ESA Science & Technology - Publication Archive
V5116 Sgr (Nova Sgr 2005 No. 2), discovered on 2005 July 4, was observed with XMM-Newton in 2007 March, 20 months after the optical outburst. The X-ray spectrum shows that the nova had evolved to a pure supersoft X-ray source, with no significant emission at energies above 1 keV. The X-ray light curve shows abrupt decreases and increases of the flux by a factor ~8. It is consistent with a periodicity of 2.97 hr, the orbital period suggested by Dobrotka and coworkers, although the observation lasted just a little more than a whole period. We estimate the distance to V5116 Sgr to be 11±3 kpc. A simple blackbody model does not fit correctly the EPIC spectra, with Chi²>4. In contrast, ONe-rich white dwarf atmosphere models provide a good fit, with NH=(1.3±0.1)x1021 cm-2, T=(6.1±0.1)x105 K, and L=(3.9±0.8)x1037 (D/10 kpc)² erg s-1 (during the high-flux periods). This is consistent with residual hydrogen burning in the white dwarf envelope. The white dwarf atmosphere temperature is the same both in the low- and the high-flux periods, ruling out an intrinsic variation of the X-ray source as the origin of the flux changes. We speculate that the X-ray light curve may result from a partial coverage by an asymmetric accretion disk in a high-inclination system.
Published: 10 March 2008
Saturn's moon Rhea had been considered massive enough to retain a thin, externally generated atmosphere capable of locally affecting Saturn's magnetosphere. The Cassini spacecraft's in situ observations reveal that energetic electrons are depleted in the moon's vicinity. The absence of a substantial exosphere implies that Rhea's magnetospheric interaction region, rather than being exclusively induced by sputtered gas and its products, likely contains solid material that can absorb magnetospheric particles. Combined observations from several instruments suggest that this material is in the form of grains and boulders up to several decimetres in size and orbits Rhea as an equatorial debris disk. Within this disk may reside denser, discrete rings or arcs of material.
Published: 07 March 2008
LISA is perhaps the most ambitious space mission envisaged for the coming decades. A recent assessment of astrophysics research proposed for NASA's Beyond Einstein programme by America's National Research Council (NRC) gave the "extraordinarily original and technically bold" project its highest scientific ranking. Under joint development by NASA and the European Space Agency (ESA), LISA's goal is to detect gravitational waves - fluctuations in the fabric of space and time - by measuring the relative motions of three spacecraft with great precision. Although that assessment carried out by the NRC saw LISA as the "least scientifically risky" of the proposals for future flagship missions, there is no getting away from the fact that no one has ever tried anything remotely similar before.
Published: 05 March 2008
Inspired by the success of the Hubble Space Telescope, NASA, ESA and the Canadian Space Agency have collaborated since 1996 on the design and construction of a scientifically worthy successor. Due to be launched from Kourou in 2013 on an Ariane-5 rocket, the James Webb Space Telescope is expected to have as profound and far-reaching an impact on astrophysics as did its famous predecessor.
Published: 01 March 2008
Observations taken as part of the INTEGRAL Galactic bulge monitoring program (see ATel #1385) on 2008, February 23 13:32-17:13 (UT) showed bursting activity from the X-ray transient, the Rapid Burster (MXB 1730-335). During one of the 1800-sec pointings when the source was in the field-of-view of JEM-X a series of 6 X-ray bursts were observed, once every 250-350 sec, with durations of 30-60 sec and peak fluxes of about 0.6-0.9 Crab (3-10 keV).
