GW190521 formation scenarios via relativistic accretion

The recent gravitational wave transient GW190521 has been interpreted by the LIGO-Virgo collaboration (LVC) as sourced by a binary black hole (BH) merger. According to the LVC parameter estimation, at least one of these progenitors falls into the so-called pair-instability supernova mass gap. This raises the important question of how and when these progenitors formed. In this paper, we analyse the scenario that the GW190521 original progenitors (OPs) formed at lower masses and grew to their estimated LVC parameters by relativistic accretion. We consider the cosmic plasma, or the environment where the binary system is immersed, has density gradients as well as a dependence on the Mach number of the gas. Taking the LVC parameter estimation at $z=0.82$ as the endpoint of the accretion evolution, we estimate the initial masses of the progenitors at three different redshifts $z=100, \ 50$, and $20$, using the relativistic accretion formulas computed from general relativistic hydrodynamics simulations. We found three distinct possible types of OPs: $(i)$ $10^{-4} M_{\odot} - 3 M_{\odot}$ primordial BHs, for an early universe, $z\sim 100$, origin; $(ii)$ $3 M_{\odot} - 40M_{\odot}$ stellar mass BHs; $(iii)$ $40M_{\odot} - 60M_{\odot}$ BHs, which could originate from the collapse of high mass Pop III stars. The mass spread is due to varying the density gradient and the relativistic Mach number of the cosmic plasma; the variation of the masses due to the origin at different redshifts, on the other hand, is negligible, $\sim 2\%$ ...