* Astronomy

Title: Spitzer Microlens Measurement of a Massive Remnant in a Well-Separated Binary
Author: Y. Shvartzvald, A. Udalski, A. Gould, C. Han, V. Bozza, M. Friedmann, M. Hundertmark, C. Beichman, G. Bryden, S. Calchi Novati, S. Carey, M. Fausnaugh, B. S. Gaudi, C. B. Henderson, T. Kerr, R. W. Pogge, W. Varricatt, B. Wibking, J. C. Yee, W. Zhu, R. Poleski, M. Pawlak, M. K. Szymanski, J. Skowron, P. Mróz, S. Kozlowski, L. Wyrzykowski, P. Pietrukowicz, G. Pietrzynski, I. Soszynski, K. Ulaczyk, J.-Y.Choi, H. Park, Y. K. Jung, I.-G. Shin, M. D. Albrow, B.-G. Park, S.-L. Kim, C.-U. Lee, S.-M. Cha, D.-J. Kim, Y. Lee, D. Maoz, S. Kaspi, R. A. Street, Y. Tsapras, E. Bachelet, M. Dominik, D. M. Bramich, Keith Horne, C. Snodgrass, I. A. Steele, J. Menzies, R. Figuera Jaimes, J. Wambsganss, R. Schmidt, A. Cassan, C. Ranc, S. Mao, Subo Dong, G. D'Ago, G. Scarpetta, et al. (4 additional authors not shown)

We report the detection and mass measurement of a binary lens OGLE-2015-BLG-1285La,b, with the more massive component having M1>1.35 solar masses (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±0.4AU 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 1.25AU 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.

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