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<SUB>E</SUB> = 491.41<SUB>−84.94</SUB><SUP>+128.31</SUP> days for the best solution and t<SUB>E</SUB> = 453.74<SUB>−105.74</SUB><SUP>+178.69</SUP> days for the second best. Assuming Galaxy priors, this translates to the most probable lens masses of M<SUB>L</SUB> = 2.65<SUB>−1.48</SUB><SUP>+5.09</SUP> M<SUB>⊙</SUB> and M<SUB>L</SUB> = 1.71<SUB>−1.06</SUB><SUP>+3.78</SUP> M<SUB>⊙</SUB>, 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 <A href=http://cdsarc.u-strasbg.fr/">cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/662/A59">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/662/A59</A>"