Radio emission from coronal streamers

Different models of coronal streamers are used to calculate the radio brightness temperature at the wavelengths of observation of the Nançays Radioheliograph. Calculation are performed assuming the location of the streamer both on the disk and at the limb. Their comparison with observations show that a satisfactory agreement with a particular model can be found in the shape and in the relative enhacement of the streamer with respect to the quiet Sun, although the absolute values of the computed brightness temperatures are much higher than the observed ones.     

Detection of iron lines in the persistent emission of some X-ray burst sources observed by EXOSAT

We have observed the X-ray burst sources 4U1728-33 and 4U1813-14 with the ME detectors aboard EXOSAT, and present here results of a spectral analysis of their persistent emissions. For both sources the data can be well fitted by a double blackbody continuum and a Gaussian emission line. The two spectral components can be interpreted in terms of a blackbody radiation from the neutron star, and emission from the inner part of an accretion disc. The line feature is consistent with the 6.7 keV iron emission line expected in the presence of a hot extended object (X-ray corona) around the neutron star.     

X-ray spectrum of the Tycho SNR observed with EXOSAT

The Gas Scintillation and Medium Energy instruments onboard EXOSAT measured the continuum and line emission spectrum in the Tycho supernova remnant. For the first time the iron line at 6.53 keV is clearly resolved from the continuum, in addition to the lines from S, Ar and Ca. The comparison with a non-equilibrium ionisation model (Hamilton et al 1983) confirms the over-abundance of S and Ca, whereas the Fe abundance is found to be solar, which provides additional support that the predicted 0.1–1 M₀ Fe in type I supernovae is presently not shocked to X-ray emitting temperatures.     

EXOSAT observations of late-type stars: The application of coronal loop models

We apply solar-type coronal loop models to X-ray and UV observations of late-type stars. We derive from EXOSAT and IUE observations constraints on the temperature, pressure and size of the emitting structures.     

EXOSAT observations of bursts from 2S1636-536: Burst morphology

In two observations with the EXOSAT ME a total of 11 bursts were detected; 7 in July and 4 in May 1984. In terms of rise time and peak count rate there are only 2 kinds, those with a sharp rise and peak count rate of 500 cts/sec/detector and those with a slow rise and a peak count rate of 200 cts/sec/detector. The spectral analysis of these bursts shows a similar situation: for the sharp bright bursts the black body radius increases rapidly to > 20 km and the temperature lags. For the slow dim bursts the temperature rises ahead of the black body radius expansion which is small and slow. The time t _1/2 at which half the total burst energy has been released is 3.5 seconds for bright bursts and 8 seconds for dim ones. These gross differences may be related to the predominance of helium or hydrogen in the thermonuclear flash.     

EXOSAT observations of the bright X-ray source GX9+1

We present the results of the spectral and timing analysis of an observation of GX9+1/4U1758-205 performed with the Medium Energy Experiment aboard EXOSAT. During our observation the source flux varied irregularly in time scales from minutes to hours. No periodic emission in the period range from 16 msec to 2000 sec was found with an upper limit of around 1% (3 ) for the pulsed fraction. The hardness ratio shows a correlated change with the flux intensity (Sco X-1 behaviour). The spectrum could be fitted by a double component model, a black body component (kT=1.16–1.26 keV) together with a thermal bremsstrahlung law (kT=13–15keV). The black-body temperature-black-body flux relation follows a Stefan Boltzmann law with R_BB=15.3 km^*D/10 kpc. No iron line was detected. The upper limit for the line equivalent width of a 6.7 keV iron emission line is 40 eV (1). The X-ray spectral behaviour of GX9+1 indicates, that this source belongs to the class of Low-Mass X-ray Binaries (LMXB).     

EXOSAT observations of the X-ray pulsar 4U1145-619

EXOSAT observations of the X-ray pulsar 4U1145-619 during June and July 1983 and July 1984 confirm that this source shows a regular 187 day outburst cycle in X-rays. The results from pulse timing and X-ray spectroscopy are discussed in terms of a model for 4U1145-619 involving an eccentric binary system in which there is variable accretion from the Be primary star onto a companion neutron star.     

EXOSAT results on the X-ray burster 2S1254-690

EXOSAT observations of 2S1254-690 in August 1984 have confirmed the presence of periodic dips in the X-ray intensity. These dips occurred with a period of 3.9 hours and last for about 50 minutes. During individual dips the observed X-ray flux oscillated several times between the quiescent level and an undetectable level on time scales varying from a few seconds to approximately 5 minutes. In addition, a second type one burst has been observed, with characteristics which are very similar to those observed earlier. In this paper we report the results of a 20 hour EXOSAT observation made in 1984 August 6th to 7th, and discuss some properties of the dips and of the burst.affiliated to the Astrophysics Division, Space Science Department, ESA     

X-ray emission from clusters of galaxies

The X-ray emission from the intracluster gas is a rich source of information on the metal abundance, evolution and mass profile of clusters. Methods for determining the mass of gas are reviewed; the total mass is uncertain. The best data so far available concentrate on the cluster core. Cooling flows are found within the cores of a significant fraction of rich and poor clusters, as well as in relatively isolated elliptical galaxies.     

