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Register a new userOVII and OVIII line emission in the diffuse soft X-ray background: heliospheric and galactic contributions
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D. Koutroumpa
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We study the 0.57 keV (O VII triplet) and 0.65 keV (O VIII) diffuse emission generated by charge transfer collisions between solar wind (SW) oxygen ions and interstellar H and He neutral atoms in the inner Heliosphere. These lines which dominate the 0.3-1.0 keV energy interval are also produced by hot gas in the galactic halo (GH) and possibly the Local Interstellar Bubble (LB). We developed a time-dependent model of the SW Charge-Exchange (SWCX) X-ray emission, based on the localization of the SW Parker spiral at each instant. We include input SW conditions affecting three selected fields, as well as shadowing targets observed with XMM-Newton, Chandra and Suzaku and calculate X-ray emission fot O VII and O VIII lines. We determine SWCX contamination and residual emission to attribute to the galactic soft X-ray background. We obtain ground level intensities and/or simulated lightcurves for each target and compare to X-ray data. The local 3/4 keV emission (O VII and O VIII) detected in front of shadowing clouds is found to be entirely explained by the CX heliospheric emission. No emission from the LB is needed at these energies. Using the model predictions we subtract the heliospheric contribution to the measured emission and derive the halo contribution. We also correct for an error in the preliminary analysis of the Hubble Deep Field North (HDFN).
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The ubiquitous diffuse soft (1/4 keV) X-ray background was one of the earliest discoveries of X-ray astronomy. At least some of the emission may arise from charge exchange between solar wind ions and neutral atoms in the heliosphere, but no detailed models have been fit to the available data. Here we report on a new model for charge exchange in the solar wind, which when combined with a diffuse hot plasma component filling the Local Cavity provides a good fit to the only available high-resolution soft X-ray and extreme ultraviolet (EUV) spectra using plausible parameters for the solar wind. The implied hot plasma component is in pressure equilibrium with the local cloud that surrounds the solar system, creating for the first time a self-consistent picture of the local interstellar medium.
Most of cosmic baryons predicted by the big-bang nucleosynthesis has evaded the direct detection. Recent numerical simulations indicate that approximately 30 to 50 percent of the total baryons in the present universe is supposed to take a form of warm/hot intergalactic medium (WHIM) whose X-ray continuum emission is very weak. To identify those missing baryons, we consider in detail the detectability of WHIM directly through emission lines of OVII (561, 568, 574, 665eV) and OVIII (653eV). For this purpose, we create mock spectra of the emission lines of WHIM using a light-cone output of the cosmological hydrodynamic simulations. Since the predicted fluxes are generally below the current detection limit, the unambiguous detection requires a dedicated X-ray satellite mission that we also discuss in detail. We find that our proposed mission is sensitive to the WHIM with gas temperature $T=10^{6-7}$K and overdensity $delta=10-100$ up to a redshift of 0.3 without being significantly contaminated by the cosmic X-ray background and the Galactic emissions. Thus such a mission provides a unique and important tool to identify a large fraction of otherwise elusive baryons in the universe.
We constrain the origin of Fermi Bubbles using 2D hydrodynamical simulations of both star formation driven and black hole accretion driven wind models. We compare our results with recent observations of OVIII to OVII line ratio within and near Fermi Bubbles. Our results suggest that independent of the driving mechanisms, a low luminosity ($mathcal{L} sim 0.7-1times 10^{41}$ erg s$^{-1}$) energy injection best reproduces the observed line ratio for which the shock temperature is $approx 3times 10^6$ K. Assuming the Galactic halo temperature to be $2times 10^6$K, we estimate the shock velocity to be $sim 300$ km s$^{-1}$ for a weak shock. The corresponding estimated age of the Fermi bubbles is $sim 15-25$ Myr. Such an event can be produced either by a star formation rate of $sim 0.5$ M$_odot$ yr$^{-1}$ at the Galactic centre or a very low luminosity jet/accretion wind arising from the central black hole. Our analysis rules out any activity that generates an average mechanical luminosity $gtrsim 10^{41}$ ergps as a possible origin of the Fermi Bubbles.
We present measurements of the Galactic halos X-ray emission for 110 XMM-Newton sight lines, selected to minimize contamination from solar wind charge exchange emission. We detect emission from few million degree gas on ~4/5 of our sight lines. The temperature is fairly uniform (median = 2.22e6 K, interquartile range = 0.63e6 K), while the emission measure and intrinsic 0.5--2.0 keV surface brightness vary by over an order of magnitude (~(0.4-7)e-3 cm^-6 pc and ~(0.5-7)e-12 erg cm^-2 s^-1 deg^-2, respectively, with median detections of 1.9e-3 cm^-6 pc and 1.5e-12 erg cm^-2 s^-1 deg^-2, respectively). The high-latitude sky contains a patchy distribution of few million degree gas. This gas exhibits a general increase in emission measure toward the inner Galaxy in the southern Galactic hemisphere. However, there is no tendency for our observed emission measures to decrease with increasing Galactic latitude, contrary to what is expected for a disk-like halo morphology. The measured temperatures, brightnesses, and spatial distributions of the gas can be used to place constraints on models for the dominant heating sources of the halo. We provide some discussion of such heating sources, but defer comparisons between the observations and detailed models to a later paper.
This paper reports the analysis procedure and results of simultaneous spectral fits of the Suzaku archive data for Sagittarius (Sgr) A East and the nearby Galactic center X-ray emission (GCXE). The results are that the mixed-morphology supernova remnant Sgr A East has a recombining plasma (RP) with Cr and Mn He$alpha$ lines, and a power-law component (PL) with an Fe I K$alpha$ line. The nearby GCXE has a $sim$1.5-times larger surface brightness than the mean GCXE far from Sgr A East, although the spectral shape is almost identical. Based on these results, we interpret that the origins of the RP and the PL with the Fe I K$alpha$ line are past big flares of Sgr A$^*$.
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