Using the gravitational microlensing event OGLE-2014-BLG-1186 as an instructive example , we present a systematic methodology for identifying the nature of localised deviations from single-lens point-source light curves , which ensures that 1 ) the claimed signal is substantially above the noise floor , 2 ) the inferred properties are robustly determined and their estimation not subject to confusion with systematic noise in the photometry , 3 ) there are no alternative viable solutions within the model framework that might have been missed .
Assessing the photometric noise by means of an effective model significantly increases the sensitivity arising from an analysis of the total microlensing data set to more subtle perturbations , and thereby in particular to low-mass planets .
With a time-scale t _ { \mathrm { E } } \sim 300 ~ { } \mbox { d } and the brightness being significantly above baseline for four years , OGLE-2014-BLG-1186 is particularly long .
Consequently , annual parallax and binarity could be separated and robustly measured from the wing and the peak data , respectively .
While we were able to establish the presence of binarity , we find model light curves matching the features indicated by the acquired data ( within the estimated noise ) that involve either a binary lens or a binary source .
Our binary-lens models indicate a planet of mass M _ { 2 } = ( 45 \pm 9 ) ~ { } M _ { \oplus } , orbiting a star of mass M _ { 1 } = ( 0.35 \pm 0.06 ) ~ { } M _ { \odot } , located at a distance D _ { \mathrm { L } } = ( 1.7 \pm 0.3 ) ~ { } \mbox { kpc } from Earth , whereas our binary-source models suggest a brown-dwarf lens of M = ( 0.046 \pm 0.007 ) ~ { } M _ { \odot } , located at a distance D _ { \mathrm { L } } = ( 5.7 \pm 0.9 ) ~ { } \mbox { kpc } , with the source potentially being a ( partially ) eclipsing binary involving stars predicted to be of similar colour given the ratios between the luminosities and radii .
The ambiguity in the interpretation would be resolved in favour of a lens binary by observing the luminous lens star separating from the source at the predicted proper motion of \mu = ( 1.6 \pm 0.3 ) ~ { } \mbox { mas } \mbox { yr } ^ { -1 } , whereas it would be resolved in favour of a source binary if the source could be shown to be a ( partially ) eclipsing binary matching the obtained model parameters .
We experienced that close binary source stars pose a challenge for claiming the detection of planets by microlensing in events where the source trajectory passes close to the central caustic near the lens star hosting the planet .