  X-ray binaries are known to launch powerful accretion disk winds that can
have significant impact on the binary systems and their surroundings. To
quantify the impact and determine the launching mechanisms of these outflows,
we need to measure the wind plasma number density, an important ingredient in
the theoretical disk wind models. While X-ray spectroscopy is a crucial tool to
understanding the wind properties, such as their velocity and ionization, in
nearly all cases, we lack the signal-to-noise to constrain the plasma number
density, weakening the constraints on outflow location and mass outflow rate.
We present a new approach to determine this number density in the X-ray binary
Hercules X-1 by measuring the speed of the wind ionization response to
time-variable illuminating continuum. Hercules X-1 is powered by a highly
magnetized neutron star, pulsating with a period of 1.24 s. We show that the
wind number density in Hercules X-1 is sufficiently high to respond to these
pulsations by modeling the ionization response with the time-dependent
photoionization model TPHO. We then perform a pulse-resolved analysis of the
best-quality XMM-Newton observation of Hercules X-1 and directly detect the
wind response, confirming that the wind density is at least $10^{12}$
cm$^{-3}$. Finally, we simulate XRISM observations of Hercules X-1 and show
that they will allow us to accurately measure the number density at different
locations within the outflow. With XRISM we will rule out $\sim3$ orders of
magnitude in density parameter space, constraining the wind mass outflow rate,
energetics, and its launching mechanism.
