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Analysis of the Cosmic X-ray Background for constraining the Warm-Hot Intergalactic Medium baryon density

Presentation #116.03 in the session “LSS, Intergalactic Medium, and QSO Absorption Line Systems”.

Published onJun 18, 2021
Analysis of the Cosmic X-ray Background for constraining the Warm-Hot Intergalactic Medium baryon density

Measurements of the baryon density of the Local Universe are unable to account for half of the baryon density expected from observations of the CMB, and measurements at high redshift (omegab = 0.04). Cosmological hydrodynamical simulations suggest that the Warm-Hot Intergalactic Medium (WHIM), an intergalactic filamentary gas with 105 < T < 107, accounts for this discrepancy. Detection of the WHIM is difficult due to weak thermal emission, and insufficient neutral H for Ly-alpha absorption (most emission in UV, soft X-ray). On the other hand, resonant scattering of radiation from background point sources (which can be individually resolved) has absorption amplitude which scales linearly with WHIM column density, and so is used to find density of scattering ions in WHIM. As the O VII w resonance line is prominent, is located in the soft X-ray range and is resonantly scattered by the WHIM, with a survey (of ~2 deg2 and ~5 Ms exposure) that can suppress the point sources, we can trace the intergalactic oxygen independent of the temperature and density of the intergalactic gas. We use the CXB spectrum from the 2.15 deg2, 4.6 Ms exposure, Chandra COSMOS Legacy Survey (CCLS) field. After removing X-ray sources, we model the spectrum using a powerlaw (for unresolved sources), APEC (for the Local Hot Bubble) and a broken powerlaw (for WHIM resonant scattering) with photon spectral index Gamma ~ -3.7. Using the normalization of the broken powerlaw, we are able to constrain the O VII abundance in the WHIM to under 0.2. This is an upper limit as emission can also be due to the Solar Wind Charge Exchange, or even direct thermal emission from the WHIM.


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