Table of contents

These proceedings contain papers presented at the Nordic Conference in Theoretical High Energy Astrophysics, held in Copenhagen 14–16 September, 1997. The conference was organised jointly by the Danish Space Research Institute (DSRI) and the Nordic Institute for Theoretical Physics (NORDITA). Financial support was also received from the Nordic Academy for Advanced Study (Nordisk Forskerutdanningsakademi, NORFA).

The aim of the meeting was to summarize current issues in the field as well as to show the excellent opportunities that exist in the Nordic countries to perform high level research in x- and gamma-ray astronomy. Special focus was placed on the missions Spectrum-Röntgen-Gamma (SRG) and INTEGRAL, where several Nordic countries are involved. These new generation observatories will be launched soon after NASA's Advanced X-ray Astronomy Facility (AXAF) and ESA's X-ray Multiple Mirror (XMM) mission. Together these four missions will boost x- and gamma-ray astronomy into its most exciting era so far.

Astrophysicists are now routinely using satellite observations covering the whole electromagnetic spectrum, including the x- and gamma-rays. However, it should be kept in mind that somebody must plan, design, and build the science instruments, and most importantly obtain the funding for those instruments. Here DSRI has played the major role in the Nordic countries.

Following the WATCH instruments on board GRANAT and EURECA, DSRI is currently building the two mirror systems, the Bragg spectrometer, and four proportional counters of the SODART telescope on board SRG. DSRI is also the PI-institute for the x-ray monitor JEM-X of INTEGRAL (ESA). Finland has joined DSRI in these hardware tasks, and the University of Helsinki Observatory and the METOREX company are currently finalizing the SIXA spectrometer and gas systems for the proportional counters of the SODART telescope, and the microstrip proportional counters for JEM-X.

Norway participates in INTEGRAL's imaging detector IBIS and has traditions in studying high energy phenomena on the Sun. The Stockholm Observatory has a strong theoretical group involved in the SRG and INTEGRAL software and science. NORDITA in Copenhagen has close contacts with the Nordic theoretical physics community. Together with the neighbouring institutes, DSRI, the Niels Bohr Institute, the Theoretical Astrophysics Center (TAC), and Copenhagen University Observatory, they form a strong group for the interpretation of data coming from SRG and INTEGRAL.

During the conference, concerns were expressed regarding the difficulties of national funding agencies to follow their committments of their funding plans for satellite missions. In particular, the notice of further delays of the SRG launch was considered as a disaster. For this reason, the participants of the conference decided to send a signed communique to the prime minister of the Russian federation, Mr Viktor Chernomyrdin, to express their concern and with the hope that the previously agreed plan for the launch of the SRG could keep its timeline. This communique is included in these proceedings.

The organizers wish to thank all the invited and contributing speakers, and the poster paper authors for creating a very exciting conference. Let us hope that the present proceedings will further stimulate theoretical and observational work in high energy astrophysics, resulting in efficient scientific return from the SRG and INTEGRAL observatories as well as AXAF and XMM.

Finally, we thank those who helped us in the editorial work of these proceedings: Allan Hornstrup, Hans Ulrik Nørgaard-Nielsen, Martin Pohl, Jamila Oukbir, Kristian Pedersen, Pasi Hakala, Diana Hannikainen, Juhani Huovelin, Panu Muhli, Jukka Nevalainen, and Tuomo Tikkanen.

Construction of the Flight Model Joint European X-ray Telescope (JET-X) for the Russian SPECTRUM RÖNTGEN GAMMA mission has been completed. Measurements of the responses of the x-ray mirrors, the CCD detectors and the optical filters already indicate that JET-X will achieve spatial resolutions of around 20 arcsec, an on-axis collecting area of 310 cm² at 1.5 keV and an energy resolution of 130 eV at 6 keV. The final calibration of the telescope assembly has been performed in the x-ray beam line facility at MPE Garching and the in-orbit sensitivity responses have been determined for each of the scientific operating modes of JET-X.

