Limits on hierarchical black hole mergers from the most negative χeff systems

Maya Fishbach, Chase Kimball, Vicky Kalogera

Submitted on 6 July 2022


It has been proposed that some black holes (BHs) in binary black hole (BBH) systems are born from ``hierarchical mergers" (HM); i.e. earlier mergers of smaller BHs. These HM products have spin magnitudes χ0.7, and, if they are dynamically assembled into BBH systems, their spin orientations will be sometimes anti-aligned with the binary orbital angular momentum. In fact, as Baibhav et al. (2020) showed, 16% of BBH systems that include HM products will have an effective inspiral spin parameter, χeff<0.3. Nevertheless, the LIGO-Virgo-Kagra (LVK) gravitational-wave (GW) detectors have yet to observe a BBH system with χeff0.2, leading to upper limits on the fraction of HM products in the population. We fit the astrophysical mass and spin distribution of BBH systems and measure the fraction of BBH systems with χeff<0.3, which implies an upper limit on the HM fraction. We find that fewer than 26% of systems in the underlying BBH population include HM products (90\%. credibility). Even among BBH systems with primary masses m1=60M, the HM fraction is less than 69\%, which may constrain the location of the pair-instability mass gap. With 300 GW events (to be expected in the LVK's next observing run), if we fail to observe a BBH with χeff<0.3, we can conclude that the HM fraction is smaller than 2.52.2+9.1%.


Comment: 10 pages, 4 figures

Subjects: Astrophysics - High Energy Astrophysical Phenomena; General Relativity and Quantum Cosmology