Mass gap between neutron stars and black holes

Question

From the detection of gravitational waves in GW190814, the merging of a 2.6 solar mass compact object with a heavier object has been inferred. The lighter object is in the "mass gap" between the heaviest neutron stars and the lightest stellar mass black holes, making astrophysicists wonder what kind of an object this was.

The theoretical lower bound of the gap is probably not much higher than 2.16 solar masses (Using Gravitational-wave Observations and Quasi-universal Relations to Constrain the Maximum Mass of Neutron Stars, In between neutron stars and black holes). This would exclude a neutron star as the 2.6 solar mass object.

Regarding the upper bound of the mass gap, no black hole candidates have (indirectly) been "observed" below 5 solar masses. However theoretically, much lighter black holes could exist, e.g. primordial black holes.

My question: why this 5 solar mass upper bound of the mass gap? does the evolution of massive stars preclude the formation of a 2.6 solar mass black hole?

Answer 1

It is presently not known. There are basically two alternatives.

The first is that something in the mechanics of core collapse supernovae prevents the formation of a low-mass black hole. For example it could be that below a certain progenitor mass, the supernova explosions are always successful, blowing off the envelope and leaving a neutron star remnant. At higher masses, the explosion may be unsuccessful and a substantial fraction of the envelope is accreted (recall that these progenitors will be at least 10 solar masses) resulting in a much higher mass black hole. An example of this class of explanation can be found in Kochanek (2014), which proposes a class of "failed supernovae" with progenitor masses of $16<M/M_{\odot}<25$, that do manage to eject their envelopes in weak transient events, but leave behind their helium cores to form the lowest mass $5-8M_{\odot}$ black holes. Lower mass progenitors are then responsible for the neutron stars.

A second possibility is that it is just difficult to find black holes with masses of 2.5-5 solar masses (which is why it is important that one appears to have been found). For example, prior to GW detectors, the masses of (stellar-mass) black holes could only be found in binary systems and then, only if the dark companion was identified by its accretion activity. If low mass black holes have a continuous low accretion rate, as opposed to a more "bursty" behaviour shown by higher mass (or higher mass-ratio) X-ray binaries, there might be a strong observational selection bias against finding them in the first place and the continuous accretion luminosity masks the spectrum of the companion, making a dynamical mass measurement impossible.