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| 2XMM J083026+524133: the most X-ray luminous cluster at redshift 1 |
Aims. In the distant universe X-ray luminous clusters of galaxies are rare objects. Large area surveys are therefore needed to probe the high luminosity end of the cluster population at redshifts z>~1.
Methods. We correlated extended X-ray sources from the second XMM-Newton source catalogue (2XMM) with the SDSS in order to identify new clusters of galaxies. Distant cluster candidates in empty SDSS fields were imaged in the r- and z-bands with the Large Binocular Telescope. We extracted the X-ray spectra of the cluster candidates and fitted thermal plasma models to the data.
Results. We determined the redshift 0.99 ± 0.03 for 2XMM J083026.2+524133 from its X-ray spectrum. With a bolometric luminosity of 1.8 × 1045 erg s-1 this is the most X-ray luminous cluster at redshifts z>1. We measured a gas temperature of 8.2 ± 0.9 keV and estimate a cluster mass M500 = 5.6 × 1014 MSun. The optical imaging revealed a rich cluster of galaxies. |
| Publication date: 25 Aug 2008 |
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| Wave activity inside hot flow anomaly cavities |
| Low-frequency wave properties inside two hot flow anomalies (HFAs) at different stages of evolution are, for the first time, studied applying the k-filtering technique on multipoint measurements from the Cluster satellites. The observed wave activity in an HFA cavity in an early stage of its evolution was interpreted as the combination of inherent fluctuation in the solar wind and those of a plasma component specularly reflected at the Earth's bow shock, where the amplitude of the fluctuations had been enhanced by a plasma beam instability. The wave field of a more evolved HFA was found to be less complex but contained a periodicity in the wave number distribution with a period that is suggested to come from the geometry of the HFA cavity. |
| Publication date: 23 Aug 2008 |
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| Large-scale spring deposits on Mars? |
| We present a large-scale spring hypothesis for the formation of various enigmatic light-toned deposits (LTDs) on Mars. Layered to massive LTDs occur extensively in Valles Marineris, chaotic terrains, and several large craters, in particular, those located in Arabia Terra. Most of these deposits are not easily explained with either a single process or multiple ones, either in combination or occurring sequentially. Spring deposits can have a very wide range of internal facies and exhibit complex architectural variations. We propose the concept of large-scale spring deposits for explaining LTDs on Mars. Stable volcano-tectonic settings, such as the ones typical on Mars, are compatible with a large-scale, long-term, multistage formation of spring deposits. The large-scale spring deposit model can explain the formation of LTDs with a common process, although active in different times and locations, compatible with coeval local or regional processes and deposits, such as volcaniclastic ones. LTDs, if formed as spring deposits derived from subsurface fluids, could potentially offer favorable conditions both to life and to the fossilization of past life forms.
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| Publication date: 22 Aug 2008 |
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| Observations of aurorae by SPICAM ultraviolet spectrograph on board Mars Express: Simultaneous ASPERA-3 and MARSIS measurements |
| We present a new set of observations of Martian aurorae obtained by Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) on board Mars Express (MEX). Using nadir viewing, several auroral events have been identified on the Martian nightside, all near regions of crustal magnetic fields. For most of these events, two to three consecutive events with variable intensities and separated by a few seconds to several tens of seconds have been observed, whereas simultaneous observations with Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) have been possible. In this paper, we present the data set for these events and discuss the possible correlation between the measured UV emission by SPICAM, the measured downward and/or upward flux of electrons by ASPERA-3 and the total electron content recorded by MARSIS. Despite the limited coverage of SPICAM ultraviolet spectrograph (UVS) on the Martian nightside (essentially in regions of high crustal magnetic fields), there is however a very good correlation between the regions with the locally smallest probability to be on closed crustal magnetic field lines, as derived from Mars Global Surveyor/Electron Reflectometer (MGS/MAG-ER), and the position of an aurora event. This suggests that the crustal magnetic fields, when organized into cusp-like structure, can trigger the few aurorae identified by SPICAM UVS. It confirms also the good probability, in the cases where SPICAM UVS measured UV emissions, that the increase in the measured total electron content by MARSIS and the simultaneous measured precipitating electron flux by the ASPERA-3/Electron Spectrometer may be related to each other. |
