The Milky Way's million degree gaseous halo contains a considerable amount of mass that, depending on its structural properties, can be a significant mass component. In order to analyze the structure of the Galactic halo, we use XMM-Newton Reflection Grating Spectrometer archival data and measure O VII Kα absorption-line strengths toward 26 active galactic nuclei, LMC X-3, and two Galactic sources (4U 1820-30 and X1735-444). We assume a β-model as the underlying gas density profile and find best-fit parameters of n • = 0.46 +0.74 −0.35 cm −3 , r c = 0.35 +0.29 −0.27 kpc, and β = 0.71 +0.13 −0.14 . These parameters result in halo masses ranging between M (18 kpc) = 7.5 +22.0 −4.6 × 10 8 M ⊙ and M (200 kpc) = 3.8 +6.0 −0.5 × 10 10 M ⊙ assuming a gas metallicity of Z = 0.3 Z ⊙ , which are consistent with current theoretical and observational work. The maximum baryon fraction from our halo model of f b = 0.07 +0.03−0.01 is significantly smaller than the universal value of f b = 0.171, implying the mass contained in the Galactic halo accounts for 10% -50% of the missing baryons in the Milky Way. We also discuss our model in the context of several Milky Way observables, including ram pressure stripping in dwarf spheroidal galaxies, the observed X-ray emission measure in the 0.5 -2 keV band, the Milky Way's star formation rate, spatial and thermal properties of cooler gas (∼10 5 K) and the observed Fermi bubbles toward the Galactic center. Although the metallicity of the halo gas is a large uncertainty in our analysis, we place a lower limit on the halo gas between the Sun and the Large Magellanic Cloud (LMC). We find that Z 0.2 Z ⊙ based on the pulsar dispersion measure toward the LMC.

The Milky Way hosts a hot (≈ 2 × 10 6 K), diffuse, gaseous halo based on detections of z = 0 O VII and O VIII absorption lines in quasar spectra and emission lines in blank-sky spectra. Here we improve constraints on the structure of the hot gas halo by fitting a radial model to a much larger sample of O VII and O VIII emission line measurements from XMM-Newton/EPIC-MOS spectra compared to previous studies (≈650 sightlines). We assume a modified β-model for the halo density distribution and a constant-density Local Bubble from which we calculate emission to compare with the observations. We find an acceptable fit to the O VIII emission line observations with χ 2 red (dof) = 1.08 (644) for best-fit parameters of n o r 3β c = 1.35 ± 0.24 cm −3 kpc 3β and β = 0.50 ± 0.03 for the hot gas halo and negligible Local Bubble contribution. The O VII observations yield an unacceptable χ 2 red (dof) = 4.69 (645) for similar best-fit parameters, which is likely due to temperature or density variations in the Local Bubble. The O VIII fitting results imply hot gas masses of M (<50 kpc) = 3.8 +0.3 −0.3 × 10 9 M ⊙ and M (<250 kpc) = 4.3 +0.9 −0.8 × 10 10 M ⊙ , accounting for 50% of the Milky Way's missing baryons. We also explore our results in the context of optical depth effects in the halo gas, the halo gas cooling properties, temperature and entropy gradients in the halo gas, and the gas metallicity distribution. The combination of absorption and emission line analyses implies a sub-solar gas metallicity that decreases with radius, but that also must be ≥ 0.3Z ⊙ to be consistent with the pulsar dispersion measure toward the Large Magellanic Cloud.

We summarize and reanalyze observations bearing upon missing galactic baryons, where we propose a consistent picture for halo gas in L L* galaxies. The hot X-ray emitting halos are detected to 50-70 kpc, where typically, M hot (< 50 kpc) ∼ 5 × 10 9 M , and with density n ∝ r −3/2 . When extrapolated to R 200 , the gas mass is comparable to the stellar mass, but about half of the baryons are still missing from the hot phase. If extrapolated to 1.9-3R 200 , the baryon to dark matter ratio approaches the cosmic value. Significantly flatter density profiles are unlikely for R < 50 kpc and they are disfavored but not ruled out for R > 50 kpc. For the Milky Way, the hot halo metallicity lies in the range 0.3-1 solar for R < 50 kpc. Planck measurements of the thermal Sunyaev-Zeldovich effect toward stacked luminous galaxies (primarily early-type) indicate that most of their baryons are hot, near the virial temperature, and extend beyond R 200 . This stacked SZ signal is nearly an order of magnitude larger than that inferred from the X-ray observations of individual (mostly spiral) galaxies with M * > 10 11.3 M . This difference suggests that the hot halo properties are distinct for early and late type galaxies, possibly due to different evolutionary histories. For the cooler gas detected in UV absorption line studies, we argue that there are two absorption populations: extended halos; and disks extending to ∼ 50 kpc, containing most of this gas, and with masses a few times lower than the stellar masses. Such extended disks are also seen in 21 cm HI observations and in simulations.

For increasing the life of sensor networks, each node must conserve energy as much as possible. In this paper, we propose a protocol in which energy is conserved by amortizing the energy cost of communication over multiple packets. In addition, we allow sensors to control the amount of buffered packets since storage space is limited. To achieve this, a two-radio architecture is used which allows a sensor to "wakeup" a neighbor with a busy tone and send its packets for that destination. However, this process is expensive because all neighbors must awake and listen to the primary channel to determine who is the intended destination. Therefore, triggered wakeups on the primary channel are proposed to avoid using the more costly wakeup procedure. We present a protocol for efficiently determining how large the period for these wakeups should be such that energy consumption is reduced.

The Fermi bubbles are two lobes filled with non-thermal particles that emit gamma rays, extend ≈10 kpc vertically from the Galactic center, and formed from either nuclear star formation or accretion activity on Sgr A*. Simulations predict a range of shock strengths as the bubbles expand into the surrounding hot gas halo distribution (T halo ≈ 2 × 10 6 K), but with significant uncertainties in the energetics, age, and thermal gas structure. The bubbles should contain thermal gas with temperatures between 10 6 and 10 8 K, with potential X-ray signatures. In this work, we constrain the bubbles' thermal gas structure by modeling the O VII and O VIII emission line strengths from archival XMM-Newton and Suzaku data. Our emission model includes a hot thermal volume-filled bubble component cospatial with the gamma-ray region, and a shell of compressed material. We find that a bubble/shell model with n ≈ 1×10 −3 cm −3 and with log(T ) ≈ 6.60-6.70 is consistent with the observed line intensities. In the framework of a continuous Galactic outflow, we infer a bubble expansion rate, age, and energy injection rate of 490 +230 −77 km s −1 , 4.3 +0.8 −1.4 Myr, and 2.3 +5.1 −0.9 × 10 42 erg s −1 . These estimates are consistent with the bubbles forming from a Sgr A* accretion event rather than from nuclear star formation.