Published: 26 February 2008
Contents of presentation:
- Cosmic Vision Cycle 1
- Selected Missions
- Assessment Studies
- National Activities Parallel to Cosmic Vision studies
- Technology Developments
Published: 26 February 2008
Magnetic fluctuations in the solar wind are distributed according to Kolmogorov's power law f -5/3 below the ion cyclotron frequency fci. Above this frequency, the observed steeper power law is usually interpreted in two different ways, as a dissipative range of the solar wind turbulence, or another turbulent cascade, the nature of which is still an open question. Using the Cluster magnetic data we show that after the spectral break the intermittency increases toward higher frequencies, indicating the presence of nonlinear interactions inherent to a new inertial range and not to the dissipative range. At the same time the level of compressible fluctuations rises. We show that the energy transfer rate and intermittency are sensitive to the level of compressibility of the magnetic fluctuations within the small-scale inertial range. We conjecture that the time needed to establish this inertial range is shorter than the eddy-turnover time, and is related to dispersive effects. A simple phenomenological model, based on the compressible Hall MHD, predicts the magnetic spectrum ~k-7/3 + 2 alpha, which depends on the degree of plasma compression alpha.
Published: 20 February 2008
Published: 19 February 2008
We present analysis of a bright haze observed inside Valles Marineris, which formed in mid northern spring. The data were collected by the High Resolution Stereo Camera (HRSC) and the imaging spectrometer, Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA), aboard Mars Express. This study provides a case example of the power of simultaneous multiple emission angle and hyperspectral imaging for study of aerosols and clouds in the Martian atmosphere. The haze appeared thinner after three days and disappeared in nine days. It was limited to a 2-km layer at the bottom of the canyon. The color was redder than the underlying surface. The analysis of the OMEGA spectra indicates that this haze was composed of dust particles. The dust layer appeared brighter with the HRSC stereo channels than the nadir channel due to longer scattering paths. We have estimated the optical depth of the haze by fitting both HRSC and OMEGA data with radiative transfer calculations. The retrieval of the optical depth is very sensitive to the aerosol scattering model used and the reflectance of the surface. Applying an aerosol scattering model derived from sky surveys at a constant elevation by the Imager for Mars Pathfinder, the optical depth of the haze is estimated from HRSC data to be within 1.7 to 2.3 at the wavelength of 0.675 micron. The wavelength dependence is obtained from OMEGA spectrum. It increases to 2.2-2.6 at lambda = 1.35 micron and moderately decreases to 1.2-1.8 at lambda = 2.4 micron.
Published: 09 February 2008
One of the spectacular discoveries of the Cassini spacecraft was the plume of water vapour and icy particles (dust) originating near the south pole of Saturn's moon Enceladus. The data imply considerably smaller velocities for the grains than for the vapour, which has been difficult to understand. The gas and dust are too dilute in the plume to interact, so the difference must arise below the surface. Here we report a model for grain condensation and growth in channels of variable width. We show that repeated wall collisions of grains, with re-acceleration by the gas, induce an effective friction, offering a natural explanation for the reduced grain velocity. We derive particle speed and size distributions that reproduce the observed and inferred properties of the dust plume. The gas seems to form near the triple point of water; gas densities corresponding to sublimation from ice at temperatures less than 260 K are generally too low to support the measured particle fluxes. This in turn suggests liquid water below Enceladus' south pole.
Published: 07 February 2008
Published: 07 February 2008
We present observations of ionospheric convection in the Northern Hemisphere made by the SuperDARN radar network during a 3 h period on 3 December 2001. The interplanetary magnetic field (IMF) during the time of observations is predominately northward with the By component changing from positive to slightly negative. During this period Cluster is skimming the southern high latitude dusk magnetopause and reveals that reconnection is going on quasi-continuously with the reconnection site being most of the time tailward of the southern cusp and always near the satellite location (Retinò, et al., 2005). Detailed analysis of the three dimensional distribution function indicates that Cluster samples magnetosheath lines connected with geomagnetic field lines tailward of the cusps in both hemispheres (Bavassano Cattaneo et al., 2006). The evolution of the ionospheric convection measured by SuperDARN, together with IMAGE FUV observations of aurorae and DMSP particle precipitation data, confirms Cluster observations and shows that simultaneous reconnection poleward of both the northern and southern cusps occurs at a variable rate on the dusk part of the magnetosphere when the IMF clock angle is small.
