An Overview of Compact Star Populations and Some of Its Open Problems

Abstract

The study of compact object populations has come a long way since the determination of the mass of the Hulse–Taylor pulsar, and we now count on more than 150 known Galactic neutron stars and black hole masses, as well as another 180 objects from binary mergers detected from gravitational-waves by the Ligo–Virgo–KAGRA Collaboration. With a growing understanding of the variety of systems that host these objects, their formation, evolution and frequency, we are now in a position to evaluate the statistical nature of these populations, their properties, parameter correlations and long-standing problems, such as the maximum mass of neutron stars and the black hole lower mass gap, to a reasonable level of statistical significance. Here, we give an overview of the evolution and current state of the field and point to some of its standing issues. We focus on Galactic black holes, and offer an updated catalog of 35 black hole masses and orbital parameters, as well as a standardized procedure for dealing with uncertainties.

Mass Distribution and Maximum Mass of Neutron Stars: Effects of Orbital Inclination Angle

“…The discrepancy between the two approaches we treat in this work revealed that it is necessary to be careful when making assumptions over i, since different hypotheses over this quantity can lead to drastically different pulsar masses, consequently influencing the m max determination. An ideal approach would involve implementing the "accuracydependent" model while accounting for the distribution of i for each particular system, constrained from observational data, as performed for the case of black hole masses in [62], where each object was analyzed individually and it was checked that the different Keplerian parameter values employed in each mass estimate did not rely on inconsistent assumptions. A similar treatment for NS masses remains a subject for future work.…”