The universe contains an extremely wide variety of temperature structures from 3K to 1 billion K and even beyond. This symposium focuses on the hot part of the universe. The “hot universe” is by far the best place to study high-energy astrophysics. In this overview, I shall be based mainly on the results in the X-ray band that best manifests the hot universe. However, needless to say that multi-wavelength investigations, from radio, infrared through gamma-rays, are essential for comprehensive understanding.

Recent development on the theory and numerical modeling of solar flares and jets is reviewed with emphasis on the magnetic reconnection model. Application to protostellar flares and jets is also discussed.

A number of major advances in stellar coronal physics have occurred since 1990 mainly as a consequence of imaging observations by ROSAT and spectroscopic observations by ASCA. These can be summarised as follows:

1. 1. an all-sky survey has been performed by ROSAT at a sensitivity of ~ 2 × 10−13 erg cm−2 s−1, complemented by pointed observations an order of magnitude deeper;

2. 2. complete mapping and deeper pointings have been obtained for virtually all open clusters closer than ~ 500 pc, and covering the age range from ~ 30 Myr to ~ 700 Myr;

3. 3. complete mapping and deeper paintings have been obtained for several Star Forming Regions (SFRs) covering the age range ~ 1 to ~ 10 Myr;

4. 4. spectroscopic observations of bright coronal sources have been obtained with EUVE and ASCA allowing the derivation of the temperature structure and elemental abundances.

The field of low-mass star formation and early evolution has made rapid progress in recent years, thanks in particular to observations in the IR and mm ranges. The current evolutionary scheme calls for two main stages, themselves divided into two substages (e.g., André & Montmerle 1994): (i) protostars, comprizing the newly discovered so-called “Class 0 sources”, detected mostly or only in the mm range, which are young protostars with estimated ages ~ 104 yrs, and “Class I sources”, visible in the near- to mid-IR, which are evolved protostars with estimated ages ~ 105 yrs; (ii) T Tauri stars, which are visible in the IR but also in the optical, the younger being the “classical” T Tauri stars (called “Class II” in the IR), and the “weak-line” T Tauri stars (“Class III” in the IR), with a large age spread of ~ 106 – 107 yrs. According to current models (e.g., Shu et al. 1987), protostars consist of a forming star surrounded by an extended envelope (up to ~ 10,000 AU in radius); the star forms via an accretion disk inside a cavity ~ several 100 AU in radius. The disk probably plays an important role in generating molecular outflows, running through the envelope. Classical T Tauri stars are only surrounded by a disk, which disappears at the weak-line T Tauri stage.

Prototypes of magnetic actions in producing and shedding the X-ray-emitting high temperature plasmas in various astrophysical objects are witnessed in the spatially-resolved form on the Sun by the Solar X-ray Satellite “Yohkoh”. The most prominent of those are arcade flarings seen as powerful arcade flares in active regions with strong magnetic field. Larger scale fainter X-ray arcade formation observed at high latitudes, shedding a large amount of mass and energy as CME's (coronal mass ejections) also belongs to this category. Since some features found by the new observation by Yohkoh are incompatible with the so-called “classical model of arcade flarings”, we advance an alternative model based on the quadruple magnetic sources in the photosphere.

Among the mechanisms of X-ray emissions from supernovae, we focus on the circumstellar interaction. In particular, we relate the new X-ray features of SN1993J to the hydrodynamical instabilities in the ejecta. First, we model the early-time two component spectral feature by invoking instabilities in the dense cooling shell in the ejecta. Second, we model the gradual increase in the X-ray light curve as X-rays emitted from the reverse shocked ejecta; here the model requires a large scale change in the density distribution due to Rayleigh-Taylor instabilities around the core-envelope interface.

The detection of the relatively short-lived (~106 yr) radioactive 26Al in our Galaxy, more than 12 years ago, convincingly showed that nucleosynthesis is still active today on a large scale. Still, despite considerable efforts in the past decade, the site of this nucleosynthetic activity cannot be identified from theory alone; several potential sites are still on the list, been able to provide the 2-3 M⊙ of 26Al per Myr, implied by the detected 1.8 MeV flux of ~4 10−4 cm−2 s−1 from the galactic plane (see for a review).

Comptel aboard NASA's Compton Observatory has led to a major progress in the field of astronomical gamma-ray line spectroscopy. Highlights are the all-sky map of the 1.8 MeV line from radioactive 26Al, the first detection of the 1.156 MeV line from radioactive 44Ti from a Supernova remnant (Cas A), and the detection of excessive MeV emission from the Orion complex that may be ascribed to excitation of 12C and 16O nuclei.

