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| Extended Hard X-Ray Emission from the Vela Pulsar Wind Nebula |
| The nebula powered by the Vela pulsar is one of the best examples of an evolved pulsar wind nebula, allowing access to the particle injection history and the interaction with the supernova ejecta. We report on the INTEGRAL discovery of extended emission above 18 keV from the Vela nebula. The northern side has no known counterparts and it appears larger and more significant than the southern one, which is in turn partially coincident with the cocoon, the soft X-ray, and TeV filament toward the center of the remnant. We also present the spectrum of the Vela nebula in the 18-400 keV energy range as measured by IBIS/ISGRI and SPI on board the INTEGRAL satellite. The apparent discrepancy between IBIS/ISGRI, SPI, and previous measurements is understood in terms of the point-spread function, supporting the hypothesis of a nebula more diffuse than previously thought. A break at ~25 keV is found in the spectrum within 6' from the pulsar after including the Suzaku XIS data. Interpreted as a cooling break, this points out that the inner nebula is composed of electrons injected in the last ~2000 years. Broadband modeling also implies a magnetic field higher than 10 mG in this region. Finally, we discuss the nature of the northern emission, which might be due to fresh particles injected after the passage of the reverse shock. |
| Publication date: 21 Nov 2011 |
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| A reservoir of ionized gas in the Galactic halo to sustain star formation in the Milky Way |
| Without a source of new gas, our Galaxy would exhaust its supply of gas through the formation of stars. Ionized gas clouds observed at high velocity may be a reservoir of such gas, but their distances are key for placing them in the galactic halo and unraveling their role. We have used the Hubble Space Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the foreground of galactic stars. We show that iHVCs with 90 < |vLSR| <~ 170 kilometers per second (where vLSR is the velocity in the local standard of rest frame) are within one galactic radius of the Sun and have enough mass to maintain star formation, whereas iHVCs with |vLSR| >~ 170 kilometers per second are at larger distances. These may be the next wave of infalling material. |
| Publication date: 18 Nov 2011 |
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| Electron Temperature Gradient Scale at Collisionless Shocks |
| Shock waves are ubiquitous in space and astrophysics. They transform directed flow energy into thermal energy and accelerate energetic particles. The energy repartition is a multiscale process related to the spatial and temporal structure of the electromagnetic fields within the shock layer. While large scale features of ion heating are known, the electron heating and smaller scale fields remain poorly understood. We determine for the first time the scale of the electron temperature gradient via electron distributions measured in situ by the Cluster spacecraft. Half of the electron heating coincides with a narrow layer several electron inertial lengths (c/omega_pe) thick. Consequently, the nonlinear steepening is limited by wave dispersion. The dc electric field must also vary over these small scales, strongly influencing the efficiency of shocks as cosmic ray accelerators. |
| Publication date: 18 Nov 2011 |
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| The hidden mass and large spatial extent of a post-starburst galaxy outflow |
| Outflowing winds of multiphase plasma have been proposed to regulate the buildup of galaxies, but key aspects of these outflows have not been probed with observations. By using ultraviolet absorption spectroscopy, we show that 'warm-hot' plasma at 105.5 Kelvin contains 10 to 150 times more mass than the cold gas in a post-starburst galaxy wind. This wind extends to distances > 68 kiloparsecs, and at least some portion of it will escape. Moreover, the kinematical correlation of the cold and warm-hot phases indicates that the warm-hot plasma is related to the interaction of the cold matter with a hotter (unseen) phase at >>106 Kelvin. Such multiphase winds can remove substantial masses and alter the evolution of post-starburst galaxies. |
| Publication date: 18 Nov 2011 |
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| The large, oxygen-rich halos of star-forming galaxies are a major reservoir of Galactic metals |
| The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained by observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we detected ubiquitous, large (150-kiloparsec) halos of ionized oxygen surrounding star-forming galaxies; we found much less ionized oxygen around galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. Our data indicate that it is a basic component of nearly all star-forming galaxies that is removed or transformed during the quenching of star formation and the transition to passive evolution. |
| Publication date: 18 Nov 2011 |
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| MarcoPolo-R CDF study - External Final Presentation |
| Summary of the study performed at ESA's Concurrent Design Facility (CDF) into the M-class candidate mission MarcoPolo-R. |
| Publication date: 08 Nov 2011 |
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| MarcoPolo-R Preliminary Mission Design |
| Presentation of the preliminary mission design for MarcoPolo-R, based on the mission study performed at ESA's Concurrent Design Facility (CDF) in October and November 2011.
