Abstract

We present the analysis of the event OGLE-2017-B1,G-1186 from the 2017.Spitzer microlensing campaign. This is a remarkable microlensing event because its source is photometrically bright and variable, which makes it possible to perform an asteroseismic analysis using ground-based data. We find that the source star is an oscillating red giant with average time scale of similar to 9 d. The asteroseismic analysis also provides us source properties including the source angular size (similar to 27 as) and distance (similar to 11.5 kpc), which are essential for inferring the properties of the lens. When fitting the light curve, we test the feasibility of Gaussian processes (GPs) in handling the correlated noise caused by the variable source. We find that the parameters from the GP model are generally more loosely constrained than those from the traditional chi(2) minimization method. We note that this event is the first microlensing system for which asteroseismology and GPs have been used to account for the variable source. With both finite-source effect and microlens parallax measured, we find that the lens is likely a similar to 0,045 M-circle dot brown dwarf at distance similar to 9.0 kpc, or a similar to 0.073 M-circle dot ultracool dwarf at distance similar to 9.8 kpc. Combining the estimated lens properties with a Bayesian analysis using a Galactic model, we find a similar to 35 per cent probability for the lens to be a bulge object and similar to 65 per cent to be a background disc object.