Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. V. Detection Rates of Multiplanetary Systems in High Magnification Microlensing Events

TL;DR

This study evaluates the Nancy Grace Roman Space Telescope's capability to detect multiplanet microlensing systems, specifically triple-lens configurations with two bound planets, using high-cadence simulations of Roman-like observations. By generating ~1.3 million synthetic high-magnification light curves and comparing binary- and triple-lens fits via a Δχ^2 threshold, the authors quantify detection efficiencies across regime geometries and planet mass ratios. They find that roughly 66% of high-magnification triple-lens events are detectable, with efficiencies near 90% when one or both planets are relatively massive and in resonant configurations near the Einstein radius, and markedly lower for low-mass companions. Translating this to the mission level, they predict about 64 triple-lens detections (~4.5% of the total) over Roman’s six-season microlensing survey, underscoring Roman’s potential to reveal complex planetary architectures at kiloparsec distances and guiding future survey strategy and modeling efforts.

Abstract

The Nancy Grace Roman Space Telescope will expand the reach of gravitational microlensing surveys by increasing the number of events monitored and the precision of their light curves. We investigate Roman's ability to detect triple-lens microlensing systems, cases where a foreground star with two bound exoplanets produces detectable anomalies in a microlensing event, using its planned high-cadence observations toward the Galactic bulge. We simulate a large set of high-magnification microlensing light curves based on Roman's expected survey characteristics. A detection criterion, based on a required $χ^2$ improvement for a two-planet model, is applied to determine whether the second planet can be reliably distinguished from a single-planet (binary-lens) model. Our simulations show that the majority of two-planet microlensing events would be detectable with Roman. Events in which both planets are relatively massive (planet-star mass ratios of order $10^{-3}$), or in which the more massive planet occupies a favorable resonant configuration, produce strong central perturbations, resulting in detection efficiencies of roughly 90\%. By contrast, systems with only low-mass planets ($q \sim 10^{-4}$) or with less favorable alignments generate much weaker signals, which often fall below the detection threshold. In general, the planetary mass ratios and the resulting caustic geometry (e.g., central caustic size in resonant versus wide/close orbits) are the dominant factors governing detectability. Taking into account the expected frequency of planetary systems and the fraction of high-magnification events, we estimate that Roman will detect a high-magnification triple-lens event in approximately 4.5\% of multi-planet microlensing events, corresponding to about 64 events over the course of the full survey.

Figures (8)

• Figure 1: Schematic convention used for the geometry of triple-lens configurations. The parameters shown in the figure are defined in Section \ref{['sec:parametrization']}.
• Figure 2: Detection efficiency of triple-lens microlensing events as a function of various physical and geometric parameters. Each panel shows the normalized distribution of all simulated events (light blue), events with significant signals ($\Delta\chi^2 > 160$, dark blue), events with very strong signals ($\Delta\chi^2 > 1000$, red line), and the corresponding detection success rate (orange curve). The efficiency increases with the outer planet’s mass ratio $q_3$, its projected separation $s_3$, and favorable source-caustic configurations.
• Figure 3: Detection efficiency as a function of planetary mass ratios. The color-coded map shows the fraction of detectable triple-lens events in each $(q_1, q_2)$ bin, where $q_1$ and $q_2$ are the mass ratios of the two planetary companions. Detection probability is highest when both planets are relatively massive ($q \gtrsim 10^{-3}$), while systems with low-mass companions ($q \lesssim 10^{-4}$) are generally undetectable. This trend reflects the dependence of caustic size and light curve anomalies on planetary mass.
• Figure 4: Detection efficiency of a triple-lens system as a function of the mass ratio of the second planet ($q_3$). The efficiency is plotted for three different values of the first planet's mass ratio ($q=10^{-2}$, $q=10^{-3}$, and $q=10^{-4}$). The plot shows that when the second planet is massive ($q_3=10^{-3}$), the detection efficiency is high regardless of the first planet's mass. However, as $q_3$ decreases, the efficiency drops, and this effect is more pronounced when the first planet is more massive.
• Figure 5: Simulated light curves for triple-lens systems where at least one planet lies in the resonant regime, as defined by the mass-ratio-dependent boundaries (Equations \ref{['eq:s_min_max']}) These configurations generally show strong central caustic perturbations and yield the highest detection efficiencies in our sample. For each case, the two smaller panels show the triple-lens magnification map and the map of the relative difference between the triple- and binary-lens models near the central caustic.