2017ApJ...836..243G

Hot, million degree gas appears to pervade the Milky Way halo, containing a large fraction of the Galactic missing baryons. This circumgalactic medium (CGM) is probed effectively in X-rays, both in absorption and in emission. The CGM also appears to be anisotropic, so we have started a program to determine CGM properties along several sightlines by combining absorption and emission measurements. Here we present the emission measure close to the Mrk 509 sightline using new Suzaku and XMM-Newton observations. We also present new analysis and modeling of Chandra HETG spectra to constrain the absorption parameters. The emission measure in this sightline is high, EM = 0.0165 ± 0.0008 ± 0.0006 cm^–6 pc, five times larger than the average. The observed O VII column density N(O VII) = 2.35±0.4×10¹⁶ cm^–2, however, is close to the average. We find that the temperature of the emitting and absorbing gas is the same: logT(K) = 6.33 ± 0.01 and logT(K) = 6.33 ± 0.16 respectively. We fit the observed column density and emission measure with a β-model density profile. The central density is constrained to be between n₀ = 2.8-6.0 x 10^–4 cm^–3 and the core radius of the density profile has a lower limit of 40 kpc. This shows that the hot gas is mostly in the CGM of the galaxy, not in the Galactic disk. Our derived density profile is close to the Maller & Bullock profile for adiabatic gas in hydrostatic equilibrium with an NFW dark-matter potential well. Assuming this density profile, the minimum mass of the hot CGM is 3.2×10¹⁰ M_☉.