Spitzer Microlens Measurement of a Massive Remnant in a Well-Separated Binary

TL;DR

This paper reports the detection of a binary system in the Galactic bulge with a primary likely being a neutron star or black hole, using microlensing parallax measurements from Spitzer and ground surveys, highlighting methods to find dark compact objects.

Contribution

It demonstrates the use of microlensing parallax with Spitzer and ground data to identify and measure a massive remnant in a wide binary system, advancing methods for detecting dark compact objects.

Findings

Primary mass > 1.35 solar masses with 80% probability

System located in the Galactic bulge at 6.1 AU separation

Future AO imaging will refine the primary mass measurement

Abstract

We report the detection and mass measurement of a binary lens OGLE-2015-BLG-1285La,b, with the more massive component having $M_1>1.35\,M_\odot$ (80% probability). A main-sequence star in this mass range is ruled out by limits on blue light, meaning that a primary in this mass range must be a neutron star or black hole. The system has a projected separation $r_\perp= 6.1\pm 0.4\,{\rm AU}$ and lies in the Galactic bulge. These measurements are based on the "microlens parallax" effect, i.e., comparing the microlensing light curve as seen from $Spitzer$, which lay at $1.25\,{\rm AU}$ projected from Earth, to the light curves from four ground-based surveys, three in the optical and one in the near infrared. Future adaptive optics imaging of the companion by 30m class telescopes will yield a much more accurate measurement of the primary mass. This discovery both opens the path and defines the challenges to detecting and characterizing black holes and neutron stars in wide binaries, with either dark or luminous companions. In particular, we discuss lessons that can be applied to future $Spitzer$ and $Kepler$ K2 microlensing parallax observations.