First simultaneous microlensing observations by two space telescopes: $Spitzer$ & $Swift$ reveal a brown dwarf in event OGLE-2015-BLG-1319

TL;DR

This paper reports the first simultaneous microlensing observations by $Spitzer$ and $Swift$, leading to the discovery of a brown dwarf in the Galactic bulge, demonstrating the potential of space telescopes in characterizing lensing systems despite degeneracies.

Contribution

It presents the first use of $Swift$ in microlensing, combining multiple space-based observations to measure physical properties of a lensing system and analyze observational sensitivities.

Findings

Discovered a 30-55 M_J brown dwarf orbiting a K dwarf.

Measured the system at approximately 5 kpc in the Galactic bulge.

Identified an 8-fold degeneracy in the event parameters.

Abstract

Simultaneous observations of microlensing events from multiple locations allow for the breaking of degeneracies between the physical properties of the lensing system, specifically by exploring different regions of the lens plane and by directly measuring the "microlens parallax". We report the discovery of a 30-55$M_J$ brown dwarf orbiting a K dwarf in microlensing event OGLE-2015-BLG-1319. The system is located at a distance of $\sim$5 kpc toward the Galactic bulge. The event was observed by several ground-based groups as well as by $Spitzer$ and $Swift$, allowing the measurement of the physical properties. However, the event is still subject to an 8-fold degeneracy, in particular the well-known close-wide degeneracy, and thus the projected separation between the two lens components is either $\sim$0.25 AU or $\sim$45 AU. This is the first microlensing event observed by $Swift$, with the UVOT camera. We study the region of microlensing parameter space to which $Swift$ is sensitive, finding that while for this event $Swift$ could not measure the microlens parallax with respect to ground-based observations, it can be important for other events. Specifically, for detecting nearby brown dwarfs and free-floating planets in high magnification events.