We present the discovery of a Neptune-mass planet orbiting a 0.8 +- 0.3 M_Sun star in the Galactic bulge. The planet manifested itself during the microlensing event MOA 2011-BLG-028/OGLE-2011-BLG-0203 as a low-mass companion to the lens star. The analysis of the light curve provides the measurement of the mass ratio: (1.2 +- 0.2) x 10^-4, which indicates the mass of the planet to be 12-60 Earth masses. The lensing system is located at 7.3 +- 0.7 kpc away from the Earth near the direction to Baade's Window. The projected separation of the planet, at the time of the microlensing event, was 3.1-5.2 AU. Although the "microlens parallax" effect is not detected in the light curve of this event, preventing the actual mass measurement, the uncertainties of mass and distance estimation are narrowed by the measurement of the source star proper motion on the OGLE-III images spanning eight years, and by the low amount of blended light seen, proving that the host star cannot be too bright and massive. We also discuss the inclusion of undetected parallax and orbital motion effects into the models, and their influence onto the final physical parameters estimates.
@article{arxiv.1512.03422,
title = {MOA 2011-BLG-028Lb: a Neptune-mass Microlensing Planet in the Galactic Bulge},
author = {J. Skowron and A. Udalski and R. Poleski and S. Kozłowski and M. K. Szymański and Ł. Wyrzykowski and K. Ulaczyk and P. Pietrukowicz and G. Pietrzyński and I. Soszyński and F. Abe and D. P. Bennett and A. Bhattacharya and I. A. Bond and M. Freeman and A. Fukui and Y. Hirao and Y. Itow and N. Koshimoto and C. H. Ling and K. Masuda and Y. Matsubara and Y. Muraki and M. Nagakane and K. Ohnishi and N. Rattenbury and To. Saito and D. J. Sullivan and T. Sumi and D. Suzuki and P. J. Tristram and A. Yonehara and M. Dominik and U. G. Jørgensen and V. Bozza and K. Harpsøe and M. Hundertmark and J. Skottfelt},
journal= {arXiv preprint arXiv:1512.03422},
year = {2016}
}