The TAUVEX instrument on board SRG shall provide images and photometry from ~135 nm to ~280 nm on targets observed with high-energy imagers of the spacecraft. TAUVEX consists of three bore-sighted telescopes with a 55' field-of-view, 10'' resolution, and photon-counting capability. In addition to its scientific tasks, TAUVEX shall provide the aspect solution for SODART and tracking information to the SRG attitude and control system, to facilitate spacecraft pointing and stabilization. The capabilities of TAUVEX are unique in the field of UV imaging and promise very interesting science returns on the physics of hot stars, galactic structure and interstellar matter, galaxy evolution, and the compilation of a complete samples of QSOs.

MOXE (MOnitoring X-ray Experiment) is the all-sky x-ray monitor on the Spectrum-X-Gamma astrophysical observatory. The six MOXE pinhole cameras will enable almost continous observation of more than 95% of the sky in the 2–25 keV energy band. The scientific objectives of MOXE are to detect transient x-ray sources for follow-up by instruments on SXG or by other observers, as well as to study the time variability of x-ray sources. With a sensitivity of ≈ 3 mCrab (5σ) most of the observed persistent sources will be Galactic x-ray binaries. MOXE will be well suited as a partner in multi-wavelength campaigns.

The on- and off-axis imaging properties and effective area of the two SODART flight telescopes have been measured using the expanded beam x-ray facility at the Daresbury synchrotron. Following measurements have been done for both Flight Model 1 & 2, at three energies: 6.627 keV, 8.837 keV and 11.046 keV.

On-axis: Encircled Power Distribution, using one dimensional position sensitive detector (1D-detector). Point Spread Function (PSF), using 1D-detector and two-dimensional LEPC-type detector, FOV=33' (2D-detector). Effective Area, using 1D-detector.

Off-axis: Point Spread Function, at off-axis angles 0.09°, 0.18° and 0.25°, using 2D-detector. Effective Area, estimates at off-axis angles in the interval [−0.4°,+0.4°], in steps of 0.05°, using 1D-detector.

SIXA is one of the focal plane detectors of the SODART telescope on board the Spectrum-X-Gamma satellite. The detector consists of 19 circular Si(Li) elements, each with an active diameter of 9.2 mm and thickness of 3 mm. A radiative cooler will be used to bring the detector to the proper operating temperature (110–125 K). Photons can be recorded in the energy range of 0.5–20 keV with 20a 30 µs time resolution, 200 eV energy resolution and with an effective area of approximately 750 cm² at 6 keV. Unlike conventional CCD's, the detector can observe spectra at high count rates and is sensitive at energies up to 20 keV. Several hardware coded observing modes are available. Simulations, based on the current knowledge of the instrument response, can be found in these proceedings.

OCS, the Bragg spectrometer for the SODART X-ray telescope onboard the SPECTRUM–RÖNTGEN–GAMMA (SRG) satellite has been completed. Preliminary results of his performance are presented.

The SODART X-ray telescope includes an Objective Crystal Spectrometer (OCS) providing an energy resolving power around 1000 by Bragg reflection upon crystals. The SODART-OCS response function is used within the XSPEC package for the calculation of count rates for x-ray line and continuum registration from different classes of cosmic x-ray sources.

The SODART telescope with two parallel 8 m focal length mirror systems and eight different focal plane instruments, four for one mirror system and four for the other one, is the largest instrument on board the SPECTRUM-RÖNTGEN-GAMMA satellite. Among the focal plane instruments are two identical position sensitive detectors, KFRD, based on multiwire proportional counters.

The energy band of the KFRD detector is 2–25 keV.

