X-Ray spectroscopic investigation of the coronal structure of capella

The binary system Capella (G6 III + F9 III) has been observed on 1979 March 15 and on 1980 March 15–17 with the Objective Grating Spectrometer (OGS) onboard theEinstein Observatory. The spectrum measured with the 1000 l/mm grating covers the range 5–30 Å with a resolution < 1 Å. The spectra show evidence for a bimodal temperature distribution of emission measure in an optically thin plasma with one component 5 million degrees and the other one 10 million degrees. Spectral features can be identified with line emissions from O VIII, Fe XVII, Fe XVIII, Fe XXIV, and Ne X ions. Good spectral fits have been obtained assuming standard cosmic abundances. The data are interpreted in terms of emission from hot static coronal loops rather similar to the magnetic arch structures found on the Sun. It is shown that the conditions required by this model exist on Capella. Mean values of loop parameters are derived for both temperature components.     

The Rosat mission

The primary scientific objective of the ROSAT mission is to perform the first all sky survey with an imaging X-ray telescope leading to an improvement in sensitivity by several orders of magnitude compared with previous surveys. Consequently a large number of new sources (> 10⁵) will be discovered and located with an accuracy of 1 arcmin. After completion of the survey which will take about half a year the instrument will be used for detailed observations of selected targets.The X-ray telescope consists of a fourfold nested Wolter type I mirror system with 80 cm aperture and 240 cm focal length, and three focal plane detectors. In the baseline version these will be imaging proportional counters (0.1 – 2 keV) providing a field of view of 2⁰ × 2⁰.     

Spectra and time variability of GX 5-1

From a short observation of GX 5-1 with EXOSAT we have derived information on spectral and temporal behaviour in the energy range 1–20 keV. The source was found to be variable on time scales from 10 s to 1 h. Describing the spectrum one is forced to assume at least two spectral components. The best fit is reached using a spectrum composed of two blackbody functions with typical temperatures 1 keV and 2 keV, corresponding to apparent blackbody radii of 43 km and 11 km, respectively (for a distance of 10 kpc). With respect to the hot component there is evidence for variability in temperature as well as in apparent blackbody radius. No periodic variability has been found over the period range 0.25 s to 2000 s. There is no evidence for an iron emission line.     

EXOSAT medium energy observations of Cyg X-2

On July 5.–6. 1983, during the EXOSAT performance verification (PV) and calibration phase, a raster scan of Cygnus X-2 was performed. In contrast to the previously observed smooth intensity variations on timescales of hours, the source revealed a behaviour unknown until now: active periods with high energy flares recurring on time scales of 300–500 s were interrupted by quiet periods of several hours. At all intensity levels the source spectra clearly require a two component continuum (blackbody + thermal bremsstrahlung). In addition, a weak iron emission line with equivalent widths between 39 an 70 eV was detected. The source has a much harder spectrum during the flares than during quiet periods, indicating drastic temperature changes within the emission region, while the absolute iron line flux does not vary. From the spectral characteristics it becomes clear that self-comptonization of the thermal bremsstrahlung spectrum plays an important role. The time variability and spectral behaviour in this peculiar state allow Cyg X-2 to be classified as a Low Mass X-ray Binary System (LMXB) very similar to the prototype of this class, Sco X-1.     

The long term variability of 4U 1258-61 (GX304-1)

We present optical spectroscopy and photometry of 4U 1258-61 obtained over the course of six years. Evidence for the evolution of a circumstellar envelope throughout the course of the observations is presented.     

LMC X-4, AO538-66 and surrounding X-ray sources observed with EXOSAT

Repeated observations of LMC X-4 with EXOSAT were carried out in 1983/84 in order to study its 30.5 day cycle and to cover the expected outbursts of the recurrent LMC transient A0538-66. The latter source was inactive during our campaign although a variable circumstellar envelope was still present around the optical counterpart.At least ten further X-ray sources are detected in the CMA field of view around LMC X-4 including the SNR N49 which is the possible site for the March 5, 1979 -ray burster and N63A which appears to be variable in X-ray luminosity. We furthermore discuss the strongest sources that were not present in a previous EINSTEIN survey of the LMC.     

