Constraining the Density of the Accretion Disk Wind in Hercules X-1 Using its Ionization Response to X-ray Pulsations

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, which is 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 a vast majority of X-ray observations, it is unable to constrain the plasma number density. In my talk I will present a novel approach to determine the wind number density in the X-ray binary Hercules X-1 by measuring the speed of the wind ionization response to time-variable illuminating continuum. Her X-1 is powered by a highly magnetized neutron star, regularly pulsating with a period of 1.24 s. I will show that the wind number density in Her X-1 is sufficiently high to periodically respond to these pulsations by modeling the ionization response with the time-dependent photoionization model TPHO. Motivated by the photoionization simulations, we performed a pulse-resolved analysis of the best-quality XMM-Newton observation of Her X-1 and directly detect the wind response, confirming that the plasma density is at least 10¹² cm^-3. Finally, I will discuss how the future wind number density measurements with XRISM will allow us to determine the launching mechanism and accurately measure the energetics of the disk wind in Her X-1 and in other X-ray binaries.