SPI (Spectrometer for INTEGRAL) is a high spectral resolution gamma-ray telescope to be flown on board the ESA mission INTEGRAL. It consists of an array of 19 closely packed Germanium detectors surrounded by an active anticoincidence shield of BGO. The imaging capabilities of the instrument are obtained with a tungsten coded aperture mask located 1.7 m from the Ge array. The fully coded field-of-view is 16° and the angular resolution ≈2°. The energy range extends from 20 keV to 8 MeV with a typical energy resolution ≈2 keV at 1 MeV. The point-source narrow-line sensitivity is estimated to be (3–7) × 10⁻⁶ ph/cm⁻² s⁻¹ over most of the energy range of the instrument (E > 200 keV) for a 10⁶ s observing time. With these expected performances a major step forward can be expected in fine gamma-ray spectroscopy of astrophysical objects.

JEM-X is the x-ray monitor for INTEGRAL. It is being built by a large European consortium led by the Danish Space Research Institute. It consists of two identical, independent coded mask x-ray telescopes with an energy span from 3 keV to 60 keV. Each system has a microstrip gas detector and a mask with a 25% transparent hexagonal uniformly redundant pattern based on a bi-quadratic residue set situated 3.4 m above the detector. The fully illuminated field-of-view is circular and 4.8° across. The mechanical properties and source detection sensitivities are reviewed.

The INTEGRAL payload has been designed to study simultaneously gamma-ray sources in a wide field of view over many decades in energy (around 2 eV + 4 keV − 20 MeV) and thus make a major contribution to short time-scale high-energy astrophysics. The OMC will observe the optical emission from the prime targets of the gamma-ray instruments with the support of the x-ray monitor. This capability will provide invaluable diagnostic information on the nature and the physics of the sources over a broad wavelength range.

The main scientific objectives will be: (1) to monitor the optical emission from the sources observed by the gamma- and x-ray instruments, measuring the time and intensity structure of the optical emission for comparison with variability at high energies, and (2) to provide the brightness and position of the optical counterpart of any gamma- or x-ray transient taking place within its field of view.

The OMC will be based on a refractive optics with an aperture of 50 mm focused onto a large format CCD (1024 × 2048 pixels) working in frame transfer mode (1024 × 1024 pixels imaging area). With a field of view of 5° × 5° it will be able to monitor sources down to V = 19 mag. Typical exposures will consist of 10 integrations of 100 seconds each.

The deepest ROSAT surveys have shown, that, in the energy range 0.5–2.0 keV, QSO's can account for ~ 30 per cent of the Diffuse X-ray Background (DXRB), and Narrow Emission Line Galaxies (NELG) and clusters of galaxies for about 10 per cent each. But, by assuming characteristic spectral behaviour, known from bright archetypes of these classes, the faint ROSAT sources will, due to the hardness of the DXRB spectrum, only give an insignificant contribution at say 10 keV.

By exploiting the excellent capability of the SODART telescopes to obtain broad band x-ray spectroscopy at energies > 2 keV, we propose to perform a spectroscopic survey with SODART and push these telescopes to the limit at the high energies. By selecting the four ESO Imaging Survey fields EIS (each 6 deg²) we will take full advantage of the large, systematic effort ESO is putting into the optical survey of these fields. This spectroscopic x-ray survey will provide a large, statistically complete, sample of sources detected at high energies, more than an order of magnitude fainter than obtained by previous missions. The study of these sources will significantly improve our understanding not only of the origin of DXRB, but also provide new insight into the evolution of galaxies, clusters of galaxies and AGN's.

The overall phenomenology of Active Galactic Nuclei (AGN) with particular attention to the high energy band of electromagnetic spectrum is discussed in the light of the most recent BeppoSAX and ASCA observational data. The advantages of future observations made available by the deep sensitivity broad-band coverage instruments onboard INTEGRAL and Spectrum X–γ are also addressed.

