Uncovering the Invisible: A Study of Gaia18ajz, a Candidate Black Hole Revealed by Microlensing

Identifying black holes is essential for comprehending the development of stars and uncovering novel principles of physics. Gravitational microlensing provides an exceptional opportunity to examine an undetectable population of black holes in the Milky Way. In particular, long-lasting events are likely to be associated with massive lenses, including black holes. We present an analysis of the Gaia18ajz microlensing event, reported by the Gaia Science Alerts system, which has exhibited a long timescale and features indicative of the annual microlensing parallax effect. Our objective is to estimate the parameters of the lens based on the best-fitting model. We utilized photometric data obtained from the Gaia satellite and terrestrial observatories to investigate a variety of microlensing models and calculate the most probable mass and distance to the lens, taking into consideration a Galactic model as a prior. Subsequently, we applied a mass-brightness relation to evaluate the likelihood that the lens is a main sequence star. We also describe DarkLensCode, an open-source routine which computes the distribution of probable lens mass, distance and luminosity employing the Galaxy priors on stellar density and velocity for microlensing events with detected microlensing parallax. We modelled Gaia18ajz event and found its two possible models with most likely Einstein timescale of t_E=316^+36_-30 days and t_E=299^+25_-22 days. Applying Galaxy priors for stellar density and motion, we calculated the most probable lens mass of M_L = 5.6^+7.5_-2.5 M_⊙ located at D_S = 1.05^+0.78_-0.60 kpc or M_L = 12.0^+14.9_-5.4 M_⊙ located at D_S = 1.18^+0.82_-0.63 kpc. Our analysis of the blended light suggests that the lens is likely a dark remnant of stellar evolution, rather than a main sequence star.