Published: 06 February 2008
We present a multi-instrument study of a substorm bursty bulk flow (BBF) and auroral streamer. During a substorm on 25 August 2003, which was one of a series of substorms that occurred between 00:00 and 05:00 UT, the Cluster spacecraft encountered a BBF event travelling Earthwards and duskwards with a velocity of ~500 kms-1 some nine minutes after the onset of the substorm. Coincident with this event the IMAGE spacecraft detected an auroral streamer in the substorm auroral bulge in the Southern Hemisphere near the footpoints of the Cluster spacecraft. Using FluxGate Magnetometer (FGM) data from the four Cluster spacecraft, we determine the field-aligned currents in the BBF, using the curlometer technique, to have been ~5 mA km-2. When projected into the ionosphere, these currents give ionospheric field-aligned currents of ~18 A km-2, which is comparable with previously observed ionospheric field-aligned currents associated with BBFs and auroral streamers. The observations of the BBF are consistent with the plasma "bubble" model of Chen and Wolf (1993). Furthermore, we show that the observations of the BBF are consistent with the creation of the BBF by the reconnection of open field lines Earthward of a substorm associated near-Earth neutral line.
Published: 04 February 2008
The role of the centrifugal acceleration mechanism for ion outflow at high altitude above the polar cap has been investigated. Magnetometer data from the four Cluster spacecraft has been used to obtain an estimate of magnetic field gradients. This is combined with ion moment data of the convection drift and the field-aligned particle velocity. Thus all spatial terms in the expression for the centrifugal acceleration are directly obtained from observations. The temporal variation of the unit vector of the magnetic field is estimated by predicting consecutive measurement-points through the use of observed estimates of the magnetic field gradients, and subtracting this from the consecutively observed value. The calculation has been performed for observations of outflowing O+ beams in January to May for the years 2001-2003, and covers an altitude range of about 5 to 12 RE. The accumulated centrifugal acceleration during each orbit is compared with the observed parallel velocities to get an estimate of the relative role of the centrifugal acceleration. Finally the observed spatial terms (parallel and perpendicular) of the centrifugal acceleration are compared with the results obtained when the magnetic field data was taken from the Tsyganenko T89 model instead. It is found that the centrifugal acceleration mechanism is significant, and may explain a large fraction of the parallel velocities observed at high altitude above the polar cap. The magnetic field model results underestimate the centrifugal acceleration at the highest altitudes investigated and show some systematic differences as compared to the observations in the lower altitude ranges investigated. - Remainder of abstract truncated -
Published: 04 February 2008
The dynamics of the polar cap boundary and auroral oval in the nightside ionosphere are studied during late expansion and recovery of a substorm from the region between Tromsø (66.6° cgmLat) and Longyearbyen (75.2° cgmLat) on 27 February 2004 by using the coordinated EISCAT incoherent scatter radar, MIRACLE magnetometer and Cluster satellite measurements. During the late substorm expansion/early recovery phase, the polar cap boundary (PCB) made zig-zag-type motion with amplitude of 2.5° cgmLat and period of about 30 min near magnetic midnight. We suggest that the poleward motions of the PCB were produced by bursts of enhanced reconnection at the near-Earth neutral line (NENL). The subsequent equatorward motions of the PCB would then represent the recovery of the merging line towards the equilibrium state (Cowley and Lockwood, 1992). The observed bursts of enhanced westward electrojet just equatorward of the polar cap boundary during poleward expansions were produced plausibly by particles accelerated in the vicinity of the neutral line and thus lend evidence to the Cowley-Lockwood paradigm. - Remainder of abstract truncated -
Published: 04 February 2008
This issue of Spatium is devoted to the story of the ozone hole. It aims at familiarizing the reader with the crucial importance of the ozone layer for all living organisms on Earth, and to enhance his/her motivation to contribute to what we owe to future generations: a planet Earth that is capable of supporting the peaceful evolution of mankind.
