Hot Milky Way halo seen in by Chandra HETG over eight years

The Milky Way resides in a gaseous halo made up of hot and tenuous plasma. It is important to understand the physics of this gas, because of its impact on the Galaxy and because it is an analog of halos of external galaxies, which are currently challenging to probe. In this talk we present an absorption study of the hot Milky Way halo towards radiatively efficient and feature-rich AGN NGC 4051 with 1 Ms Chandra HETG observations across two epochs, 8 years apart. In contrast to typically used featureless blazars, modeling Milky Way absorption here requires de-blending the signal from AGN absorption. We achieve this by using an agnostic Bayesian framework and modeling Milky Way and AGN absorption fully self-consistently, using all of the data available (i.e. not focusing on individual lines). Critically, we consider the photoionization by the Comic Background, which is relevant for this low density gas and crucially allows us to place first direct constraints on the density of the Milky Way hot halo. We also show that not considering this photoionization background biases abundance measurements. Our approach allows us now to use the deepest, highest S/N spectra ever taken with Chandra to map the structure of the Milky Way halo towards multiple sightlines across many visits, which is important because this instrument remains the highest resolution X-ray spectrograph in the soft X-ray band. Full potential of this method will be enabled by future high resolution X-ray missions, such as e.g. LEM or Arcus.