Figure 1: Artist’s impression of a brown dwarf.
OGLE-2015-BLG-1319 is the first gravitational microlensing event observed by two space telescopes, Spitzer and Swift, and from the ground. The foreground star responsible for this gravitational microlensing event is a K-type main sequence star that crossed the line-of-sight to a distant background star. The gravitational field of the foreground star acted as a "lens", magnifying light from the distant background star. The gravitational microlensing light curve observed from the ground shows a short anomaly over the peak due to a presence of a companion around the foreground star.
Parallax measurement of the gravitational microlensing event from Spitzer shows that the companion is a brown dwarf with 30 to 55 times the mass of Jupiter. The small separation of Swift from Earth means that the baseline is not long enough to allow Swift to provide an independent parallax measurement. As a result, there are two possible solutions for the projected separation of the brown dwarf from its host star. Due to the wide/close degeneracy, the brown dwarf is either 0.23 to 0.28 AU or 40 to 52 AU from its host star.
Figure 2: Light curve of OGLE-2015-BLG-1319 with data from Spitzer (red), and various ground-based observatories. The inset shows the anomalous region over the peak of the event, revealing the presence of the companion brown dwarf. Shvartzvald et al. (2016)
Shvartzvald et al. (2016), "First simultaneous microlensing observations by two space telescopes: Spitzer & Swift reveal a brown dwarf in event OGLE-2015-BLG-1319", arXiv:1606.02292 [astro-ph.EP]