| Publication date: 22 Aug 2008 |
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| Magnetic support of the optical emission line filaments in NGC 1275 |
| The giant elliptical galaxy NGC 1275, at the centre of the Perseus cluster, is surrounded by a well-known giant nebulosity of emission-line filaments, which are plausibly in excess of 108 years old. The filaments are dragged out from the centre of the galaxy by radio-emitting 'bubbles' rising buoyantly in the hot intracluster gas, before later falling back. They act as markers of the feedback process by which energy is transferred from the central massive black hole to the surrounding gas. The mechanism by which the filaments are stabilized against tidal shear and dissipation into the surrounding extremely hot (4x107 K) gas has been unclear. Here we report observations that resolve thread-like structures in the filaments. Some threads extend over 6 kpc, yet are only 70 pc wide. We conclude that magnetic fields in the threads, in pressure balance with the surrounding gas, stabilize the filaments, so allowing a large mass of cold gas to accumulate and delay star formation. |
| Publication date: 21 Aug 2008 |
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| Ground-based and Venus Express Coordinated Campaign |
Beyond their intrinsic interest, ground-based observations have proven their usefulness in supporting spacecraft observations of Solar System bodies. Probably the most spectacular illustration ever was provided during the descent of the Huygens Probe on Titan, when the radio astronomy segment detected the "channel A" carrier signal from Huygens and allowed the recovery of the Doppler Wind Experiment that had been compromised by the failure of the corresponding Cassini channel (Lebreton et al., 2005). Furthermore, ground-based science observations performed during or around the Huygens mission provided new, complementary information on Titan's atmosphere and surface, helping to put the Huygens observations into context (Witasse et al., 2006). Another example of a successful ground-based campaign is the Deep Impact event, when numerous Earth-based and Earth-orbiting observatories monitored comet 9P/Tempel 1 when it was hit by the impactor (Meech et al., 2005).
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| Publication date: 15 Aug 2008 |
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| Transport of transient solar wind particles in Earth's cusps |
| An important problem in space physics still not understood well is how the solar wind enters the Earth's magnetosphere. Evidence is presented that transient solar wind particles produced by solar disturbances can appear in the Earth's mid-altitude (~5 RE geocentric) cusps with densities nearly equal to those in the magnetosheath. That these are magnetosheath particles is established by showing they have the same "flattop" electron distributions as magnetosheath electrons behind the bow shock. The transient ions are moving parallel to the magnetic field (B) toward Earth and often coexist with ionospheric particles that are flowing out. The accompanying waves include electromagnetic and broadband electrostatic noise emissions and Bernstein mode waves. Phase-space distributions show a mixture of hot and cold electrons and multiple ion species including field-aligned ionospheric O+ beams. |
| Publication date: 11 Aug 2008 |
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| A magnetic null geometry reconstructed from Cluster spacecraft observations |
| This paper reports for the first time the identification of a magnetic structure around a magnetic null in a magnetic reconnection region in the magnetotail. Magnetic reconnection is one of the fundamental processes in astrophysical and solar-terrestrial plasmas. Though the concept of reconnection has been studied for many years, the process that really occurs has not been fully revealed by direct measurements. In particular, the lack of a description of three-dimensional (3-D) reconnecting magnetic field from observations makes the task more difficult. The Cluster spacecraft array provide an opportunity to reconstruct the 3-D magnetic reconnection structure based on magnetic field vectors simultaneously measured at four positions. The identification of this structure comes from a new method of analysis of in situ measurements proposed here. Applying a fitting model of 10 spherical harmonic functions and a Harris current sheet function, plus a constant field, we reconstruct a 3-D magnetic field configuration around the magnetic null in an reconnection event observed by Cluster in the geo-magnetotail. |
| Publication date: 08 Aug 2008 |
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| Statistical analysis of plasmaspheric plumes with Cluster/WHISPER observations |
