Most of the baryons from galaxies have been "missing" and several studies have attempted to map the circumgalactic medium (CGM) of galaxies in their quest. We report on X-ray observations made with the Chandra X-ray Observatory probing the warm-hot phase of the CGM of our Milky Way at about 10 6 K. We detect O vii and O viii absorption lines at z = 0 in extragalactic sight lines and measure accurate column densities using both Kα and Kβ lines of O vii. We then combine these measurements with the emission measure of the Galactic halo from literature to derive the density and the pathlength of the CGM. We show that the warm-hot phase of the CGM is massive, extending over a large region around the Milky Way, with a radius of over 100 kpc. The mass content of this phase is over ten billion solar masses, many times more than that in cooler gas phases and comparable to the total baryonic mass in the disk of the Galaxy. The missing mass of the Galaxy appears to be in this warm-hot gas phase.

A high spectral resolution observation of the diffuse X-ray background in the 60 -1000 eV energy range has been made using an array of thirty-six 1 mm 2 microcalorimeters flown on a sounding rocket. Detector energy resolution ranged from 5-12 eV FWHM, and a composite spectrum of 1 steradian of the background centered at l = 90°, b = +60° was obtained with a net resolution of ~ 9 eV. The target area includes bright 1/4 keV regions, but avoids Loop I and the North Polar Spur. Lines of C VI, O VII, and O VIII are clearly detected with intensities of 5.4 ± 2.3, 4.8 ± 0.8, and 1.6 ± 0.4 photons cm -2 s -1 sr -1 , respectively. The oxygen lines alone account for a majority of the diffuse background observed in the ROSAT R4 band that is not due to resolved extragalactic discrete sources. We also have a positive detection of the Fe-M line complex near 70 eV at an intensity consistent with previous upper limits that indicate substantial gas phase depletion of iron. We include a detailed description of the instrument and its detectors.

High-resolution X-ray spectroscopy with Hitomi was expected to resolve the origin of the faint unidentified » E 3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the Hitomi first-light observation of the Perseus cluster. The emission line expected for Perseus based on the XMM-Newton signal from the large cluster sample under the dark matter decay scenario is too faint to be detectable in the Hitomi data. However, the previously reported 3.5 keV flux from Perseus was anomalously high compared to the sample-based prediction. We find no unidentified line at the reported high flux level. Taking into account the XMM measurement uncertainties for this region, the inconsistency with Hitomi is at a 99% significance for a broad dark matter line and at 99.7% for a narrow line from the gas. We do not find anomalously high fluxes of the nearby faint K line or the Ar satellite line that were proposed as explanations for the earlier 3.5 keV detections. We do find a hint of a broad excess near the energies of high-n transitions of S XVI (  E 3.44 keV rest-frame)-a possible signature of charge exchange in the molecular nebula and another proposed explanation for the unidentified line. While its energy is consistent with XMM pn detections, it is unlikely to explain the MOS signal. A confirmation of this interesting feature has to wait for a more sensitive observation with a future calorimeter experiment.

We analyze two XMM-Newton observations toward the high-density, high-latitude, neutral hydrogen cloud MBM 20 and a nearby low-density region that we call the Eridanus hole. MBM 20 lies at a distance between 100 and 200 pc from the Sun, and its density is sufficient to shield about 75% of the foreground emission in the 3 4 keV energy band. The combination of the two observations makes it possible to separate the foreground component, due to the Local Bubble and, possibly, charge exchange within the solar system, from the background component, due primarily to the Galactic halo and unidentified point sources. The two observations are in good agreement with each other and with ROSAT observations of the same part of the sky; the O vii and O viii intensities are 3.89 AE 0.56 and 0.68 AE 0.24 photons cm À2 s À1 sr À1 for MBM 20, respectively, and 7.26 AE 0.34 and 1.63 AE 0.17 photons cm À2 s À1 sr À1 for the Eridanus hole. The spectra agree with a simple three-component model: one unabsorbed and one absorbed plasma component, and a power law, due to unresolved distant point sources. Assuming that the two plasma components are in thermal equilibrium, we obtain a temperature of 0.096 keV for the foreground component and 0.197 keV for the background one. Assuming the foreground component is due solely to Local Bubble emission, we obtain lower and upper limits for the plasma density of 0.0079 and 0.0095 cm À3 and limits of 16,200 and 19,500 cm À3 K for the plasma pressure, in good agreement with theoretical predictions. Similarly, assuming that the absorbed plasma component is due to Galactic halo emission, we obtain a plasma density ranging from 0.0009 to 0.0016 cm À3 and a pressure between 3.8 ; 10 3 and 6.7 ; 10 3 cm À3 K.

Both heliophysics and planetary physics seek to understand the complex nature of the solar wind's interaction with solar system obstacles like Earth's magnetosphere, the ionospheres of Venus and Mars, and comets. Studies with this objective are frequently conducted with the help of single or multipoint in situ electromagnetic field and particle observations, guided by the predictions of both local and global numerical simulations, and placed in con- text by observations from far and extreme ultraviolet (FUV, EUV), hard X-ray, and energetic neutral atom imagers (ENA). Each proposed interaction mechanism (e.g., steady or transient magnetic reconnection, local or global magnetic reconnection, ion pick-up, or the KelvinHelmholtz instability) generates diagnostic plasma density structures. The significance of each mechanism to the overall interaction (as measured in terms of atmospheric/ionospheric loss at comets, Venus, and Mars or global magnetospheric/ionospheric convection at Earth) remains to be determined but can be evaluated on the basis of how often the density signatures that it generates are observed as a function of solar wind conditions. This paper reviews efforts to image the diagnostic plasma density structures in the soft (low energy, 0.1-2.0 keV) X-rays produced when high charge state solar wind ions exchange electrons with the exospheric neutrals surrounding solar system obstacles. The introduction notes that theory, local, and global simulations predict the characteristics of plasma boundaries such the bow shock and magnetopause (including location, density gradient, and motion) and regions such as the magnetosheath (including density and width) as a function of location, solar wind conditions, and the particular mechanism operating. In situ measurements confirm the existence of time-and spatial-dependent plasma density structures like the bow shock, magnetosheath, and magnetopause/ionopause at Venus, Mars, comets, and the Earth. However, in situ measurements rarely suffice to determine the global extent of these density structures or their global variation as a function of solar wind conditions, except in the form of empirical studies based on observations from many different times and solar wind conditions. Remote sensing observations provide global information about auroral ovals (FUV and hard X-ray), the terrestrial plasmasphere (EUV), and the terrestrial ring current (ENA). ENA instruments with low energy thresholds (∼ 1 keV) have recently been used to obtain important information concerning the magnetosheaths of Venus, Mars, and the Earth. Recent technological developments make these magnetosheaths valuable potential targets for high-cadence wide-field-of-view soft X-ray imagers.Section 2 describes proposed dayside interaction mechanisms, including reconnection, the Kelvin-Helmholtz instability, and other processes in greater detail with an emphasis on the plasma density structures that they generate. It focuses upon the questions that remain as yet unanswered, such as the significanc...