Several galactic black holes show transitions between spectral states. The nature of these transitions is not fully understood yet. None of the dynamical accretion disk models can fully describe spectral transitions. In this paper we present a unifying radiation transfer model that can fit the spectral data in both states. Since Cyg X-1 has the best available data, we focus here on modeling this object. We fit individual broad-band (from 1 keV up to 4 MeV) spectral data for the `hard' and `soft' states of Cyg X-1 using an emission model where a central Comptonizing corona/cloud is illuminated by the soft photon emission from a cold, outer disk that does not penetrate much in to the corona. We assume that the energy is injected to the corona by two channels: a non-thermal one that injects energetic (> MeV) electrons into the coronal region, and a thermal one that heats injected and ambient electrons once they cool sufficiently to form a Maxwellian distribution, i.e., we consider a hybrid thermal/non-thermal model. The process of photon-photon pair production is included in the model, and the number of pairs produced in the coronal region can be substantial.

Using simple scaling laws for the luminosity of the cold disk, the thermal dissipation/heating rate in the corona, and the rate of energy injection from a non-thermal source, all as functions of radius of the corona, we explain the hard-to-soft transition as the result of a decrease in the size of the corona and the inner radius of the cold disk by a factor ~ 5. For the case of Cyg X-1, we show that the bolometric luminosity of the source (mass accretion rate) does not change significantly during the transition, and thus the transition is probably the result of a disk instability.

This paper summarizes the spectral and timing properties of narrow-line Seyfert 1 galaxies (NLS1) in view of recent ROSAT and ASCA observations. NLS1 have generally steeper soft (0.1–2.4 keV) x-ray continuum slopes than Seyfert 1s with broader lines, and there exists an anticorrelation between the 0.1–2.4 keV continuum slope and the FWHM of the Hβ line. Objects with steep 0.1–2.4 keV continuum slopes and Hβ FWHM > 3000 km s⁻¹ are clearly discriminated against by nature. When simple power-law models are fit to ROSAT spectra, photon indices reach values up to about 5, much higher than is usually seen in Seyfert 1s. Interestingly, the hard (2–10 keV) x-ray continuum slopes tend also to be steeper in NLS1 compared to broad-lined Seyfert 1 galaxies. We also report evidence for giant and rapid soft x-ray variability in the narrow-line Seyfert 1 galaxy IRAS 13224–3809 and the narrow-line quasar PHL 1092. The advantages of Spectrum-Röntgen-Gamma (SRG) and INTEGRAL for the study of NLS1 are discussed.

Some results on Seyfert galaxies obtained by the Italian/Dutch x-ray satellite BeppoSAX are briefly reviewed.

Broad-absorption line quasars are found to have extremely weak soft x-ray emission when compared with other optically selected quasars. In the only example of PHL 5200 for which a detailed x-ray spectrum has been obtained with ASCA, strong absorption in the source appears to be responsible for the lack of soft x-ray emission. Broad-band x-ray observations of a sample of BAL QSOs are proposed with a high throughput mission SPECTRUM-RÖNTGEN-GAMMA (SRG), to find out whether these sources are intrinsically weak over the entire bandwidth of x-rays or only in the soft x-rays due to absorption resulting from the line of sight passing through large column density clouds. Simultaneous UV observations will help to constrain the ionization state of the absorbers, and also improve the overall UV to x-ray continuum measurements in them.

We discuss the observational phenomenology of Seyferts from the point of view of the disk model, and argue that the spectral variability may be related to geometrical changes of the cold matter which provides a source of seed photons for Comptonization in a hot central disk. One possible configuration is a model with quasi-spherical accretion in the central part of the disk, with variability determined by dynamics of the transition zone between the cold and hot disk. In such a physical picture the soft excess component appears as a driver of spectral variability and contributes significantly to the source energetics.

The current status of the RXTE observations of the recently discovered kilohertz QPO in low-mass x-ray binaries is reviewed.

In this paper, we review the physical parameters of the eight X-ray sources which are considered to be radio-jet sources: SS433, Cyg X-3, 1E 1740.7-2942, GRS 1758-258, LSI+61°303, Cir X-1, GRS 1915+105 and GRO J1655-40. We present a summary of their properties and try to put in light commonalities between these sources.

