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Context: Gravitational microlensing is sensitive to compact-object lenses inthe Milky Way, including white dwarfs, neutron stars or black holes, and couldpotentially probe a wide range of stellar remnant masses. However, the mass ofthe lens can be determined only in very limited cases, due to missinginformation on both source and lens distances and their proper motions.  Aims: We aim at improving the mass estimates in the annual parallaxmicrolensing events found in the 8 years of OGLE-III observations towards theGalactic Bulge (Wyrzykowski et al. 2016) with the use of Gaia Data Release 2(DR2).  Methods: We use Gaia DR2 data on distances and proper motions of non-blendedsources and recompute the masses of lenses in parallax events. We also identifynew events in that sample which are likely to have dark lens; the total numberof such events is now 18.  Results: The derived distribution of masses of dark lenses is consistent witha continuous distribution of stellar remnant masses. A mass gap betweenneutron-star and black-hole masses in the range between 2 and 5 solar masses isnot favoured by our data, unless black holes receive natal-kicks above 20-80km/s. We present 8 candidates for objects with masses within the putative massgap, including a spectacular multi-peak parallax event with mass of$2.4^{+1.9}_{-1.3}\ M_\odot$ located just at 600 pc. The absence of anobservational mass gap between neutron stars and black holes, or, conversely,the evidence for black hole natal kicks if a mass gap is assumed, can informfuture supernova modelling efforts.

Constraining the masses of microlensing black holes and the mass gap with Gaia DR2