We present an analysis of the anomalous microlensing event , MOA-2010-BLG-073 , announced by the Microlensing Observations in Astrophysics survey on 2010-03-18 .
This event was remarkable because the source was previously known to be photometrically variable .
Analyzing the pre-event source lightcurve , we demonstrate that it is an irregular variable over time scales > 200 d. Its dereddened color , ( V - I ) _ { S, 0 } , is 1.221 \pm 0.051 mag and from our lens model we derive a source radius of 14.7 \pm 1.3 R _ { \odot } , suggesting that it is a red giant star .
We initially explored a number of purely microlensing models for the event but found a residual gradient in the data taken prior to and after the event .
This is likely to be due to the variability of the source rather than part of the lensing event , so we incorporated a slope parameter in our model in order to derive the true parameters of the lensing system .
We find that the lensing system has a mass ratio of q =0.0654 \pm 0.0006 .
The Einstein crossing time of the event , t _ { E } =44.3 \pm 0.1 d , was sufficiently long that the lightcurve exhibited parallax effects .
In addition , the source trajectory relative to the large caustic structure allowed the orbital motion of the lens system to be detected .
Combining the parallax with the Einstein radius , we were able to derive the distance to the lens , D _ { L } =2.8 \pm 0.4 kpc , and the masses of the lensing objects .
The primary of the lens is an M-dwarf with M _ { L, 1 } =0.16 \pm 0.03 M _ { \odot } while the companion has M _ { L, 2 } =11.0 \pm 2.0 M _ { J } , putting it in the boundary zone between planets and brown dwarfs .