We discuss a problem of the formation of neutron stars and black holes from massive stars.

It appears that several independent lines of evidence indicate that a considerable fraction of the stars more massive than 20–25 M_⊙ leave black holes as remnants, while, on the other hand, the neutron star in at least one high mass x-ray binary had a progenitor more massive than 50 M_⊙. We suggest that if the collapsing core has a strong magnetic field and rotates rapidly it will produce a neutron star which expells the remaining matter of the star producing an observable Supernova, whereas if it is not strongly magnetized and/or is slowly rotating it collapses silently to a black hole.

This model allows one to understand: (i) the large range in masses of the black holes in the soft x-ray transients (SXT), from about 3 M_⊙ to > 12 M_⊙ (ii) the fact that in some x-ray binaries of very high mass (> 50 M_⊙) one still sometimes finds neutron stars, (iii) the observed fact that the birthrates of SXT with neutron stars and black holes are of the same order of magnitude and (iv) that galactic nucleosynthesis requires no contribution from stars more massive than 20–25 M_⊙

Cooling of neutron stars (NSs) with standard and enhanced neutrino energy losses is simulated taking into account the effect of nucleon superfluidity in the stellar cores on heat capacity and neutrino luminosity. The superfluidity affects x-ray emission from the surfaces of cooling NSs which gives a method to constrain the critical temperatures T_cn and T_p of neutron and proton superfluids. The method is applied for interpretation of observations of PSRB0656+14 and the Geminga pulsar. The prospects of interpretation of future observations are outlined.

GROJ1655-40 and GRS 1915+105 are the two confirmed superluminal radio jet X-ray sources in the Galaxy. They have both exhibited plasmoid ejections at relativistic velocities.

An optical counterpart has been observed for GROJ1655-40 and subsequent radial velocity measurements revealed the mass of the compact object to be greater than the theoretical and observational upper limit to the mass of a neutron star. As yet, no optical counterpart to GRS 1915+105 has been observed, probably due to the heavy extinction towards the source, and thus its true nature is still in question. We have searched for a possible periodicity in the light curves of GRS 1915+105, based on the repetitions of the plasmoid ejections, which could link these ejections to an orbital period of a binary system.

The results of hard x-ray observations of GRS 1758-258 in 1990–1997 by GRANAT/SIGMA are presented. The long-term light curve of the source shows variations in the 40–200 keV luminosity by a factor of more than ~ 7 — from less than 1.5 × 10³⁶ erg s⁻¹ up to 10 × 10³⁶ erg s⁻¹. The average 40–200 keV luminosity is ~ 6 × 10³⁶ erg s⁻¹ and the averaged spectrum has best-fit power-law photon index α = 2.00 ± 0.06 with a steepening above ~ 100 keV. The source long-term light curve and the spectrum are similar to that of 1E1740.7-2942 and Cyg X-1.

Combined ASCA and SIGMA data of 1E1740.7-2942 during its standard state (September 1993 and 1994) were successfully fitted with a two-phase model ISM iterative scattering method) [1]. The classical cold accretion disk does not extend up to the innermost stable orbit, but is truncated at ~ 30GM/c². A hot inner disk has electron temperature T_e ≈ 60 keV and radial Thomson optical depth τ ≈ 2.4. The cold disk radiates ≈ 40% of the total luminosity with ≈ 0.04_Edd of 10M_⊙.

Broad, asymmetric iron K lines have been detected in the x-ray spectra of about a dozen Active Galactic Nuclei (AGN). The distorted line profiles are most probably due to strong relativistic effects in the innermost parts of an optically thick accretion disk around a central black hole. However, none of the Galactic Black Hole Candidates (GBHC) has exhibited any clear evidence of a relativistic iron line profile. Observations of the persistently bright black hole candidate Cyg X-1 have only revealed a narrow Gaussian line. Weak relativistic features may not have been detected due to insufficient signal-to-noise and energy resolution.

