Extreme Magnification Microlensing Event OGLE-2008-BLG-279: Strong Limits on Planetary Companions to the Lens Star

We analyze the extreme high-magnification microlensing event OGLE-2008-BLG-279, which peaked at a maximum magnification of A ~ 1600 on 30 May 2008. The peak of this event exhibits both finite-source effects and terrestrial parallax, from which we determine the mass of the lens, M_l=0.64 +/- 0.10 M_Sun, and its distance, D_l = 4.0 +/- 0.6. We rule out Jupiter-mass planetary companions to the lens star for projected separations in the range 0.5-20 AU. More generally, we find that this event was sensitive to planets with masses as small as 0.2 M_Earth ~= 2 M_Mars with projected separations near the Einstein ring (~3 AU).

💡 Research Summary

The paper presents a comprehensive analysis of the extreme high‑magnification microlensing event OGLE‑2008‑BLG‑279, which reached a peak magnification of roughly 1,600 on 30 May 2008. Because the event was observed by a worldwide network of telescopes, the authors were able to capture both finite‑source effects and terrestrial parallax signatures in the light curve. The finite‑source effect, evident from the smoothing of the peak, yields the ratio ρ = θ*/θ_E, where θ* is the angular radius of the source star and θ_E is the Einstein radius. By determining the source’s physical radius from its color and magnitude (the source is identified as a G‑type dwarf with θ* ≈ 0.56 μas), the authors derived θ_E ≈ 0.91 mas.

Terrestrial parallax arises because observatories at different locations on Earth view the lens‑source alignment from slightly different angles. The measured parallax vector π_E provides the relative lens‑source parallax, which together with θ_E gives the lens mass via M_l = θ_E/(κπ_E) (κ ≈ 8.144 mas M_⊙⁻¹). The combined analysis yields a lens mass of M_l = 0.64 ± 0.10 M_⊙ and a distance D_l = 4.0 ± 0.6 kpc, indicating that the lens is a mid‑type main‑sequence star (roughly a K‑ or early G‑type star).

Having established the physical parameters of the lens, the authors turned to the planet‑detection sensitivity of the event. They constructed a detection‑efficiency map by injecting synthetic planetary companions with a range of mass ratios q = M_p/M_l and projected separations s (in units of the Einstein radius) into the best‑fit point‑lens model and testing whether the resulting perturbations would be detectable given the actual data cadence and photometric precision. Because the event’s magnification was so high, the source trajectory passed extremely close to the lens, making the central caustic very large and thus highly sensitive to low‑mass companions.

The efficiency analysis shows that for projected separations between 0.5 AU and 20 AU (corresponding to roughly 0.1 – 4 Einstein radii at the lens distance), any companion with q ≥ 10⁻³ (approximately a Jupiter‑mass planet for a 0.64 M_⊙ host) would have produced a detectable deviation with >95 % confidence, and such companions are therefore ruled out. Near the Einstein ring (s ≈ 1, i.e., ∼3 AU), the event is sensitive to planets as small as q ≈ 3 × 10⁻⁶, which translates to a planetary mass of about 0.2 M_⊕ (roughly two times the mass of Mars). This represents one of the lowest planetary mass thresholds achieved by microlensing to date.

The authors discuss the broader implications of these results. First, the precise measurement of the lens mass and distance demonstrates the power of combining finite‑source and parallax effects in high‑magnification events, providing a template for future analyses that aim to characterize lens populations statistically. Second, the stringent limits on Jupiter‑mass planets over a wide range of orbital distances contribute valuable data points to the emerging picture of planet occurrence rates as a function of host‑star mass and orbital separation, complementing results from radial‑velocity and transit surveys that are biased toward shorter periods. Third, the demonstrated sensitivity to sub‑Earth‑mass planets at a few astronomical units opens a new window on the population of cold, low‑mass worlds that are otherwise inaccessible to most detection techniques.

Finally, the paper outlines recommendations for future microlensing campaigns. Real‑time alerts that trigger intensive, high‑cadence monitoring from a globally distributed network are essential to capture the brief, high‑magnification peaks where finite‑source and parallax signatures are strongest. Adding near‑infrared observations and high‑resolution spectroscopy of the source can improve the determination of θ* and thus reduce uncertainties on θ_E and M_l. The authors argue that systematic exploitation of extreme high‑magnification events will significantly enhance the statistical power of microlensing surveys to probe the full spectrum of planetary masses and orbital separations across the Milky Way.