Publication archive

Publication archive

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.
Published: 20 January 2008
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.
Published: 18 January 2008
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.
Published: 15 January 2008
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.
Published: 12 January 2008
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.
Published: 10 January 2008
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'.
Published: 08 January 2008
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.
Published: 29 December 2007
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.
Published: 21 December 2007
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 By and Bz 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 Bz remained negative. Strong magnetic waves are also generated in the magnetosheath-cusp region.
Published: 19 December 2007
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.
Published: 18 December 2007
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
Published: 15 December 2007
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.
Published: 15 December 2007
The concept of an electrical current encircling the Earth at high altitudes was first proposed in 1917 to explain the depression of the horizontal component of the Earth's magnetic field during geomagnetic storms. In situ measurements of the extent and composition of this current were made some 50 years later and an image was obtained in 2001. Ring currents of a different nature were observed at Jupiter and their presence inferred at Saturn. Here we report images of the ring current at Saturn, together with a day-night pressure asymmetry and tilt of the planet's plasma sheet, based on measurements using the magnetospheric imaging instrument (MIMI) on board Cassini. The ring current can be highly variable with strong longitudinal asymmetries that corotate nearly rigidly with the planet. This contrasts with the Earth's ring current, where there is no rotational modulation and initial asymmetries are organized by local time effects.
Published: 13 December 2007
In the outer regions of Saturn's main rings, strong tidal forces balance gravitational accretion processes. Thus, unusual phenomena may be expected there. The Cassini spacecraft has recently revealed the strange "flying saucer" shape of two small satellites, Pan and Atlas, located in this region, showing prominent equatorial ridges. The accretion of ring particles onto the equatorial surfaces of already-formed bodies embedded in the rings may explain the formation of the ridges. This ridge formation process is in good agreement with detailed Cassini images showing differences between rough polar and smooth equatorial terrains. We propose that Pan and Atlas ridges are kilometers-thick "ring-particle piles" formed after the satellites themselves and after the flattening of the rings but before the complete depletion of ring material from their surroundings.
Published: 07 December 2007
Cassini images of Saturn's small inner satellites (radii of less than 100 kilometers) have yielded their sizes, shapes, and in some cases, topographies and mean densities. This information and numerical N-body simulations of accretionary growth have provided clues to their internal structures and origins. The innermost ring-region satellites have likely grown to the maximum sizes possible by accreting material around a dense core about one-third to one-half the present size of the moon. The other small satellites outside the ring region either may be close to monolithic collisional shards, modified to varying degrees by accretion, or may have grown by accretion without the aid of a core. We derived viscosity values of 87 and 20 square centimeters per second, respectively, for the ring material surrounding ring-embedded Pan and Daphnis. These moons almost certainly opened their respective gaps and then grew to their present size early on, when the local ring environment was thicker than it is today.
Published: 07 December 2007
The occurrence of lightning in a planetary atmosphere enables chemical processes to take place that would not occur under standard temperatures and pressures. Although much evidence has been reported for lightning on Venus, some searches have been negative and the existence of lightning has remained controversial. A definitive detection would be the confirmation of electromagnetic, whistler-mode waves propagating from the atmosphere to the ionosphere. Here we report observations of Venus' ionosphere that reveal strong, circularly polarized, electromagnetic waves with frequencies near 100 Hz. The waves appear as bursts of radiation lasting 0.25 to 0.5 s, and have the expected properties of whistler-mode signals generated by lightning discharges in Venus' clouds.
Published: 29 November 2007
The atmosphere and ionosphere of Venus have been studied in the past by spacecraft with remote sensing or in situ techniques. These early missions, however, have left us with questions about, for example, the atmospheric structure in the transition region from the upper troposphere to the lower mesosphere (50-90 km) and the remarkably variable structure of the ionosphere. Observations become increasingly difficult within and below the global cloud deck (<50 km altitude), where strong absorption greatly limits the available investigative spectrum to a few infrared windows and the radio range. Here we report radio-sounding results from the first Venus Express Radio Science (VeRa) occultation season. We determine the fine structure in temperatures at upper cloud-deck altitudes, detect a distinct day-night temperature difference in the southern middle atmosphere, and track day-to-day changes in Venus' ionosphere.
Published: 29 November 2007
Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.
Published: 29 November 2007
25-Oct-2020 19:52 UT

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