Double Compact Objects as Low-frequency Gravitational Wave Sources
  We study the Galactic field population of double compact objects (NS-NS,
BH-NS, BH-BH binaries) to investigate the number (if any) of these systems that
can potentially be detected with LISA at low gravitational-wave frequencies. We
calculate the Galactic numbers and physical properties of these binaries and
show their relative contribution from the disk, bulge and halo. Although the
Galaxy hosts 10^5 double compact object binaries emitting low-frequency
gravitational waves, only a handful of these objects in the disk will be
detectable with LISA, but none from the halo or bulge. This is because the bulk
of these binaries are NS-NS systems with high eccentricities and long orbital
periods (weeks/months) causing inefficient signal accumulation (small number of
signal bursts at periastron passage in 1 yr of LISA observations) rendering
them undetectable in the majority of these cases. We adopt two evolutionary
models that differ in their treatment of the common envelope phase that is a
major (and still mostly unknown) process in the formation of close double
compact objects. Depending on the adopted evolutionary model, our calculations
indicate the likely detection of about 4 NS-NS binaries and 2 BH-BH systems
(model A; likely survival of progenitors through CE) or only a couple of NS-NS
binaries (model B; suppression of the double compact object formation due to CE
mergers).
