MOA-2011-BLG-293Lb: A test of pure survey microlensing planet detections

Because of the development of large-format, wide-field cameras, microlensing surveys are now able to monitor millions of stars with sufficient cadence to detect planets. These new discoveries will span the full range of significance levels including planetary signals too small to be distinguished from the noise. At present, we do not understand where the threshold is for detecting planets. MOA-2011-BLG-293Lb is the first planet to be published from the new surveys, and it also has substantial follow-up observations. This planet is robustly detected in survey+follow-up data (Δχ² 5400). The planet/host mass ratio is q = (5.3 ± 0.2) × 10^-3. The best-fit projected separation is s = 0.548 ± 0.005 Einstein radii. However, due to the s↔s ^-1 degeneracy, projected separations of s ^-1 are only marginally disfavored at Δχ² = 3. A Bayesian estimate of the host mass gives M_L = 0.43^+0.27 _{- 0.17} M, with a sharp upper limit of M_L < 1.2 M from upper limits on the lens flux. Hence, the planet mass is m_p = 2.4^+1.5 _{- 0.9} M _Jup, and the physical projected separation is either r ≃ 1.0AU or r ≃ 3.4AU. We show that survey data alone predict this solution and are able to characterize the planet, but the Δχ² is much smaller (Δχ² 500) than with the follow-up data. The Δχ² for the survey data alone is smaller than for any other securely detected planet. This event suggests a means to probe the detection threshold, by analyzing a large sample of events like MOA-2011-BLG-293, which have both follow-up data and high-cadence survey data, to provide a guide for the interpretation of pure survey microlensing data.