Coordinated EXOSAT and optical observations of the X-ray burster 4U1735-44

We present the first results of an EXOSAT observation of the low-mass X-ray burster 4U1735-44. The ME data show low-amplitude variations in the persistent flux including two 5% dips separated by 4 hours. The structure of the single observed burst is briefly described. Five hours of simultaneous B-band photometry were obtained at SAAO with 12 minute time resolution; a strong anti-correlation is shown to exist between the X-ray and optical flux, with a high level of significance. A model for this behaviour is suggested, based on reprocessing of the X-ray flux in a corona or stellar wind.     

Mapping of the Cygnus Loop with EXOSAT

Three 2.10⁴ s observations were carried out with EXOSAT on three fields following the shock from the north to the east of the Cygnus Loop supernova remnant. Due to the softness of the source, most of the photons were collected with the LE package (CMA). For each exposure we used at least three filters (4000 Å lexan, Al-Par, boron) in order to extract the maximum spectral information from the data. The few photons gathered with the boron filter are particularly important in that respect. The total count number collected with the other filters allows a statistically significant overall mapping of the fields with 1*1 pixels, but a better resolution can be achieved on the brightest areas of the Loop. Interesting details are revealed, such as bright small spots. Irregularities are also evident both in the shock front and inside the remnant, specially in the northern and eastern fields.     

Soft X-ray emission from radio Loops I and III compared

We examine X-ray maps of the whole sky covering energies from 0.1 to 6 keV. We model Loop I X-ray emission as being caused by an old supernova that exploded into an already warm interstellar medium. After comparison with Loop III we deduce that there may be a general temperature gradient in the gas as we move away from the plane in the northern galactic hemisphere.     

EXOSAT observations of the contact binary VW Cephei

EXOSAT observations of the contact binary VW Cephei on 19th March 1984 are presented. The L1-telescope with CMA+thick Lexan filter was used. The observations cover one orbital revolution showing an asymmetrical X-ray light curve. This can be modelled by an active neck, connecting the two stars, and with enhanced coronal regions on the primary star. Nearly simultaneous IUE observations are also presented. The observations form a part of the program to observe contact binaries with EXOSAT.     

Observability of magnetically strongly shifted iron line emission from X-ray pulsars

As an application of our extensive calculations of energies and intensities of atomic lines in very intense magnetic fields of the order of 10¹¹-10¹³ G we discuss the possibility of observing magnetically strongly shifted iron lines in the spectra of pulsating X-ray sources. Careful estimates of the relevant parameters lead us to the conclusion that it would be profitable to look for magnetically shifted iron line enission in magnetic neutron stars of low luminosity using spectrometers working in the energy range 10 – 100 keV with sensitivities of 10^–4 cm^–2 s^–1 and energy resolutions E/E 10 – 100.This work was supported in part by the Deutsche Forschungsgemeinschaft (DFG).     

A new view of the galactic plane with EXOSAT

A new X-ray image of the galactic plane has been produced using the 45 arcmin square field of view of the Medium Energy Instrument on EXOSAT. This image shows a total of 64 sources including 18 new ones which include the first observation of persistent emission from the globular cluster bursters Terzan 1 and Terzan 5. The most important discovery from this image is a 2° wide ridge of diffuse emission symmetrical about the plane and extending from the galactic centre to 1=±40°. The spectrum of this emission appears to be hard ( 1.2) with no significant absorption.     

Observations of the solar wind from coronal holes

The solar wind emanating from coronal holes (CH) constitutes a quasi-stationary flow whose properties change only slowly with the evolution of the hole itself. Some of the properties of the wind from coronal holes depend on whether the source is a large polar coronal hole or a small near-equatorial hole. The speed of polar CH flows is usually between 700 and 800 km/s, whereas the speed from the small equatorial CH flows is generally lower and can be <400 km/s. At 1 AU, the average particle and energy fluxes from polar CH are 2.5×10⁸ cm^–2 sec^–1 and 2.0 erg cm^–2 s^–1. This particle flux is significantly less than the 4×10⁸ cm^–2 sec^–1 observed in the slow, interstream wind, but the energy fluxes are approximately the same. Both the particle and energy fluxes from small equatorial holes are somewhat smaller than the fluxes from the large polar coronal holes.Many of the properties of the wind from coronal holes can be explained, at least qualitatively, as being the result of the effect of the large flux of outward-propagating Alfvén waves observed in CH flows. The different ion species have roughly equal thermal speeds which are also close to the Alfvén speed. The velocity of heavy ions exceeds the proton velocity by the Alfvén speed, as if the heavy ions were surfing on the waves carried by the proton fluid.The elemental composition of the CH wind is less fractionated, having a smaller enhancement of elements with low first-ionization potentials than the interstream wind, the wind from coronal mass ejections, or solar energetic particles. There is also evidence of fine-structure in the ratio of the gas and magnetic pressures which maps back to a scale size of roughly 1° at the Sun, similar to some of the fine structures in coronal holes such as plumes, macrospicules, and the supergranulation.