Black holes formed by direct collapse: observational evidences

TL;DR

This paper reviews observational evidence for the formation of binary black holes through direct collapse, emphasizing in situ formation without energetic kicks, based on kinematic analysis of Galactic black hole binaries.

Contribution

It provides observational evidence supporting direct collapse formation of black holes in binaries, challenging models requiring energetic natal kicks.

Findings

Black holes in Cygnus X-1 and GRS 1915+105 formed in situ without kicks

Binary black holes with ~10 solar mass components may be common in the universe

Kinematic data supports direct collapse formation scenarios

Abstract

Binary black holes as the recently detected sources of gravitational waves can be formed from massive stellar binaries in the field or by dynamical interactions in clusters of high stellar density, if the black holes are the remnants of massive stars that collapsed without natal kicks that would disrupt the binary system or eject the black holes from the cluster before binary black hole formation. Here are summarized and discussed the kinematics in three dimensions of space of five Galactic black hole X-ray binaries. For Cygnus X-1 and GRS 1915+105 it is found that the black holes of ~15 and ~10 solar masses in these sources were formed in situ, without energetic kicks. These observations suggest that binary black holes with components of ~10 solar masses may have been prolifically produced in the universe.