Detectability of stochastic gravitational wave background from weakly hyperbolic encounters

¹ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1a, 141 00 Prague, Czech Republic
e-mail: kerachian.morteza@gmail.com
² Birla Institute of Technology and Science Pilani, Rajasthan 333031, India
³ Inter-University Centre for Astronomy and Astrophysics (IUCAA), Ganeshkhind, Pune, Maharashtra 411007, India

Received: 27 November 2023
Accepted: 31 January 2024

Abstract

We compute the stochastic gravitational wave (GW) background generated by black hole–black hole (BH–BH) hyperbolic encounters with eccentricities close to one and compare them with the respective sensitivity curves of planned GW detectors. We use the Keplerian potential to model the orbits of the encounters and the quadrupole formula to compute the emitted GWs. We take into account hyperbolic encounters that take place in clusters up to redshift 5 and with BH masses spanning from 5 M_⊙ to 55 M_⊙. We assume the clusters to be virialized and study several cluster models with different mass and virial velocity, and finally obtain an accumulative result, displaying the background as an average. Using the maxima and minima of our accumulative result for each frequency, we provide analytical expressions for both optimistic and pessimistic scenarios. Our results suggest that the background from these encounters is likely to be detected by the third-generation detectors Cosmic explorer and Einstein telescope, while the tail section at lower frequencies intersects with DECIGO, making it a potential target source for both ground- and space-based future GW detectors.