gives the distribution of observed values.
• Figure 5: The distribution of white noise (WN) (essentially pulsar timing precision) for 10 realizations of the disk MSP population in the virtual LAT. As in previous plots, the solid line indicates the mean while the shaded region indicates the range of the simulations. The green trace gives the distribution of observed WN values.