Probing the Warm-Hot Circumgalactic Medium with broad OVI and X-rays
  Most of the baryonic mass in the circumgalactic medium (CGM) of a spiral
galaxy is believed to be warm-hot, with temperature around $10^6$K. The narrow
OVI absorption lines probe a somewhat cooler component at $\log \rm T(K)= 5.5$,
but broad OVI absorbers have the potential to probe the hotter CGM. Here we
present 376 ks Chandra LETG observations of a carefully selected galaxy in
which the presence of broad OVI together with the non-detection of Lya was
indicative of warm-hot gas. The strongest line expected to be present at
$\approx 10^6$K is OVII $\lambda 21.602$. There is a hint of an absorption line
at the redshifted wavelength, but the line is not detected with better than
$2\sigma$ significance. A physical model, taking into account strengths of
several other lines, provides better constraints. Our best-fit absorber model
has $\log \rm T(K) =6.3\pm 0.2$ and $\log \rm N_{H}
(cm^{-2})=20.7^{+0.3}_{-0.5}$. These parameters are consistent with the
warm-hot plasma model based on UV observations; other OVI models of cooler gas
phases are ruled out at better than $99$% confidence. Thus we have suggestive,
but not conclusive evidence for the broad OVI absorber probing the warm-hot gas
from the shallow observations of this pilot program. About 800ks of XMM-Newton
observations will detect the expected absorption lines of OVII and OVIII
unequivocally. Future missions like XRISM, Arcus and Athena will revolutionize
the CGM science.