This document has been superseded by the MarcoPolo-R CDF study - External Final Presentation. |
| Publication date: 07 Nov 2011 |
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| MarcoPolo-R Payload Definition Document |
Reference: SRE-PA/2011.079 This document describes the proposed model payload of MarcoPolo-R, a sample return mission to a near-Earth asteroid (NEA), which is currently under assessment as a candidate mission in ESA's Cosmic Vision 2015-2025 Plan. |
| Publication date: 04 Nov 2011 |
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| Evidence for ultra-fast outflows in radio-quiet Active Galactic Nuclei. II. Detailed photoionization modeling of Fe K-shell absorption lines |
X-ray absorption line spectroscopy has recently shown evidence for previously unknown Ultra-fast Outflows (UFOs) in radio-quiet active galactic nuclei (AGNs). These have been detected essentially through blueshifted Fe XXV/XXVI K-shell transitions. In the previous paper of this series we defined UFOs as those highly ionized absorbers with an outflow velocity higher than 10,000 km/s and assessed the statistical significance of the associated blueshifted absorption lines in a large sample of 42 local radio-quiet AGNs observed with XMM-Newton. The present paper is an extension of that work. First, we report a detailed curve of growth analysis of the main Fe XXV/XXVI transitions in photoionized plasmas. Then, we estimate an average spectral energy distribution for the sample sources and directly model the Fe K absorbers in the XMM-Newton spectra with the detailed Xstar photoionization code. We confirm that the frequency of sources in the radio-quiet sample showing UFOs is >35 per cent and that the majority of the Fe K absorbers are indeed associated with UFOs. The outflow velocity distribution spans from ~10,000 km/s (~0.03c) up to ~100,000 km/s (~0.3c), with a peak and mean value of ~42,000 km/s (~0.14c). The ionization parameter is very high and in the range log xi~3-6 erg s-1 cm, with a mean value of log xi~4.2 erg s-1 cm. The associated column densities are also large, in the range NH~1022-1024 cm-2, with a mean value of NH~1023 cm-2. We discuss and estimate how selection effects, such as those related to the limited instrumental sensitivity at energies above 7 keV, may hamper the detection of even higher velocities and higher ionization absorbers.
- The remainder of the abstract is truncated - |
| Publication date: 03 Nov 2011 |
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| Subsurface water and clay mineral formation during the early history of Mars |
| Clay minerals, recently discovered to be widespread in Mars's Noachian terrains, indicate long-duration interaction between water and rock over 3.7 billion years ago. Analysis of how they formed should indicate what environmental conditions prevailed on early Mars. If clays formed near the surface by weathering, as is common on Earth, their presence would indicate past surface conditions warmer and wetter than at present. However, available data instead indicate substantial Martian clay formation by hydrothermal groundwater circulation and a Noachian rock record dominated by evidence of subsurface waters. Cold, arid conditions with only transient surface water may have characterized Mars's surface for over 4 billion years, since the early-Noachian period, and the longest-duration aqueous, potentially habitable environments may have been in the subsurface. |
| Publication date: 03 Nov 2011 |
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| A layer of ozone detected in the nightside upper atmosphere of Venus |
Made available online 25 August 2011 To date, ozone has only been identified in the atmospheres of Earth and Mars. This study reports the first detection of ozone in the atmosphere of Venus by the SPICAV ultraviolet instrument onboard the Venus Express spacecraft. Venusian ozone is characterized by a vertically confined and horizontally variable layer residing in the thermosphere at a mean altitude of 100 km, with local concentrations of the order of 107-108 molecules cm-3. The observed ozone concentrations are consistent with values expected for a chlorine-catalyzed destruction scheme, indicating that the key chemical reactions operating in Earth's upper stratosphere may also operate on Venus. |
| Publication date: 01 Nov 2011 |
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