ESA Science & Technology - Publication Archive

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 M_sun 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.

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.

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 T_{perp. e} > T_{parallel 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 T_{perp. e} > T_{parallel e} anisotropy, whereas deeper into the magnetosheath the electrons exhibit a significant T_{perp. e} > T_{parallel 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.

A new kind of magnetohydrodynamic waves are analyzed for a current sheet in a presence of a small normal magnetic field component varying along the sheet. As a background, two simplified models of a current sheet are considered with a uniform and nonuniform current distributions in the current sheet. On a basis of these two models, the flapping-type waves are obtained which are related to a coexistence of two gradients of the tangential and normal magnetic field components along the normal and tangential directions with respect to the current sheet. A stable situation for the current sheet is associated with a positive result of the multiplication of the two magnetic gradients, and unstable (wave growth) condition corresponds to a negative result of the product. In the stable region, the "kink"-like wave mode is interpreted as so called flapping waves observed in the Earth's magnetotail current sheet.

The atmospheres of the gas giant planets (Jupiter and Saturn) contain jets that dominate the circulation at visible levels. The power source for these jets (solar radiation, internal heat, or both) and their vertical structure below the upper cloud are major open questions in the atmospheric circulation and meteorology of giant planets. Several observations and in situ measurements found intense winds at a depth of 24 bar, and have been interpreted as supporting an internal heat source. This issue remains controversial, in part because of effects from the local meteorology. Here we report observations and modelling of two plumes in Jupiter's atmosphere that erupted at the same latitude as the strongest jet (23° N). The plumes reached a height of 30 km above the surrounding clouds, moved faster than any other feature (169 m s^-1), and left in their wake a turbulent planetary-scale disturbance containing red aerosols. On the basis of dynamical modelling, we conclude that the data are consistent only with a wind that extends well below the level where solar radiation is deposited.

Cluster multisatellite observations provide snapshots of electron distributions around the magnetic neutral line. An isotropic flat-top-type electron distribution in phase space is frequently observed around the X line, together with large ion velocities and a Hall quadrupole-like magnetic field inside the hot and tenuous plasma sheet in the magnetotail. The flat-top distributions are also associated with a finite magnetic field in the direction normal to the neutral sheet, and the cross-tail current density is sometimes very small. These results indicate that the flat-top-type distribution is mainly located near the outer boundary of the ion diffusion region in the plasma sheet outflow region, before reaching the pileup region with large normal component of the magnetic field. Simultaneously with the flat-top distributions, strong field-aligned electron beams mainly toward the X line are occasionally observed. This type of beam is mainly observed in the off-equatorial plasma sheet and also appears well inside the plasma sheet. Typical energies of the beam are 410 keV, which is comparable to the upper edge of flat-top energy. These highly accelerated electron distributions have a steep decrease in phase space density at the high-energy end, and it is found that they are not correlated with the increase of the higher-energy electrons related to suprathermal acceleration (>30 keV). This result indicates that the electron acceleration processes for the flat-top-type distributions are different from the suprathermal components, both of which are beyond the conventional MHD outflow acceleration and considered to be associated with some kinetic processes.

Context. If relativistic particle acceleration takes place in colliding-wind binaries, hard X-rays and gamma-rays are expected through inverse Compton emission, but to date these have never been unambiguously detected.
Aims. To detect this emission, observations of eta Carinae were performed with INTEGRAL, leveraging its high spatial resolution.
Methods. Deep hard X-ray images of the region of eta Car were constructed in several energy bands.
Results. The hard X-ray emission previously detected by BeppoSax around eta Car originates from at least 3 different point sources. The emission of eta Car itself can be isolated for the first time, and its spectrum unambiguously analyzed. The X-ray emission of eta Car in the 22-100 keV energy range is very hard (Gamma ~ 1±0.4 ) and its luminosity is 7x10³³erg s^-1.
Conclusions. The observed emission is in agreement with the predictions of inverse Compton models, and corresponds to about 0.1% of the energy available in the wind collision. Eta Car is expected to be detected in the GeV energy range.

