It has been known for decades that the observed number of baryons in the local Universe falls about 30-40 per cent short
of the total number of baryons predicted
by Big Bang nucleosynthesis, as ...inferred
from density fluctuations of the cosmic microwave background and seen during the first 2-3 billion years of the Universe in the so-called 'Lyman α forest'
(a dense series of intervening H I Lyman α absorption lines in the optical spectra of background quasars). A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas-hydrogen-is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios
. Indeed, despite large observational efforts, only a few marginal claims of detection have been made so far
. Here we report observations of two absorbers of highly ionized oxygen (O VII) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar's intrinsic outflow or the host galaxy's interstellar medium implausible. The O VII systems lie in regions characterized by large (four times larger than average
) galaxy overdensities and their number (down to the sensitivity threshold of our data) agrees well with numerical simulation predictions for the long-sought warm-hot intergalactic medium. We conclude that the missing baryons have been found.
We present the redshift lower limit of z > or =, slanted 0.6035 for the very high energy (VHE; E > or =, slanted 100 GeV) emitting blazar PKS 1424+240 (PG 1424+240). This limit is inferred from Ly ...beta and Ly gamma absorption observed in the far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph. No VHE-detected blazar has shown solid spectroscopic evidence of being more distant. At this distance, VHE observations by VERITAS are shown to sample historically large gamma-ray opacity values at 500 GeV, extending beyond tau = 4 for low-level models of the extragalactic background light (EBL) and beyond tau = 5 for high levels. The majority of the z = 0.6035 absorption-corrected VHE spectrum appears to exhibit a lower flux than an extrapolation of the contemporaneous Large Area Telescope power-law fit beyond 100 GeV. However, the highest energy VERITAS point is the only point showing agreement with this extrapolation, possibly implying the overestimation of the gamma-ray opacity or the onset of an unexpected VHE spectral feature. A curved log parabola is favored when fitting the full range of gamma-ray data (0.5-500 GeV). While fitting the absorption-corrected VHE data alone results in a harder differential power law than that from the full range, the indices derived using three EBL models are consistent with the physically motivated limit set by Fermi acceleration processes.
As a bright gamma-ray source, 3C 66A is of great interest to the high-energy astrophysics community, having a potential for placing cosmological constraints on models for the extragalactic background ...light (EBL) and the processes which contribute to this photon field. No firm spectroscopic redshift measurement has been possible for this blazar due to a lack of intrinsic emission and absorption features in optical spectra. We present new far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 3C 66A covering the wavelength range 1132-1800 Angstrom. The data show a smooth continuum with intergalactic medium absorption features which can be used to place a firm lower limit on the blazar redshift of z > or =, slanted 0.3347. An upper limit is set by statistically treating the non-detection of additional absorbers beyond z = 0.3347, indicating a redshift of less than 0.41 at 99% confidence and ruling out z > or =, slanted 0.444 at 99.9% confidence. We conclude by showing how the redshift limits derived from the COS spectra remove the potential for this gamma-ray emitting blazar to place an upper limit on the flux of the EBL using high energy data from a flare in 2009 October.
We present the first results from our pilot 500 ks Chandra Low Energy Transmission Grating Large Program observation of the soft X-ray brightest source in the z > or = 0.4 sky, the blazar 1ES ...1553+113, aimed to secure the first uncontroversial detections of the missing baryons in the X-rays. We identify a total of 11 possible absorption lines, with single-line statistical significances between 2.2sigma and 4.1sigma. The true statistical significances of these three X-ray absorption systems, after properly accounting for the number of redshift trials, are 5.8sigma, 3.9sigma, and 3.8sigma, respectively.
A variety of advanced ceramic components were fabricated using the fused deposition of ceramics (FDC) process. In FDC, ceramic loaded polymer filaments are used to build parts in a layer-by-layer ...fashion. A process map, based on the compressive strength and modulus of the FDC feedstock, was developed to predict the feasibility of deposition with a variety of FDC filaments. Alumina structures with photonic bandgap properties were deposited for high frequency (GHz) applications. Net shape bismuth titanate components with oriented grains were fabricated by pre-alignment of small volume of seeds in green FDC parts, followed by grain growth treatment. Piezoelectric actuators with novel structures such as spiral and bellows were manufactured and studied.