We present simulated observations of a relativistic iron K line in the spectrum of Cyg X-1 obtained with SIXA onboard SPECTRUM-RÖNTGEN-GAMMA. We studied the feasibility of detecting the relativistic effects in the line profile both in the hard and soft state of the source. Our results show that a relativistic line profile can be resolved in Cyg X-1 within a 50–100 ksec observation if the accretion disk extends within ~ 100 r_g from the compact object and the equivalent width of the relativistic component exceeds ~ 30–40 eV.

The recent detection of the super-Eddington persistent luminosity of the bursting pulsar GRO J1744-28 suggests that the x-ray radiation in this object is emitted by the side walls of high accretion columns standing on the magnetic polar caps. We show that the observed energy dependence of the pulsed fraction in the x-ray flux of GRO J1744-28 can result from a non-uniform distribution of the effective emission temperature over the height of the column. The effect of gravitational light bending near the surface of the neutron star must be taken into account.

Despite recent progress with the detection of afterglows of Gamma Ray Bursts (GRBs), the nature of these events is unknown. However, important clues to understanding what the GRBs are, may very well be found by studying the x-ray afterglows. The combination on SRG of the MOXE all-sky monitor for detecting GRBs, and the powerful pointed SODART telescopes will be a unique tool for studying the long-term behavior of GRB afterglows. It is shown that SODART will be able to follow the temporal and spectral development of a GRB similar to the now famous GRB 970228 event for well over 20 days in order to provide constraints on theoretical models.

Strong flux of x-ray emission with steep power-law spectrum was detected by Mir-Kvant from transient x-ray source GRS1739-278. The source was observed during survey of Galactic Center field in February–March of 1996. Later observations by Sigma/Granat and RXTE experiments allowed to classify the source as a typical soft x-ray Nova with unusually slow rise of x-ray flux during major outburst. The observations of the source by TTM/Kvant before the peak of brightness did not reveal the presense of strong soft component in the spectrum.

Plasma with temperatures above a million Kelvin is common in the Universe and exists for a large part in the intergalactic space, but is also found in a variety of discrete cosmic sources that range from optically thin to thick. I consider briefly the general outline of x-ray spectral modelling of hot plasmas, in particular the optically thin model applicable to sources like stellar coronae, clusters of galaxies, and supernova remnants. I deal with the diagnostics of plasma parameters to be obtained from optically thin line and continuum x-ray spectra and give some examples of spectral simulations for various spectrometers on board Spectrum-XG.

We discuss the morphology and substructure in clusters of galaxies and their relation to large scale structures such as filaments. As predicted by numerical simulations, the small scale structures in clusters are co-aligned with the much larger filamentary structures in which the clusters are embedded and from which they formed. For regular clusters, we review the potential of x-ray observations to derive gravitating masses to large radii. Using a small sample of observations, we show that the baryon fraction rises to a typical value of about 25% which, in the standard Big Bang scenario, argues for a low value of Ω. We briefly indicate the potential for future x-ray studies of clusters with AXAF, XMM, and SRG.

Clusters of galaxies are rare objects and as such, their properties are particularly sensitive to the underlying density fluctuations. Therefore, the cluster population provides very stringent constraints on models of galaxy formation. We here show how self-consistent modeling of x-ray galaxy clusters can be used to constrain the power spectrum of the initial density perturbations assumed to seed galaxy formation. We also demonstrate that the combined knowledge of the redshift distribution of x-ray selected galaxy clusters and that of the x-ray luminosity-temperature correlation at high redshifts offers a powerful method to constrain the mean density of the universe. Considering its large effective area, SODART aboard SPECTRUM-RG is extremely well suited to temperature measurement of distant clusters and the Ω₀ test we propose is well within the capabilities of the near future x-ray telescopes.