Published: 01 February 2008
We report the successful identification of the type of the supernova responsible for the supernova remnant SNR 0509-675 in the Large Magellanic Cloud (LMC) using Gemini spectra of surrounding light echoes. The ability to classify outbursts associated with centuries-old remnants provides a new window into several aspects of supernova research and is likely to be successful in providing new constraints on additional LMC supernovae as well as their historical counterparts in the Milky Way Galaxy (MWG). The combined spectrum of echo light from SNR 0509-675 shows broad emission and absorption lines consistent with a supernova (SN) spectrum. We create a spectral library consisting of 28 SNe Ia and 6 SN Ib/c that are time-integrated, dust-scattered by LMC dust, and reddened by the LMC and MWG. We fit these SN templates to the observed light echo spectrum using chi² minimization as well as correlation techniques, and we find that overluminous 91T-like SNe Ia with delta m15<0.9 match the observed spectrum best.
Published: 01 February 2008
We examine the dynamics and X-ray spectrum of the young Type Ia supernova remnant 0509-67.5 in the context of the recent results obtained from the optical spectroscopy of its light echo. Our goal is to estimate the kinetic energy of the supernova explosion using Chandra and XMM-Newton observations of the supernova remnant, thus placing the birth event of 0509-67.5 in the sequence of dim to bright Type Ia supernovae. We base our analysis on a standard grid of one-dimensional delayed detonation explosion models, together with hydrodynamic and X-ray spectral calculations of the supernova remnant evolution. From the remnant dynamics and the properties of the O, Si, S, and Fe emission in its X-ray spectrum we conclude that 0509-67.5 was originated ~400 years ago by a bright, highly energetic Type Ia explosion similar to SN 1991T. Our best model has a kinetic energy of 1.4x1051 erg and synthesizes 0.97 Msun of 56Ni. These results are in excellent agreement with the age estimate and spectroscopy from the light echo. We have thus established the first connection between a Type Ia supernova and its supernova remnant based on a detailed quantitative analysis of both objects.
Published: 01 February 2008
We investigated the ambient plasma and magnetic field conditions at high latitudes, as well as the macroparameters of the magnetopause. For this purpose we used Cluster spacecraft plasma and magnetic field data when all the interspacecraft distances were less than 300 km. We analyzed 154 magnetosheath-magnetosphere transitions which allow to distinguish different boundaries between the magnetosphere and the magnetosheath. First, we found transitions similar to the low-latitude boundary layer, the plasma mantle, and cusp-associated transitions. Second, we estimated the length of these transitions. Third, we found with high statistical evidence sub-Alfvénic magnetosheath plasma flows just above the plasma mantle. These flows are supposed to stabilize magnetopause reconnection. Fourth, we carried out an analysis of the magnetopause pressure balance. We found a group of 24 transitions during which both the thermal and the magnetic magnetosheath pressure exceeded the magnetospheric pressure, providing conditions for unusual magnetopause formation. Fifth, for 52 magnetopause crossings we obtained the orientation as well as distributions of velocity, thickness, and current density of the magnetopause. It was found that the magnetopause with an attached plasma mantle moves slower, is thinner, and reaches higher current densities than the one with an adjacent low-latitude-like boundary layer. A comparison with the magnetopause at low latitudes revealed that the high-latitude magnetopause is about two times thicker.
Published: 31 January 2008
Electron velocity distributions in Earth's magnetosheath exhibit a number of nonequilibrium characteristics, including a flat-topped shape at thermal energies and anisotropies relative to the magnetic field, with Tperp. e > Tparallel e. These features are related to processes at the bow shock and within the magnetosheath. In the present investigation we focus our attention on the development of the electron temperature anisotropy in the magnetosheath, employing the simultaneous multispacecraft data obtained by the Plasma Electron and Current Experiment (PEACE) on board Cluster. Such studies remove the temporal ambiguities introduced by single-spacecraft analyses. We find that the electron velocity space distributions just behind the bow shock are nearly isotropic with a slight Tperp. e > Tparallel e anisotropy, whereas deeper into the magnetosheath the electrons exhibit a significant Tperp. e > Tparallel e anisotropy. We find observational evidence for two processes that contribute to the sheath anisotropy. There is a clear decrease of suprathermal electrons at 0° and 180° pitch angles, suggesting that this population suffers losses (e.g., into the upstream region) related to the global configuration. Additionally, an ongoing local mechanism inflates the 90° pitch angle suprathermal electron population.
Published: 31 January 2008
— 20 Items per Page