We report the detection of the cool , Jovian-mass planet MOA-2007-BLG-400Lb .
The planet was detected in a high-magnification microlensing event ( with peak magnification A _ { max } = 628 ) in which the primary lens transited the source , resulting in a dramatic smoothing of the peak of the event .
The angular extent of the region of perturbation due to the planet is significantly smaller than the angular size of the source , and as a result the planetary signature is also smoothed out by the finite source size .
Thus the deviation from a single-lens fit is broad and relatively weak ( \sim few percent ) .
Nevertheless , we demonstrate that the planetary nature of the deviation can be unambiguously ascertained from the gross features of the residuals , and detailed analysis yields a fairly precise planet/star mass ratio of q = 2.6 \pm 0.4 \times 10 ^ { -3 } , in accord with the large significance ( \Delta \chi ^ { 2 } = 1070 ) of the detection .
The planet/star projected separation is subject to a strong close/wide degeneracy , leading to two indistinguishable solutions that differ in separation by a factor of \sim 8.5 .
Upper limits on flux from the lens constrain its mass to be M < 0.75 M _ { \odot } ( assuming it is a main-sequence star ) .
A Bayesian analysis that includes all available observational constraints indicates a primary in the Galactic bulge with a mass of \sim 0.2 - 0.5 M _ { \odot } and thus a planet mass of \sim 0.5 - 1.3 M _ { Jup } .
The separation and equilibrium temperature are \sim 0.6 - 1.1 ~ { } { AU } ( \sim 5.3 - 9.7 ~ { } { AU } ) and \sim 103 ~ { } { K } ( \sim 34 ~ { } { K } ) for the close ( wide ) solution .
If the primary is a main-sequence star , follow-up observations would enable the detection of its light and so a measurement of its mass and distance .