Neutron stars and black holes are two types of compact stellar remnants. The most massive known neutron star is about 2 times the Sun’s mass, while the least massive known black hole is about 4 times the Sun’s mass. There appears to be an absence of compact stellar remnants between 2 to 4 times the Sun’s mass. Neutron stars are unlikely to populate this mass gap since a neutron star more than twice the Sun’s mass is expected to gravitationally collapse into a black hole. However, black holes with 2 to 4 times the Sun’s mass are possible.
Low mass X-ray binaries (LMXB) are a class of binary systems that are comprised of compact stellar remnants, either a neutron star or a black hole, that accretes material from a low mass companion star. Observations of a particular LMXB known as V1408 Aquilae show that the compact stellar remnant in this binary system is quite certainly a low mass black hole. Material stripped from the companion star forms an accretion disk around the black hole. Irradiation from the black hole’s accretion disk heats up one hemisphere of the companion star, resulting in a modulation in the light curve of the LMXB as the heated hemisphere of the companion star rotates in and out of view.
Both the black hole and the companion star in V1408 Aquilae circle around each other every 9.3 hours. Models indicate that the black hole most likely has 3 times the Sun’s mass. However, the mass of the black hole is not well constrained and can be as high as 6 times the Sun’s mass, with a 90 percent probability of not exceeding this value. Nevertheless, the black hole in V1408 Aquilae is still a good candidate for a compact object that lies within the mass gap of 2 to 4 times the Sun’s mass.
Gomez, Mason & Robinson (2015), “The Case for a Low Mass Black Hole in the LMXB V1408 Aquilae (4U 1957+115)”, American Astronomical Society, AAS Meeting #221, #142.25