Blocking low-eccentricity EMRIs: A statistical direct-summation N-body
  study of the Schwarzschild barrier
  The capture of a compact object in a galactic nucleus by a massive black hole
(MBH), an extreme-mass ratio inspiral (EMRI), is the best way to map space and
time around it. Recent work on stellar dynamics has demonstrated that there
seems to be a complot in phase space acting on low-eccentricity captures, since
their rates decrease significantly by the presence of a blockade in the rate at
which orbital angular momenta change takes place. This so-called "Schwarzschild
barrier" is a result of the impact of relativistic precession on to the stellar
potential torques, and thus it affects the enhancement on lower-eccentricity
EMRIs that one would expect from resonant relaxation. We confirm and quantify
the existence of this barrier using a statistical sample of 2,500
direct-summation N-body simulations using both a post-Newtonian and also for
the first time in a direct-summation code a geodesic approximation for the
relativistic orbits. The existence of the barrier prevents low-eccentricity
EMRIs from approaching the central MBH, but high-eccentricity EMRIs, which have
been wrongly classified as "direct plunges" until recently, ignore the presence
of the barrier, because they are driven by two-body relaxation. Hence, since
the rates are significantly affected in the case of low-eccentricity EMRIs, we
predict that a LISA-like observatory such as eLISA will predominantly detect
high-eccentricity EMRIs.
