 NSFPAR ID:
 10326079
 Date Published:
 Journal Name:
 Monthly Notices of the Royal Astronomical Society
 Volume:
 507
 Issue:
 3
 ISSN:
 00358711
 Page Range / eLocation ID:
 3362 to 3380
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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Abstract It has been proposed that some black holes (BHs) in binary black hole (BBH) systems are born from “hierarchical mergers” (HMs), i.e., earlier mergers of smaller BHs. These HM products have spin magnitudes χ ∼ 0.7, and, if they are dynamically assembled into BBH systems, their spin orientations will sometimes be antialigned with the binary orbital angular momentum. In fact, as Baibhav et al. showed, ∼16% of BBH systems that include HM products will have an effective inspiral spin parameter, χ eff < −0.3. Nevertheless, the LIGO–Virgo–KAGRA (LVK) gravitationalwave (GW) detectors have yet to observe a BBH system with χ eff ≲ −0.2, 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 χ eff < −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 m 1 = 60 M ⊙ , the HM fraction is less than 69%, which may constrain the location of the pairinstability mass gap. With 300 GW events (to be expected in the LVK’s next observing run), if we fail to observe a BBH with χ eff < −0.3, we can conclude that the HM fraction is smaller than 2.5 − 2.2 + 9.1 % .more » « less

Abstract With about one hundred mergers of binary black holes (BBHs) detected via gravitational waves by the LIGOVirgoKAGRA (LVK) Collaboration, our understanding of the darkest objects in the universe has taken unparalleled steps forward. While most of the events are expected to consist of black holes (BHs) directly formed from the collapse of massive stars, some may contain the remnants of previous BBH mergers. In the most massive globular clusters and in nuclear star clusters, successive mergers can produce second (2G) or highergeneration BHs, and even form intermediatemass BHs (IMBHs). Overall, we predict that up to ∼10%, ∼1%, or ∼0.1% of the BBH mergers have one component being a 2G, 3G, or 4G BH, respectively. Assuming that ∼500 BBH mergers will be detected in O4 by LVK, this means that ∼50, ∼5, or ∼0.5 events, respectively, will involve a 2G, 3G, or 4G BH, if most sources are produced dynamically in dense star clusters. With their distinctive signatures of higher masses and spins, such hierarchical mergers offer an unprecedented opportunity to learn about the BH populations in the densest stellar systems and to shed light on the elusive IMBHs that may form therein.more » « less

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