Most stars form as members of large associations within dense, very cold (10 to 100 Kelvin) molecular clouds. The nearby giant molecular cloud in Orion hosts several thousand stars of ages less than a few million years, many of which are located in or around the famous Orion Nebula, a prominent gas structure illuminated and ionized by a small group of massive stars (the Trapezium). We present x-ray observations obtained with the X-ray Multi-Mirror satellite XMM-Newton, revealing that a hot plasma with a temperature of 1.7 to 2.1 million kelvin pervades the southwest extension of the nebula. The plasma flows into the adjacent interstellar medium. This x-ray outflow phenomenon must be widespread throughout our Galaxy.

Magnetic reconnection is the underlying process that releases impulsively an enormous amount of magnetic energy in solar flares, flares on strongly magnetized neutron stars and substorms in the Earth's magnetosphere. Studies of energy release during solar flares, in particular, indicate that up to 50% of the released energy is carried by accelerated 20-100 keV suprathermal electrons. How so many electrons can gain so much energy during reconnection has been a long-standing question. A recent theoretical study suggests that volume-filling contracting magnetic islands formed during reconnection can produce a large number of energetic electrons. Here we report the first evidence of the link between energetic electrons and magnetic islands during reconnection in the Earth's magnetosphere. The results indicate that energetic electron fluxes peak at sites of compressed density within islands, which imposes a new constraint on theories of electron acceleration.

In this paper we present observational evidence of a flux transfer event observed simultaneously at low-latitude by Polar and at high-latitude by Cluster. This event occurs on 21 March 2002, when both Cluster and Polar are located near local noon but with a large latitudinal separation. During the event, Cluster is moving outbound from the polar cusp to the magnetosheath and Polar is in the magnetosheath near the equatorial magnetopause. The observations show that a flux transfer event occurs between the equator and the northern cusp. Polar and Cluster observe the FTE's two open flux tubes: Polar encounters the southward moving flux tube near the equator and Cluster encounters the northward moving flux tube at high latitude. The low-latitude FTE appears to be a flux rope with helical magnetic field lines as it has a strong core field and the magnetic field component in the boundary normal direction exhibits a strong bipolar variation. Unlike the low-latitude FTE, the high-latitude FTE observed by Cluster does not exhibit the characteristic bipolar perturbation in the magnetic field. However, the plasma data clearly reveal its open flux tube configuration. It shows that the magnetic field lines have straightened inside the FTE and become more aligned to the neighboring flux tubes as it moves to the cusp. Enhanced electrostatic fluctuations have been observed within the FTE core, both at low and high latitudes. This event provides a unique opportunity to understand high-latitude FTE signatures and the nature of time-varying reconnection. It shows that existing FTE models cannot accommodate all the features in global observations, and coordinated measurements from largely spaced multiple spacecraft place important constraints which are crucial to the development and refinement of FTE models.

Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis accreting compact objects and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk (~10-50° from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10⁴¹ positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.

We present a high resolution dark matter reconstruction of the z = 0.16 Abell 901/902 supercluster from a weak lensing analysis of the STAGES HST survey. We detect the four main structures of the supercluster at high significance, resolving substructure within and between the clusters. We find that the distribution of dark matter is well traced by the cluster galaxies, with the brightest cluster galaxies marking out the strongest peaks in the dark matter distribution. We also find a significant extension of the dark matter distribution of Abell 901a in the direction of an infalling X-ray group Abell 901-alpha. We present mass, mass-to-light and massto-stellar mass ratio measurements of the structures and substructures that we detect. We find no evidence for variation of the mass-to-light and mass-to-stellar mass ratio between the different clusters. We compare our space-based lensing analysis with an earlier ground-based lensing analysis of the supercluster to demonstrate the importance of space-based imaging for future weak lensing dark matter 'observations'.

Directional discontinuities are frequently encountered in the solar wind. This study utilizes the possibility of simultaneous four-point measurements that the Cluster satellites provide for an investigation of the waves near one such interplanetary discontinuity event. In particular, the k-filtering technique has for the first time been applied for the wave characterization in terms of frequency, wave vector, wave power, and polarization at different distances from a discontinuity. The advantages of the k-filtering method are that these parameters can be transformed into the plasma frame of reference to allow comparison with theory and that it is possible to detect more than one wave mode at each frequency in the spacecraft frame of reference. The discontinuity event in this study was chosen because its wave activity had unusually high power and because it also contains signatures of large-scale magnetic reconnection, such as a reconnection exhaust. Two wave modes are found: one which has the features of a shear Alfvén wave with propagation direction ordered by the background magnetic field direction and one which behaves like a compressional Alfvén wave and is ordered by the discontinuity normal. Both wave modes become weaker farther away from the discontinuity, but the compressional Alfvén mode is more suppressed. The shear mode dominates for long wavelengths at all distances from the discontinuity. The wave field is also found to be asymmetric on the two sides of the event.

