Constraining the masses of microlensing black holes and the mass gap with Gaia DR2 (1904.07789v4)

Abstract: 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 $2.4^{+1.9}_{-1.3}\ M_\odot$ 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.

Constraining Microlensing Black Hole Masses with Gaia DR2: Implications for the Stellar Remnant Mass Distribution

The paper "Constraining the masses of microlensing black holes and the mass gap with Gaia DR2" by {\L}ukasz Wyrzykowski and Ilya Mandel investigates the mass distribution of astrophysical compact objects via gravitational microlensing techniques, with a special focus on addressing the existence of a potential 'mass gap' between neutron stars and black holes. The paper utilizes Gaia Data Release 2 (DR2) to enhance the precision of microlensing mass estimates obtained from OGLE-III observations targeting the Galactic Bulge. This synthesis of cutting-edge observational data significantly improves constraints on the dark lens mass distribution.

Methodology and Analysis

The methodology employed in the paper involves re-calculating the masses of lensing objects in microlensing events exhibiting annual parallax signals, capitalizing on Gaia DR2 data for non-blended sources. The research team meticulously combed through data from OGLE-III and Gaia DR2 to identify 59 long-lasting microlensing events with significant parallax features. After matching these events with Gaia DR2 records, the authors narrowed down to a set of events characterized by negligible light blending, thus reducing potential sources of astrometric errors. This selection led to the identification of 18 events with likely dark lens components, i.e., lenses composed of compact remnants such as neutron stars or black holes.

For the estimation of the compact objects' masses and distances, the authors employed a Bayesian hierarchical modeling approach to derive the probability distributions of lens masses. The models considered allowed for the mixture of a neutron star population and a black hole population, where the latter's masses trace a power-law distribution. The innovative aspect of the paper is the inclusion of Gaia DR2 parameters (such as source proper motions and distances) into microlensing models to reduce uncertainties in lens properties.

Key Findings

Upon analyzing the mass distribution from the selected sample of dark lenses, the paper finds a preference against a mass gap between neutron stars and black holes. The derived mass distribution suggests a continuous distribution, characterized by a single power-law for black holes beginning from masses just above 2 solar masses. The paper finds no statistical evidence supporting a previously theorized broad mass gap between 2 and 5 solar masses. This contradicts earlier claims based on X-ray binary observations that suggested minimal black hole masses starting around 5 solar masses.

Additionally, the research identifies several lensing events consistent with compact objects situated within this 'gap,' challenging the robustness of such a gap. The results of the Bayesian analysis support a power-law index of approximately -2.7 for the black hole mass distribution, with the lowest mass in this distribution being close to 2 solar masses.

Implications and Future Work

The absence of a significant mass gap between neutron stars and black holes as indicated in this paper has critical implications for the theories of stellar evolution and supernova mechanisms. Specifically, it supports models that propose a seamless transition from neutron stars to low-mass black holes without necessitating a discrete gap. These findings necessitate a reevaluation of the evolutionary processes and conditions that lead to the formation of stellar remnants.

Future developments in microlensing surveys, particularly with the advent of the Large Synoptic Survey Telescope (LSST), will allow more extensive data collection, affording further refinement of compact-object mass distributions. Furthermore, improving astrometric precision, possibly through future Gaia data releases or complementary ground-based high-resolution observations, will be pivotal in reducing uncertainties and confirming the nature of these lenses. The use of multi-modal data analysis integrating gravitational wave observations could offer additional clarity in distinguishing individual contributions from low-mass black holes and other stellar remnants.

Overall, this paper represents a significant step forward in leveraging contemporary datasets to elucidate the mass distribution of compact stellar remnants in the Milky Way, while also highlighting potential areas for future research development and exploration within the domain.