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Register a new userDetection capabilities of the Athena X-IFU for the warm-hot intergalactic medium using gamma-ray burst X-ray afterglows
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At low redshifts, the observed baryonic density falls far short of the total number of baryons predicted. Cosmological simulations suggest that these baryons reside in filamentary gas structures, known as the warm-hot intergalactic medium (WHIM). As a result of the high temperatures of these filaments, the matter is highly ionised such that it absorbs and emits far-UV and soft X-ray photons. Athena, the proposed European Space Agency X-ray observatory, aims to detect the `missing baryons in the WHIM up to redshifts of $z=1$ through absorption in active galactic nuclei and gamma-ray burst afterglow spectra, allowing for the study of the evolution of these large-scale structures of the Universe. This work simulates WHIM filaments in the spectra of GRB X-ray afterglows with Athena using the SImulation of X-ray TElescopes (SIXTE) framework. We investigate the feasibility of their detection with the X-IFU instrument, through O VII ($E=573$ eV) and O VIII ($E=674$ eV) absorption features, for a range of equivalent widths imprinted onto GRB afterglow spectra of observed starting fluxes ranging between $10^{-12}$ and $10^{-10}$ erg cm$^{-2}$ s$^{-1}$, in the 0.3-10 keV energy band. The analyses of X-IFU spectra by blind line search show that Athena will be able to detect O VII-O VIII absorption pairs with EW$_mathrm{O VII} > 0.13$ eV and EW$_mathrm{O VIII} > 0.09$ eV for afterglows with $F>2 times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$. This allows for the detection of $approx$ 45-137 O VII-O VIII absorbers during the four-year mission lifetime. The work shows that to obtain an O VII-O VIII detection of high statistical significance, the local hydrogen column density should be limited at $N_mathrm{H}<8 times 10^{20}$ cm$^{-2}$.
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The X-ray Integral Field Unit (X-IFU) on board the Advanced Telescope for High-ENergy Astrophysics (Athena) will provide spatially resolved high-resolution X-ray spectroscopy from 0.2 to 12 keV, with 5 arc second pixels over a field of view of 5 arc minute equivalent diameter and a spectral resolution of 2.5 eV up to 7 keV. In this paper, we first review the core scientific objectives of Athena, driving the main performance parameters of the X-IFU, namely the spectral resolution, the field of view, the effective area, the count rate capabilities, the instrumental background. We also illustrate the breakthrough potential of the X-IFU for some observatory science goals. Then we briefly describe the X-IFU design as defined at the time of the mission consolidation review concluded in May 2016, and report on its predicted performance. Finally, we discuss some options to improve the instrument performance while not increasing its complexity and resource demands (e.g. count rate capability, spectral resolution). The X-IFU will be provided by an international consortium led by France, The Netherlands and Italy, with further ESA member state contributions from Belgium, Finland, Germany, Poland, Spain, Switzerland and two international partners from the United States and Japan.
The Athena+ mission concept is designed to implement the Hot and Energetic Universe science theme submitted to the European Space Agency in response to the call for White Papers for the definition of the L2 and L3 missions of its science program. The Athena+ science payload consists of a large aperture high angular resolution X-ray optics and twelve meters away, two interchangeable focal plane instruments: the X-ray Integral Field Unit (X-IFU) and the Wide Field Imager (WFI). The X-IFU is a cryogenic X-ray spectrometer, based on a large array of Transition Edge Sensors (TES), offering 2.5 eV spectral resolution, with ~5 pixels, over a field of view of 5 arc minutes in diameter. In this paper, we briefly describe the Athena+ mission concept and the X-IFU performance requirements. We then present the X-IFU detector and readout electronics principles, the current design of the focal plane assembly, the cooling chain and review the global architecture design. Finally, we describe the current performance estimates, in terms of effective area, particle background rejection, count rate capability and velocity measurements. Finally, we emphasize on the latest technology developments concerning TES array fabrication, spectral resolution and readout performance achieved to show that significant progresses are being accomplished towards the demanding X-IFU requirements.
The Warm-Hot Intergalactic Medium (WHIM) arises from shock-heated gas collapsing in large-scale filaments and probably harbours a substantial fraction of the baryons in the local Universe. Absorption-line measurements in the ultraviolet (UV) and in the X-ray band currently represent the best method to study the WHIM at low redshifts. We here describe the physical properties of the WHIM and the concepts behind WHIM absorption line measurements of H I and high ions such as O VI, O VII, and O VIII in the far-ultraviolet and X-ray band. We review results of recent WHIM absorption line studies carried out with UV and X-ray satellites such as FUSE, HST, Chandra, and XMM-Newton and discuss their implications for our knowledge of the WHIM.
We present a survey of six low to moderate redshift quasars with Chandra and XMM-Newton. The primary goal is to search for the narrow X-ray absorption lines produced by highly ionized metals in the Warm-Hot Intergalactic Medium. All the X-ray spectra can be fitted by a power law with neutral hydrogen absorption method. The residuals that may caused by additional emission mechanisms or calibration uncertainties are taken account by polynomial in order to search for narrow absorption features. No real absorption line is detected at above 3-sigma level in all the spectra. We discuss the implications of the lack of absorption lines for our understanding of the baryon content of the universe and metallicity of the intergalactic medium (IGM). We find that the non-detection of X-ray absorption lines indicates that the metal abundance of the IGM should be smaller than ~0.3 solar abundance. We also discuss implications of the non-detection of any local (z ~ 0) X-ray absorption associated with the ISM, Galactic halo or local group, such as has been seen along several other lines of sight (LOS). By comparing a pair of LOSs we estimate a lower limit on the hydrogen number density for the (z ~ 0) 3C 273 absorber of n_H >= 4e-3 cm^-3.
X-ray absorption of $gamma$-ray burst (GRB) afterglows is prevalent yet poorly understood. X-ray derived neutral hydrogen column densities ($N_{rm H}$) of GRB X-ray afterglows show an increase with redshift, which might give a clue for the origin of this absorption. We use more than 350 X-ray afterglows with spectroscopic redshift ($z$) from the Swift XRT repository as well as over 100 Ly,$alpha$ absorption measurements in $z>1.6$ sources. The observed trend of the average optical depth $tau$ at 0.5 keV is consistent with both a sharp increase of host $N_{rm H}(z)$, and an absorbing diffuse intergalactic medium, along with decreasing host contribution to $tau$. We analyze a sub-sample of high-$z$ GRBs with $N_{rm H}$ derived both from the X-ray afterglow and the Ly,$alpha$ line. The increase of X-ray derived $N_{rm H}(z)$ is contrasted by no such increase in the Ly,$alpha$ derived column density. We argue that this discrepancy implies a lack of association between the X-ray and Ly,$alpha$ absorbers at high-$z$. This points towards the X-ray absorption at high $z$ being dominated by an intervening absorber, which lends credibility to an absorbing intergalactic medium contribution.
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