If clusters of galaxies are homologous and the intracluster gas is in global equilibrium with the dark matter a tight relation between mass and x-ray temperature is expected in any cosmological model. We here test this relation for 8 clusters with well-determined global temperatures measured with ASCA and masses inferred from weak and strong gravitational lensing. The data are well-fit by the predicted mass-temperature relation and are consistent with the empirical normalisation found by Evrard et al (1996 Astrophys. J.469 494) using gas-dynamic simulations. Thus, there is no discrepancy between lensing and x-ray derived masses using this approach. The dispersion around the relation is 22 per cent, entirely dominated by observational errors. These observational uncertainties may be brought down by the next generation of x-ray telescopes, e.g., SODART on board SPECTRUM-RG. Combined with wide-field imaging with HST such measurements could provide a sensitive test of the normalisation and intrinsic scatter of the relation resulting in a powerful and expedient way of measuring masses of clusters of galaxies. In addition, it is pointed out that the relation represents a new tool for determination of cosmological parameters, notably the cosmological constant Λ.

Using the current best understanding of the SODART mirror system, the Bragg panel and of the LEPC/HEPC responses [1] we have studied the feasibility and prospects for SODART studies of nearby clusters of galaxies. From simulated HEPC data of the cluster Abell 2256, we demonstrate that SODART observations of the hot intergalactic gas in nearby clusters offer an opportunity for efficient mapping of the hot gas temperature structure all the way to the virial radius. The temperature map is a good diagnostic of the dynamical state of a cluster, and for relaxed clusters the temperature profile derived from HEPC data allows a straight forward determination of the cluster mass profile with a formal accuracy comparable to the effects of systematics.

As a case study for assessing the feasibility of Bragg panel observations of clusters we have simulated observations of the Perseus cluster core with the LiF and RAP crystals. Using the LiF crystals, it is possible to scan each of the Fe-K lines from the hot gas in a few 30 ks exposures enabling possible detection of turbulence and/or bulk flow of the hot gas. Substantially longer exposures are required with the RAP crystals for studying lines originating in cool gas, but the line strengths are heavily dependent on the temperature of the cool gas. Hence, for the Perseus cluster, and perhaps a handful of other clusters, the unique capabilities of the Bragg panel for spectro-imaging of extended sources provide direct insight into the plasma processes in the gas.

X-ray spectroscopy of the hot Intergalactic Medium (IGM) within Clusters of Galaxies has revealed that a significant enrichment has occurred during their evolution. The astrophysical mechanisms which have driven this enrichment have not however been quantitatively clarified yet. In this contribution we shortly review the present-day knowledge of clusters' chemical abundances and we discuss how future x-ray missions, notably SPECTRUM-RÖNTGEN-GAMMA, ASCA and XMM, could contribute to extend our knowledge to redshifts larger than z ≈ 0.4.

We present simulated SIXA spectra of the hottest known cluster of galaxies Abell 2163. SIXA is an array of solid state detectors located in the focal plane of one of the two SODART telescopes onboard SPECTRUM-RÖNTGEN-GAMMA. We hereby demonstrate the capabilities of SIXA to perform spatially resolved spectroscopy of hot clusters. In a 30 ksec exposure, the temperature of the hot gas can be reliably derived from the central pixel as well as in the 2nd and 3rd pixel rings whereas the iron abundance can be determined in the central pixel and the 2nd pixel ring. SIXA will thus significantly improve current temperature and abundance measurements of the hot gas in clusters.

ROSAT has detected X-ray emission from more than 100 galactic supernova remnants with more than 100 SNR candidates, out of which a dozen sources have been confirmed by radio and optical measurements. Highlights of more than 7 years of ROSAT observations of SNRs include the discovery of progenitor fragments in the Vela SNR, the discovery of colliding SNR shells, high resolution images of shocked clouds in the Cygnus Loop, the studies of spatial and thermal structure, which favour an ambient medium of evaporating clouds. ASCA has added to this by high quality energy spectra including Doppler velocity maps of Cas-A in various emission lines, ionization non-equilibrium in the Cygnus Loop, the detection of non-thermal x-rays from sections of the SN 1006 shell and RX J1713.7-3946, an SNR discovered by ROSAT. These observations have broadened our understanding of SN explosions, the propagation of shock waves and the properties of the ambient medium as well as the acceleration of cosmic rays in SNR shells.

