Detecting the Effect of Nonthermal Sources on the Warm-hot Galactic Halo

Abstract

We report the first detection of nonthermal broadening of O vii lines in the warm-hot ≈106 K circumgalactic medium of the Milky Way. We use z = 0 absorption of O vii Kα, O vii Kβ, and O viii Kα lines in archival grating data of b > 15° quasar sightlines from Chandra and XMM-Newton. Nonthermal line broadening is evident in two-thirds of the sightlines considered, and on average is constrained at 4.6σ significance. Nonthermal line broadening dominates over thermal broadening. We extensively test whether the appearance of nonthermal line broadening could instead be because of multiple thermally broadened velocity components and robustly rule it out. Nonthermal line broadening is more evident toward sightlines at lower galactic latitude indicating the Galactic disk origin of the nonthermal sources. There is weak/no correlation between nonthermal line broadening and the angular separation of sightlines from the Galactic center, indicating that the nuclear region might not be a major source of nonthermal factors.