What is the "hypothesized lower mass gap" between 2.5 and 5 solar masses?
The &quot;mass gap&quot; is an observed deficit in the number of compact objects with mass between 2.5 and 5 solar masses.
The &quot;mass gap&quot; is/was not understood. Such objects may be rare because they are difficult to detect or because something about the supernova process leads to a bifurcation between the most massive neutron stars and the least massive black holes.
The Ozel et al. references in your question fills in the details. Also see https://astronomy.stackexchange.com/a/8280/2531