The scientific instruments on AXAF and XMM will provide far higher angular and spectral resolution than the previous generation of x-ray satellites: Einstein, Exosat, ROSAT, and ASCA. In the next few years astronomers will be able for the first time to analyze x-ray spectra of stellar coronae with a resolution of E/ΔE ≈ 1000 or greater. We summarize here the capabilities of the AXAF and XMM instruments, and identify some of the new coronal physics that these instruments will soon allow us to study.

Sun-like stars are the sites of various high-energy processes, many of which involve a release of coronal magnetic energy. Most of our knowledge on high-energy mechanisms in stellar atmospheres (other than the Sun's) is based on observations in the radio and soft x-ray regimes. They indicate a continuous presence of MeV particles, heating to high (> 10 MK) plasma temperatures, and a close interrelation between particle acceleration and heating, e.g., during chromospheric evaporation as evidenced in observations of the Neupert Effect. The present paper summarizes different aspects of high-energy processes in solar-type stellar coronae from recent, selected observing campaigns.

It is argued that x-ray observations of stellar flares are very helpful for understanding the physics of solar flares. To achieve this aim, further high-quality observations of stellar flares are urgently needed.

We study the effects of the Point Spread Function (PSF) on the performance of the Objective Crystal Spectrometer (OCS) of the SODART telescope. The advantage of OCS/SODART over XMM and AXAF instruments in studying of the diffuse sources is illustrated by modelling of the OCS/SODART observation of Tycho and Cas A Supernova Remnants (SNR).

The study of x-ray emission from co-evolving populations of stars in open clusters is extremely important for understanding the dynamo activity among the stars. With this objective, we propose to observe a number of open clusters in the x-ray and UV bands using SPECTRUM-Röntgen-Gamma. The high throughput of SPECTRUM-Röntgen-Gamma will help detect main sequence stars like Sun in middle-aged and old clusters. We will study the relationships between various parameters — age, rotation, abundance, UBV colors, x-ray luminosity, coronal temperature etc. X-ray spectra of younger and brighter populations of stars which include synchronous binaries, rapid rotators and peculiar stars, would also be carried out.

The discovery by COMPTEL of the ⁴⁴Ti line emission at 1.15 MeV from the youngest galactic supernova remnant (SNR) Cas A has opened a new window for the investigation of SNR properties. This discovery also shows the way that could help to unhide missing remnants of young galactic SNe that might have occurred during the past 300 years.

The more sensitive instruments onboard INTEGRAL (SPI, IBIS) and SPECTRUM-XG (MART-LIME) will have better chances of discovering missing young galactic SNRs by their signature in ⁴⁴Ti line emission, and providing necessary input data for the nucleosynthesis and galactic chemical evolution theory.

The recently observed nuclear γ-ray line emission from the Orion complex implies a high flux of low-energy cosmic rays (LECR) with unusual abundance. This cosmic ray component would dominate the energy density, pressure, and ionising power of cosmic rays, and thus would have a strong impact on the general structure of the interstellar medium. We propose an independent test for the existence of LECR by observing the inverse bremsstrahlung and secondary bremsstrahlung of these nucleons. The expected x-ray photon flux scales to the γ-ray photon flux with the ratio of the corresponding cross sections, thus it depends only weakly on the LECR spectrum and not on any other parameter. Observations with HEPC will allow us to derive the bremsstrahlung spectrum over the weakly extended γ-ray emission regions.

An interpretation of the highly divergent spherical `outer' eigenfunctions of the radiative transfer equation is given. It is shown that the corresponding series are analytically exact, but numerically only asymptotic.