Observations of an extremely elongated electron diffusion region occurring during fast reconnection are presented. Cluster spacecraft in situ observations of an expanding reconnection exhaust reveal a broad current layer (~10 ion skin depths thick) supporting the reversal of the reconnecting magnetic field together with an intense current embedded at the center that is due to a super-Alfvénic electron outflow jet with transverse scale of ~9 electron skin depths. The electron jet extends at least 60 ion skin depths downstream from the X-line.

Cluster made an unusual magnetosheath-exterior cusp crossing during the first 2.5 hours of a coronal mass ejection (CME) that flowed past Earth for about 7 hours on 24 October 2003. During the first 2.5 hours (1525-1802 UT) the solar wind dynamic pressure remained high and stable though the CME had a discontinuity after 40 min (1605 UT), when the azimuthal flow turned dawnward up to -100 kms^-1 and IMF B_y and B_z changed from highly negative to positive up to 25 nT. The responses of the magnetosheath-cusp region during the unusual event are presented using Cluster and ground-based (EISCAT VHF radar; 69.6°N, 19.2°E) observations. The unusual Cluster crossing (compared to the usual midaltitude cusp crossing at this time of the year) occurred owing to a large compression of the magnetosphere. Cluster, which was in the southern magnetospheric lobe, suddenly found itself in the magnetosheath at the arrival of the CME at 1524:45 UT. Cluster then crossed through the compressed magnetosheath (highly compressed after the discontinuity in the CME) for about 1.5 hours (1525-1655 UT), magnetopause with strong signatures of lobe reconnection (~1655 UT), stagnant but compressed exterior cusp for about an hour (1700-1802 UT), and then entered the dayside magnetosphere. The observations also show strong signatures of magnetosphere-ionosphere coupling through a late afternoon (~17 MLT) cusp during the first 40 min (1525-1605 UT) of the event when IMF B_z remained negative. Strong magnetic waves are also generated in the magnetosheath-cusp region.

We present magnetically-conjugate Cluster/DMSP observations of subauroral ion drifts (SAID) after the onset of a weak substorm on 18 March 2002 that confirm and expand on the previous Cluster/DMSP results. The outer side of the SAID channel is aligned with the plasmapause and its outer edge marks the dispersionless boundary of the electron precipitation. The SAID's inner edge co-locates with a sharp decrease in the flux of the injected ions whose minimum energy increases with the electric potential. Downward ionospheric FACs flow within the channel, mainly near its outer side, concentrating near the density maxima. The SAID peak co-locates either with that in the density or lies on the outer wall of the trough. The overall SAID features are consistent with a short circuiting of the substorm injection front over the plasmasphere and subsequent formation of a turbulent overlap region.

Contents:

• Hubble News Update
• ST-ECF Pre-Release of NICMOS Grism Data
• Simulating Slitless Spectroscopic Images with aXeSIM
• Virtual Observatory Services at the ST-ECF
• Inside the ESA/Hubble Internship Programme

We present the results of a statistical study of Langmuir waves observed by the Cluster spacecraft in the Earth's electron foreshock. To classify the probability density distributions of the logarithms of the wave energies, a Pearson technique is used. We show that experimental distributions can be better approximated, in a statistical sense, by Beta distribution or Pearson Type IV distribution rather than by a normal distribution predicted by the stochastic growth theory. This conclusion agrees with the results of numerical simulations that were previously performed with the use of a model for Langmuir wave propagation in unstable plasma with random inhomogeneities. The main reason for deviations of empirical distributions from a normal distribution is probably related to the effective number of regions, where the waves grow, which is not sufficiently large for the central limit theorem to be applicable under typical conditions in the Earth's electron foreshock. For two of seven events such deviations may be partially attributed to the effects of thermal Langmuir waves.