The Milky Way's bar structural properties from gravitational waves
  The Laser Interferometer Space Antenna (LISA) will enable Galactic
gravitational wave (GW) astronomy by individually resolving $ > 10^4$ signals
from double white dwarf (DWD) binaries throughout the Milky Way. In this work
we assess for the first time the potential of LISA data to map the Galactic
stellar bar and spiral arms, since GWs are unaffected by stellar crowding and
dust extinction unlike optical observations of the bulge region. To achieve
this goal we combine a realistic population of Galactic DWDs with a
high-resolution N-Body simulation a galaxy in good agreement with the Milky
Way. We then model GW signals from our synthetic DWD population and reconstruct
the structure of the simulated Galaxy from mock LISA observations. Our results
show that while the low signal contrast between the background disc and the
spiral arms hampers our ability to characterise the spiral structure, the
stellar bar will instead clearly appear in the GW map of the bulge. The bar
length and bar width derived from these synthetic observations are
underestimated, respectively within $1\sigma$ and at a level greater than
$2\sigma$, but the resulting axis ratio agrees to well within $1\sigma$, while
the viewing angle is recovered to within one degree. These are competitive
constraints compared to those from electromagnetic tracers, and they are
obtained with a completely independent method. We therefore foresee that the
synergistic use of GWs and electromagnetic tracers will be a powerful strategy
to map the bar and the bulge of the Milky Way.