| Plasmaspheric plumes have been routinely observed by the four Cluster spacecraft. This paper presents a statistical analysis of plumes observed during five years (from 1 February 2001 to 1 February 2006) based on four-point measurements of the plasmasphere (outside 4 Earth radii) as it is sampled by the spacecraft in a narrow local time sector before and after perigee. Plasmaspheric plumes can be identified from electron density profiles derived from the electron plasma frequency determined by the WHISPER wave sounder onboard Cluster. As the WHISPER instrument has a limited frequency range (2-80 kHz) only plumes with densities below 80 cm-3 can be identified in this way. Their occurrence is studied as a function of several geomagnetic indices (Kp, am and Dst). Their transverse equatorial size, magnetic local time distribution, L position and density variation are discussed. Plasmaspheric plumes are observed mostly for moderate Kp and never for small Dst. They are found mainly in the afternoon and pre-midnight MLT sectors. Comparisons are also made between the density profiles of the plumes as they are crossed on the in- and outbound legs of the orbit, before and after perigee crossing, respectively. |
| Publication date: 06 Aug 2008 |
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| Diagnosis of magnetic structures and intermittency in space-plasma turbulence using the technique of surrogate data |
| Intermittency is usually identified in turbulent flows as non-Gaussian tails of the probability density functions (PDFs) of the turbulent field derivatives. Here we investigate the role of phase coherence among the Fourier modes in creating intermittency in magnetized space plasmas using the technique of surrogate data. We apply the technique to two examples: (i) synthetic data and (ii) magnetic field fluctuations recorded in the terrestrial magnetosheath by the Cluster spacecraft. We use a set of four series of data, one observed and three surrogate, and their PDFs and moments (q<=4) as discriminating statistics. We show that the technique allows for detecting coherent structures and estimating their scales. We show furthermore that the phases, but not the amplitudes, create the non-Gaussian tails of the PDFs. We show also that the surrogate data used cannot account for asymmetries of the PDFs of the observed data. This enables us to confirm a scenario of turbulent cascade of mirror structures proposed in previous publications, by showing the existence of an approximately constant energy flux in the inertial range. |
| Publication date: 05 Aug 2008 |
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| Characteristic Parameters of Drift Vortices Coupled to Alfvén Waves in an Inhomogeneous Space Plasma |
| We present detailed measurements of ion scale vortices of drift type coupled to Alfvén waves in an inhomogeneous and collisionless space magnetoplasma. The two free parameters of a dipolar vortex, intensity and spatial radius, are measured. The vortices are driven by a strong density gradient on a boundary layer with scale size of the same order as the vortex diameter. Observations of vortices off the gradient show that symmetry-breaking conditions in a real inhomogeneous plasma can lead not only to cross-field but also to cross-boundary anomalous transport of particles and energy. |
| Publication date: 04 Aug 2008 |
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| Convective high-speed flow and field-aligned high-speed flows explored by TC-1 |
| From June 1, 2004 to October 31, 2006, a total 465 high-speed flow events are observed by the TC-1 satellite in the near-Earth region (-13 RE < x < -9 RE, |Y|<10 RE, |Z|<5 RE). Based on the angle between the flow and the magnetic field, the high-speed flow events are further divided into two types, that is, field-aligned high-speed flow (FAHF) in the plasma sheet boundary and convective bursty bulk flow (BBF) in the center plasma sheet. Among the total 465 high-speed flow events, there are 371 FAHFs, and 94 BBFs. The CHF are mainly concentrated in the plasma sheet, the intersection angle between the flow and the magnetic field is larger, the magnetic field intensity is relatively weak. The FHF are mainly distributed near the boundary layer of the plasma sheet, the intersection angle between the flow and magnetic field is smaller, and the magnetic field intensity is relatively strong. The convective BBFs have an important effect on the substorm. |
| Publication date: 02 Aug 2008 |
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| ESA's report to the 37th COSPAR meeting - LISA Pathfinder section |
| ESA's report to the 37th COSPAR meeting (13-20 July 2008) covers the missions of the Science Programme of ESA. This section contains the report on the LISA Pathfinder mission. |
| Publication date: 01 Aug 2008 |
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| Spatium 22: Solar Magnetism - Discovery and Investigation |
| The present issue of Spatium is devoted to solar magnetism. Baffled by the thought that a hot gaseous Sun should have a magnetic field, such ideas came up only very late.
It was George Ellery Hale who suggested solar magnetism in 1908. This edition of Spatium is a written version of the fascinating public lecture that was held by Professor Eugene Parker at the University of Bern on 23 January 2008, as part of the ISSI workshop on solar magnetism. |
| Publication date: 01 Aug 2008 |
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