OGLE-2017-BLG-1186: first application of asteroseismology and Gaussian processes to microlensing

Abstract

<jats:title>Abstract</jats:title> <jats:p>We present the analysis of the event OGLE-2017-BLG-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 ∼9 d. The asteroseismic analysis also provides us source properties including the source angular size (∼27 $\mu$as) and distance (∼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 χ2 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 ∼0.045 M⊙ brown dwarf at distance ∼9.0 kpc, or a ∼0.073 M⊙ ultracool dwarf at distance ∼9.8 kpc. Combining the estimated lens properties with a Bayesian analysis using a Galactic model, we find a $\sim 35{{\ \rm per\ cent}}$ probability for the lens to be a bulge object and $\sim 65{{\ \rm per\ cent}}$ to be a background disc object.</jats:p>

Pixel level decorrelation in service of the Spitzer microlens parallax survey

“…PID 10036, 11006, 12013, 12015, 13005, 14012, PI: S. Dong; PID: 13250, PI: S. Carey; PID 14121). Following the successful 2014 pilot program, Spitzer took on a new role as a 'microlens parallax satellite' with the primary objective of measuring the Galactic distribution of exoplanets towards the bulge (Calchi Novati et al 2015a,b, 2018Shvartzvald et al 2015Shvartzvald et al , 2016Shvartzvald et al , 2017Shvartzvald et al , 2019Udalski et al 2015bUdalski et al , 2018Yee et al 2015a,b;Zhu et al 2015aZhu et al ,b, 2016Zhu et al , 2017bBozza et al 2016;Han et al 2016Han et al , 2017Han et al , 2018Poleski et al 2016;Street et al 2016;Chung et al 2017Chung et al , 2019Shin et al 2017Shin et al , 2018Albrow et al 2018;Ryu et al 2018;Wang et al 2018;Jung et al 2019;Li et al 2019;Shan et al 2019;Zang et al 2019Zang et al , 2020Gould et al 2020;Hirao et al 2020).…”

Spitzer Microlensing Parallax Reveals Two Isolated Stars in the Galactic Bulge

“…The feasibility of satellite microlens parallax measurements has been demonstrated by Spitzer microlensing programs (Dong et al 2007;Calchi Novati et al 2015a;Udalski et al 2015b;Yee et al 2015b;Zhu et al 2015). Since 2014, the Spitzer satellite has observed more than 700 microlensing events toward the Galactic bulge, yielding mass measurements of eight isolated lens objects (Zhu et al 2016;Chung et al 2017;Shin et al 2018;Li et al 2019;Shvartzvald et al 2019), including two in this work.…”

“…Such discoveries are crucial for testing theories about the origin and evolution of free-floating planets (Veras & Raymond 2012;Pfyffer et al 2015;Ma et al 2016; Barclay et al 2017;Clanton & Gaudi 2017). For more massive objects (i.e., isolated brown dwarfs), five have been discovered by microlensing: OGLE-2007-BLG-224L (Gould et al 2009, OGLE-2015-BLG-1268L (Zhu et al 2016), OGLE-2015-BLG-1482 58 (Chung et al 2017), OGLE-2017-BLG-0896 (Shvartzvald et al 2019), and OGLE-2017-BLG-1186 59 (Li et al 2019). Shvartzvald et al (2019) recently announced the discovery of an isolated, extremely low-mass brown dwarf of M∼19 M J , with proper motion in the opposite direction of disk stars, which indicates that it might be a halo brown dwarf or from a different, unknown counter-rotating population.…”

Microlensing of radially pulsating stars

“…We note that in the analysis of the first microlensing event of a pulsating source star reported by Li et al (2019), the effect of a varying ρ was not considered. In this event, although the pulsation period which was estimated at ∼ 9 days is longer than t ∼ 3.7 days, the lens was crossing the source surface (u0 < ρ ).…”

“…Microlensing surveys, such as OGLE, MOA, and KMTNet, generally have not emphasized pulsating variables as source stars of microlensing events (Assef et al 2006). However, one example of microlensing of a pulsating star was discovered recently (Li et al 2019). For this microlensing event, OGLE-2017-BLG-1186, the source star was a bright and variable red giant.…”