Effect of lensing magnification on the apparent distribution of black
  hole mergers
  The recent detection of gravitational waves indicates that stellar-mass black
hole binaries are likely to be a key population of sources for forthcoming
observations. With future upgrades, ground-based detectors could detect merging
black hole binaries out to cosmological distances. Gravitational wave bursts
from high redshifts ($z \gtrsim 1$) can be strongly magnified by gravitational
lensing due to intervening galaxies along the line of sight. In the absence of
electromagnetic counterparts, the mergers' intrinsic mass scale and redshift
are degenerate with the unknown magnification factor $\mu$. Hence, strongly
magnified low-mass mergers from high redshifts appear as higher-mass mergers
from lower redshifts. We assess the impact of this degeneracy on the
mass-redshift distribution of observable events for generic models of binary
black hole formation from normal stellar evolution, Pop III star remnants, or a
primordial black hole population. We find that strong magnification ($\mu
\gtrsim 3$) generally creates a heavy tail of apparently massive mergers in the
event distribution from a given detector. For LIGO and its future upgrades,
this tail may dominate the population of intrinsically massive, but unlensed
mergers in binary black hole formation models involving normal stellar
evolution or primordial black holes. Modeling the statistics of lensing
magnification can help account for this magnification bias when testing
astrophysical scenarios of black hole binary formation and evolution.
