Context: Gravitational microlensing is sensitive to compact-object lenses in
the Milky Way, including white dwarfs, neutron stars or black holes, and could
potentially probe a wide range of stellar remnant masses. However, the mass of
the lens can be determined only in very limited cases, due to missing
information on both source and lens distances and their proper motions.
Aims: We aim at improving the mass estimates in the annual parallax
microlensing events found in the 8 years of OGLE-III observations towards the
Galactic 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-blended
sources and recompute the masses of lenses in parallax events. We also identify
new events in that sample which are likely to have dark lens; the total number
of such events is now 18.
Results: The derived distribution of masses of dark lenses is consistent with
a continuous distribution of stellar remnant masses. A mass gap between
neutron-star and black-hole masses in the range between 2 and 5 solar masses is
not favoured by our data, unless black holes receive natal-kicks above 20-80
km/s. We present 8 candidates for objects with masses within the putative mass
gap, including a spectacular multi-peak parallax event with mass of
2.4−1.3+1.9M⊙ located just at 600 pc. The absence of an
observational mass gap between neutron stars and black holes, or, conversely,
the evidence for black hole natal kicks if a mass gap is assumed, can inform
future supernova modelling efforts.Comment: 12 pages, published as Wyrzykowski&Mandel, 2020, A&A, 636, A2
