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

Context. The timescale of a microlensing event scales as a square root of a lens mass. Therefore, long-lasting events are important 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 exhibited a long timescale and features that are common for 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 investigated the range of microlensing models and used them to derive the most probable mass and distance to the lens using a Galactic model as a prior. Using a known mass-brightness relation, we determined how likely it is that the lens is a main-sequence (MS) star.
Results: This event is one of the longest ever detected, with the Einstein timescale of t_E = 491.41_−84.94^+128.31 days for the best solution and t_E = 453.74_−105.74^+178.69 days for the second best. Assuming Galaxy priors, this translates to the most probable lens masses of M_L = 2.65_−1.48^+5.09 M_⊙ and M_L = 1.71_−1.06^+3.78 M_⊙, respectively. The limits on the blended light suggest that this event was most likely not caused by a MS star, but rather by a dark remnant of stellar evolution.

Full Table A.1 is only and Table A.2 is also available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/662/A59