Is It The Smallest Ever Observed Black Hole Or The Biggest Ever Observed Neutron Star?

Gravitational wave detectors have observed what appears to be an intermediate sized black hole (26 times the mass of the Sun) collide with a "compact object" with a mass of 2.6 (2.5 to 2.64 at a 90% confidence interval) times the mass of the Sun.

Beyond a certain cutoff mass at a given radius, compact masses like neutron stars, collapse and form black holes. But, calculating the cutoff isn't a clean and simple calculation, because you have to model how tightly protons and neutrons can be squeezed together by gravity as nuclear forces push back against being squeezed too tightly, and those are complex systems involving vast numbers of protons and neutrons that can't be modeled exactly.

The most dense, large compact objects in the universe that are not black holes are neutron stars. Neutron stars are just massive, extremely dense, ordinary stars on the continuum of ordinary star behavior. 

But black holes are qualitatively different. In classical General Relativity they are mathematical singularities. In theories of quantum gravity, black holes are "almost" singularities (from which nothing can escape) but leak slightly in a theoretically described phenomena called "Hawking Radiation" which is too slight to be observed over the noise of cosmic background radiation with current means. 

This compact object is potentially more massive than any previously observed neutron star (it has a higher minimum mass within experimental uncertainties than any previously observed neutron star), but it is lighter than the lightest known black hole (see here), subject to outliers near the boundary with large error margins in their mass estimates. 

The least massive black hole ever observed has a mass of 2.72-2.82 solar masses (in a 95% confidence range). The most massive previously observed neutron stars have masses of 2.32-3.15, 1.9-3.00, 2.15-2.70 and 2.16-2.64 solar masses (in a 95% confidence range). So, the cutoff has to be somewhere in the range of 2.32 solar masses to 2.82 solar masses. This object is squarely in the middle of that range.

The paper is attracting wide attention because the object observed falls into this mass gap where we have no data to confirm if it is a neutron star or a black hole, which this result could help us to determine, shedding light on the accuracy of our predictions about the transition mass between ordinary stars and black holes. Even more exciting, but less likely, would be the possibility that the smaller object is a "primordial black hole" (discussed below).

In theory, a neutron star would emit light, which a black hole (by definition) does not, but neutron stars are dim at the best of times and this object is so far away that it would be hard to see that light even if it was being emitted. It is 241 +41 -45 megaparsecs from Earth (more than 700 light years away). No electromagnetic counterpart to the gravitational waves have been confirmed to date. This nominally favors a black hole interpretation, but not strongly, because light from a neutron star that far away is so hard to see, even with state of the art telescopes.

Knowing whether this is a black hole or a neutron star would help pin down the exact cutoff mass between black holes and neutron stars. This would in turn tell us a lot about the properties of neutrons in the ultra high pressure environment of a neutron star As an aside, note that stars can get much more massive than 3 solar masses, but only if they are much less dense than a neutron star.

It is also possible that there are some black holes out there (called "primordial black holes") which were formed not by the collapse of a star, but by the collapse of smaller amounts of matter than necessary to form a black hole now, in the early days of the Universe shortly after the Big Bang when everything was much more densely packed, providing an alternative means of creating black holes.

If primordial black holes do exist, smaller ones would have ceased to exist eons ago, because the proportion of a black hole's mass that is emitted in Hawking Radiation is a function of its total mass. Tiny black holes should evaporate via Hawking Radiation almost immediately, while primordial black holes with masses comparable to those of asteroids or larger could still be in existence 14 billion years or so after the Big Bang.

One of the main papers describing this event is LIGO Scientific Collaboration and Virgo Collaboration, "GW190814: Gravitational Waves from the Coalescence of a 23 M Black Holewith a 2.6 M Compact Object" arXiv (June 24, 2020).

UPDATE July 9, 2020: A follow up paper by different authors favors a black hole interpretation:

Although gravitational-wave signals from exceptional low-mass compact binary coalescences, like GW170817, may carry matter signatures that differentiate the source from a binary black hole system, only one out of every eight events detected by the current Advanced LIGO and Virgo observatories are likely to have signal-to-noise ratios large enough to measure matter effects, even if they are present. Nonetheless, the systems' component masses will generally be constrained precisely. Constructing an explicit mixture model for the total rate density of merging compact objects, we develop a hierarchical Bayesian analysis to classify gravitational-wave sources according to the posterior odds that their component masses are drawn from different subpopulations. Accounting for current uncertainty in the maximum neutron star mass, and adopting different reasonable models for the total rate density, we examine two recent events from the LIGO-Virgo Collaboration's third observing run, GW190425 and GW190814. For population models with no overlap between the neutron star and black hole mass distributions, we typically find that there is a ≳70% chance that GW190425 was a binary neutron star merger rather than a neutron-star--black-hole merger. On the other hand, we find that there is a ≲6% chance that GW190814 involved a slowly spinning neutron star, regardless of our assumed population model.

So does another paper:

Is the secondary component of GW190814 the lightest black hole or the heaviest neutron star ever discovered in a double compact-object system [R. Abbott et al., ApJ Lett., 896, L44 (2020)]? This is the central question animating this letter. Covariant density functional theory provides a unique framework to investigate both the properties of finite nuclei and neutron stars, while enforcing causality at all densities. By tuning existing energy density functionals we were able to: (a) account for a 2.6 Msun neutron star, (b) satisfy the original constraint on the tidal deformability of a 1.4 Msun neutron star, and (c) reproduce ground-state properties of finite nuclei. Yet, for the class of models explored in this work, we find that the stiffening of the equation of state required to support super-massive neutron stars is inconsistent with either constraints obtained from energetic heavy-ion collisions or from the low deformability of medium-mass stars. Thus, we speculate that the maximum neutron star mass can not be significantly higher than the existing observational limit and that the 2.6 Msun compact object is likely to be the lightest black hole ever discovered.