Temporal intensity fluctuations of Her X-1 around 100 sec time scales

Her X-1 has been observed with EXOSAT for one 35^d cycle between March 1 and April 5, 1984 at about 4 day intervals. During three observations, absorption dips were encountered showing fluctuations with time scales in the several hundred seconds range. The data are interpreted with a model where random superposition of absorbing blobs or inhomogeneous structures within the accretion disk, in the line of sight, cause these intensity variations. The calculations give a measure of the extent of the disk both in vertical and radial directions.     

LMC X-4: 13.5 s pulsations and an X-ray flare observed by EXOSAT

EXOSAT observed LMC X-4 on November 17/19, 1983 for one 1.4 day binary period during the high state of the 30.5 day cycle. An eclipse with sharp ingress and slow egress was detected with an eclipse angle of 27.1±1.0 dgr. In the medium energy experiment the source showed a hard power law spectrum. Outside eclipse the source was remarkably constant and only one flare was detected on November 17 at 19 UT lasting for about 1 h. The energy spectrum of the source softens considerably during that time and shows an emission line of cold iron. 13.5 sec pulsations are strongly present during the flare and have also been detected during the quiescent period and during several 1 min flares in another EXOSAT LMC X-4 observation on November 22, 1983. A pulse delay time analysis results in the determination of the pulse period (13.5019±0.0002) s and of the semimajor axis of the orbit of the X-ray star (26.0±0.6) It-sec. These results, together with other available information on LMC X-4, allowed to improve the binary parameters. The mass of the neutron star is found to be 1.34 _±0.44 ^0.48 M_o (95% confidence errors).     

Plasma transport across the heliopause

Existing heliopause models are critically rediscussed under the new aspect of possible plasma mixing between the solar wind and the ambient ionized component of the local interstellar medium (LISM). Based on current kinetic plasma theories, effective diffusion rates across the heliopause are evaluated for several models with turbulence caused by electrostatic or electromagnetic interactions that could be envisaged in this context. Some specific cases that may lead to high diffusion rates are investigated, especially in regard to their LISM magnetic field dependence.For weak fields (less than 10^–7 G), macroscopic hydrodynamic instabilities, such as of Rayleigh-Taylor or Kelvin-Helmholtz-types, can be excited. The resulting plasma mixing rates at the heliopause may amount to 20–30% of the impinging mass flow.Recently, an unconventional new approach to the problem for the case of tangential magnetic fields at the heliopause was published in which a continuous change of the plasma properties within an extended boundary layer is described by a complete set of two-fluid plasma equations including a hybrid MHD-formulation of wave-particle interaction effects. If a neutral sheet is assumed to exist within the boundary layer, the magnetic field direction is proven to be constant for a plane-parallel geometry. Considering the electric fields and currents in the layer, an interesting relationship between the field-reconnection probability and the electric conductivity can be derived, permitting a quantitative determination of either of these quantities.An actual value for the electrical conductivity is derived here on the basis of electron distribution functions given by a superposition of Maxwellians with different temperatures. Using two-stream instability theory and retaining only the most unstable modes, an exact solution for the density, velocity, and magnetic and electric fields can be obtained. The electrical conductivity is then shown to be six orders of magnitude lower than calculated by conventional formulas. Interestingly, this leads to an acceptable value of 0.1 for the reconnection coefficient.By analogy with the case of planetary magnetopauses, it is shown here for LISM magnetic fields of the order of 10^–6 G or larger that field reconnection processes may also play an important role for the plasma mixing at the heliopause. The resulting plasma mixing rate is estimated to amount to an average value of 10% of the incident mass flow. It is suggested here that the dependence of the cosmic-ray penetration into the heliosphere on the distribution of reconnecting areas at the heliopause may provide a means of deriving the strength and orientation of the LISM field.A series of observational implications for the expected plasma mixing at the heliopause is discussed in the last part of the paper. In particular, consequences are discussed for the generation of radio noise at the heliopause, for the penetration of LISM neutrals into the heliosphere, for the propagation of cosmic rays towards the inner part of the solar system and for convective electric field mergings into the heliosphere during the course of the solar cycle, depending on the solar cycle variations. With concern to a recent detection of electrostatic plasma waves by plasma receivers on Voyagers 1 and 2, we come to an interesting alternate explanation: the heliopause, rather than the heliospheric shock front, could be responsible for the generation of these waves.