On the formation of GW190521-like binary black hole merger systems

GW190521 is the most massive merging binary black hole (BBH) system detected so far. At least one of the component BHs was measured to lie within the pair-instability supernova (PISN) mass gap (∼50–135 M⊙), making its formation a mystery. However, the transient observed signal allows alternative posterior distributions. There was suggestion that GW190521 could be an intermediate-mass ratio inspiral, with the component masses m1 ∼ 170 M⊙ and m2 ∼ 16 M⊙, happening to straddle the PISN mass gap. Under this framework, we perform binary population synthesis to explore the formation of GW190521-like systems via isolated binary evolution. We numerically calculate the binding energy parameter for massive stars at different metallicities, and employ them in our calculation for common envelope evolution. Our results prefer that the progenitor binaries formed in metal-poor environment with $\rm Z\le 0.0016$. The predicted merger rate density within redshift z = 1.1 is ${\sim} 4\times 10^{-5}{\text {--}}5\times 10^{-2} \, \rm Gpc^{-3}yr^{-1}$. We expect that such events are potentially observable by upcoming both space and ground-based gravitational wave detectors.