A total of 717 sources from the Australia Telescope Compact Array (ATCA) catalogue of the Small Magellanic Cloud (SMC) have been classified in Paper II (of this series) by Payne et al. Here, we ...present a statistical analysis of all 16 confirmed supernova remnants (SNRs) and five new candidate remnants. Included is a detailed discussion of the latter and three other sources that have some SNR characteristics. We have also found a new microquasar candidate (ATCA J005523-721055) in addition to ATCA J004718-723947 reported in Paper II. Source diameter comparisons suggest that SNRs as a group are of similar size in radio, optical and X-ray with surface brightness values in the range of Galactic remnants. Remnant spectral indices, αdefined as Sυ∝υα, with Sυ (flux density) and υ (frequency), have a mean of −0.63 standard deviation (s.d.) = 0.43 and ROSAT X-ray hardness ratios confirm them to be soft X-ray sources compared to background objects. We could not find any meaningful correlation between SNR surface brightness and diameter; we also discuss the number-diameter relation. A Venn diagram summarizes that most SNRs emit radiation in all three of the radio, optical and X-ray domains. H ii region diameter comparisons between radio and optical sources show them to be a very diverse group that defies any simple relationship, preventing any meaningful calculation of flux density or spectral index. To better understand environments containing SNRs, we have scaled Hα images of four SMC regions and subtracted their flux from the ATCA 2.37-GHz radio image. These 2.37 GHz-Hα subtraction (or difference) images reveal some new sources with predominantly non-thermal emission, exposing SNRs confused with H ii regions.
X-raying the Intergalactic O VI Absorbers Yao, Y; Tripp, T. M; Wang, Q. D ...
Astrophysical journal/The Astrophysical journal,
06/2009, Letnik:
697, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The observed intergalactic O VI absorbers at z>0 have been regarded as a significant reservoir of the 'missing baryons.' However, to fully understand how these absorbers contribute to the baryon ...inventory, it is crucial to determine whether the systems are collisionally ionized or photoionized (or both). Using the identified intergalactic O VI absorbers as tracers, we search for the corresponding X-ray absorption lines, which are useful for finding the missing baryons and for revealing the nature of the O VI absorbers. Stacking the Chandra grating spectra along six AGN sight lines, we obtain three spectra with signal-to-noise ratios of 32, 28, and 10 per 12.5 mA spectral bin around the expected O VII K alpha wavelength. These spectra correspond to O VI absorbers with various dynamic properties. We find no detectable Ne IX, O VII, O VIII, N VII, or C VI absorption lines in the spectra, but the high counting statistics allows us to obtain firm upper limits on the corresponding ionic column densities (in particular on average at the 95% confidence level). Jointly analyzing these nondetected X-ray lines with the averaged O VI column density, we further limit the average temperature of the O VI-bearing gas to be T 105.7 K in collisional ionization equilibrium. We discuss the implications of these results for physical properties of the putative warm-hot intergalactic medium and its detection in future X-ray observations.
Traditional identification of supernova remnants (SNRs) include the use of radio spectral index, optical spectral studies (including strong S ii, N ii, O i, O ii and O iii lines) and X-ray ...co-identifications. Each of these can have significant limitations within the context of a particular SNR candidate and new identification methods are continually sought. In this paper, we explore subtraction techniques by Ye, Turtle and Kennicutt to remove thermal emission estimated from Hα flux from radio-continuum images. The remaining non-thermal emission allows the identification of SNRs embedded within these H ii regions. Subtraction images of the N 66 region in the Small Magellanic Cloud (SMC) using Hα wide-field optical CCD images from the Curtis Schmidt Telescope and the recent Australia Telescope Compact Array (ATCA)/Parkes radio-continuum (1420, 2370, 4800 and 8640 MHz) data are presented as an example. These show three SNRs (B0057 − 724, B0056 − 724 and B0056 − 725) separated from their surrounding H ii radio emission. 2.3-m dual-beam spectrograph long-slit spectra from selected regions within N 66 suggest the presence of an additional SNR with no radio or X-ray emission. Radio spectral index, S ii/Hα ratio and archived Chandra images of N 66 combine to give a more coherent picture of this region, confirming B0057 − 724 as an SNR. The N 66 nebula complex is divided into 10 components, composed separately of these SNRs and H ii regions.
Fused deposition of ceramics (FDC) is a technique in which green parts are fabricated directly from CAD designs. The feedstock for FDC is a 1.778 mm diameter filament that requires a low viscosity ...and high column strength. This study explores the powder processing science, as well as the rheological and mechanical properties required for a successful FDC feedstock material. GS44 Si3N4 powders were dispersed in RU9 binder using oleyl alcohol (OA). The viscosity of the RU9/OA/Si3N4 mixture was measured as a function of temperature, solids loading, and OA concentration. The mechanical properties of the filament feedstock were evaluated in compression to establish FDC process limits. The feedstock material shows a shear thinning behavior with OA acting mainly as a plasticizer. The viscosity of GS44‐filled RU9 decreases with temperature, and increases with solids content. At 185°C and 55 vol% loading, the viscosity was found to be in the range of 49–7 Pa·s for a corresponding shear rate of 70–1128 s−1. This was sufficiently low for FDC. Based on pressure requirements for FDC extrusion (ΔP), and maximum sustainable stress without buckling by the filament (σE), it has been found that for successful FDC of RU955, 1.1ΔP < σE.
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