First and second-generation black hole and neutron star mergers in 2+2 quadruples: population statistics

Understand

Recent detections of gravitational waves from mergers of neutron stars (NSs) and black holes (BHs) in the low and high-end mass gap regimes pose a puzzle to standard stellar and binary evolution theory.

• Mass-gap mergers may originate from successive mergers in hierarchical systems such as quadruples.
• Here, we consider repeated mergers of NSs and BHs in stellar 2+2 quadruple systems, in which secular evolution can accelerate the merger of one of the inner binaries.
• Subsequently, the merger remnant may interact with the companion binary, yielding a second-generation merger.