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Search for Dormant Black Holes in the OGLE Data

Kapusta M., Mróz P.

Acta Astronomica

2023

Vol. 73, No. 3

197--226

Light curves of ellipsoidal variables collected by the Optical Gravitational Lensing Experiment (OGLE) were analyzed, in order to search for dormant black hole candidates. After the preselection based on the amplitude of ellipsoidal modulation, each object was investigated by means of the spectral energy distribution fit, which allowed us to select objects that are in close agreement with the spectrum of a single stellar object. After this final step of the preselection process, we were left with only fourteen objects that were then investigated in detail. For each candidate, we estimated basic physical parameters such as temperature, mass, luminosity, and, in some cases, radial velocity semi-amplitude. One of the objects turned out to be a spotted star while the rest are considered black-hole binary candidates. In the end, we present an alternative explanation for the ellipsoidal modulation in the form of contact binaries, which are not only vast in number, contrary to black-hole binaries, but are also in much better agreement with the radial velocity estimates for some of the systems analyzed. Even if the presented arguments suggest a noncompact character of the companion objects, each of them should be observed spectroscopically in order to verify the compact object hypothesis.

Measuring the Mass Function of Isolated Stellar Remnants with Gravitational Microlensing I. Revisiting the OGLE-III Dark Lens Candidates

Mróz P., Wyrzykowski Ł.

Acta Astronomica

2021

Vol. 71, No. 2

89--102

Gravitational microlensing may detect dark stellar remnants - black holes or neutron stars - even if they are isolated. However, it is challenging to estimate masses of isolated dark stellar remnants using solely photometric data for microlensing events. A recent analysis of OGLE-III long-timescale microlensing events exhibiting the annual parallax effects claimed that a number of bright events were due to "mass-gap" objects (with masses intermediate between those of neutron stars and black holes). Here, we present a detailed description of the updated and corrected method that can be used to estimate masses of dark stellar remnants detected in microlensing events given the light curve data and the proper motion of the source. We use this updated method, in combination with new proper motions from Gaia EDR3, to revise masses of dark remnant candidates previously found in the OGLE-III data. We demonstrate that masses of "mass-gap" and black hole events identified in the previous work are overestimated and, hence, these objects are most likely main-sequence stars, white dwarfs, or neutron stars.