Lens parameters for Gaia18cbf -- a long gravitational microlensing event in the Galactic plane

Abstract

Context. The size of the Einstein Radius in a microlensing event scales as a square root of the mass of the lens. Therefore, long-lasting microlensing events are the best candidates for massive lenses, including black holes. Aims. Here we present the analysis of the Gaia18cbf microlensing event reported by the Gaia Science Alerts system. It has exhibited a long timescale and features characteristic to the annual microlensing parallax effect. We deduce the parameters of the lens based on the derived best fitting model. Methods. We used photometric data collected by the Gaia satellite as well as the follow-up data gathered by the ground-based observatories. We investigate the range of microlensing models and use them to derive the most probable mass and distance to the lens using a Galactic model as a prior. Using known mass-brightness relation we determined how likely it is that the lens is a main sequence star. Results. This event is one of the longest ever detected, with the Einstein timescale of t E = 491.41 +128.31 −84.94 days for the best solution and t E = 453.74 +178.69 −105.74 days for the second-best. This translates to a lens mass of M L = 2.91 +6.02 −1.70 M and M L = 1.88 +4.40 −1.19 M respectively. The limits on the blended light suggest that this event was most likely not caused by a main sequence star, but rather by a dark remnant of stellar evolution.

Dark lenses through the dust: parallax microlensing events in the VVV

“…Wyrzykowski et al 2016;Wyrzykowski & Mandel 2020;Golovich et al 2020). For example, the parallax lensing event Gaia18cbf has been recently analysed by Kruszyńska et al (2021), who found three possible solutions using Gaia data alone, and two possible solutions with follow-up. In most studies (Wyrzykowski et al 2016;Wyrzykowski & Mandel 2020;Kruszyńska et al 2021), the modes of the posterior are sampled and analysed separately, yielding alternative solutions for lens mass and distance.…”

“…For example, the parallax lensing event Gaia18cbf has been recently analysed by Kruszyńska et al (2021), who found three possible solutions using Gaia data alone, and two possible solutions with follow-up. In most studies (Wyrzykowski et al 2016;Wyrzykowski & Mandel 2020;Kruszyńska et al 2021), the modes of the posterior are sampled and analysed separately, yielding alternative solutions for lens mass and distance. A possible disadvantage is not returning the relative probability of those solutions and only using 𝜒 2 as an indicator of weak preference towards one of the solutions.…”

“…However, the Earth-based observer revolves around the Sun causing an acceleration effect (Alcock et al 1995). These parallax effects break the symmetry of the lightcurve, leading to mild asymmetries (e.g., Mao et al 2002;Bennett et al 2002;Poindexter et al 2005;Wyrzykowski et al 2016) or even spectacular multiple peak behaviour (e.g., Smith et al 2002;Kruszyńska et al 2021).…”