SNe are one of the major source of the heavy elements in Galaxy. They produced high temperature plasma heavily contaminated with heavy elements mixing with the interstellar matter. Due to the temperature and the density, the shock heated plasma does not reach the collisional ionization condition in young SNR, like Cassiopeia A, Tycho and Kepler. The emission is well represented with a non-equilibrium condition plasma.

Several plasma codes are available for the analysis of hot astrophysical plasmas. Among the oldest are the Raymond-Smith model (Raymond & Smith 1977) and Mewe-Gronenschild model (Mewe et al. 1985, 1986). Minor updates to this last code have resulted in the meka model (Kaastra 1992); major updates (most importantly the ionization balance and the treatment of the Fe-L complex) resulted in the mekal code (Mewe, Kaastra & Liedahl 1995). Here also the plasma codes of Masai (1984) and Landini & Monsignori Fossi (1990) should be mentioned. The RS, meka and mekal codes are included in the XSPEC fitting package, the latest mekal code is incorporated in the SPEX package (Kaastra et al. 1996). Both Masai's code and SPEX contain non-equilibrium ionization (NEI) modes. All these codes differ in details, see Brickhouse et al. (1995) for an overview. Most important for the analysis of X-ray data is the ionization balance that is used for iron and the treatment of the Fe-L complex.

In the X-ray band, we can see weak and extended X-rays along the Galactic plane and near the Galactic Bulge region, although these regions are dominated by many point sources (e.g., Warwick et al. 1985). The Tenma satellite discovered conspicuous emission lines from selected regions near the Galactic plane (Koyama et al. 1986). These lines are identified with K-shell line from He-like Fe, hence the extended emission is attributable to optically thin hot plasmas with temperatures of several keV. The origin of the thin hot plasmas, however, have been debatable, because no class of X-ray objects shows such high temperature plasma emissions. To investigate the origin of the extended X-rays, we are currently observing the Galactic plane regions with the ASCA satellite. In this paper, we report on the ASCA results: the hard X-ray imaging and spectroscopy of the hot plasma in the Galaxy.

In elliptical galaxies, where most of the stars were formed at an early epoch, the total mass, spatial distribution, and relative abundances of metals are intimately connected to the galaxy formation process. Determinations of the hot interstellar medium metallicity from X-ray spectral analysis are more direct, less model-dependent, and more radially extensive than optical estimates based on broad-band colors or line indices, and provide a view into the nucleosynthetic histories of elliptical galaxies.

The Einstein observations revealed that starburst and luminous elliptical galaxies had X-ray halo. These galaxies have quite different stellar population. Starburst galaxies contain young massive stars, while elliptical galaxies generally contain an old-metal rich population dominated by K and M giants. Therefore a question is why these two type of galaxies commonly have hot gas, in spite of quite different stellar populations. In order to address this question, we observed these galaxies with ASCA. In this paper, I would like to present observational results, then compare the physical parameters of the hot gas in these galaxies.

It is clear that there is an important interplay between galaxies and the group environment. At the velocity dispersions (~ 100 km s−1) characteristic of groups, the galaxies interact strongly, leading to triggering of star formation, and galaxy merging. We can expect to see evidence of such processes through differences in the properties of galaxies in groups compared to field galaxies. Conversely, the galaxies affect their environment, as is apparent from the presence of heavy elements in the hot intergalactic medium (IGM) in groups, which emit characteristic X-ray lines.

Investigation of evolution of X-ray properties of clusters of galaxies is a key study of cosmology. The most important result in this field before ROSAT and ASCA observatories is the detection of the negative evolution of the X-ray luminosity function at red-shift lower than 0.6 (Gioia et al., 1990). Following it, many groups have been investigating evolution of X-ray luminosity function from various surveys with ROSAT observatory (eg. Collins et al., 1997). Many of them indicate no negative evolution at the red-shift lower than 0.7, which is against the Einstein result.

SAX, an acronym for “Satellite Italiano per Astronomia a raggi X”, now renamed “BeppoSAX” in honor of Giuseppe Occhialini, is the first X-ray mission sensitive in the very broad energy range between 0.1 and 300 keV (Boella et al. 1997a). The Narrow Field Instruments (NFI) have approximately 1° fields of view and consist of the imaging low- and medium-energy concentrator spectrometers (LECS, 0.1-10 keV, Parmar et al. 1997; and MECS, 1-10 keV, Boella et al. 1997b), and the non-imaging high pressure gas scintillation proportional counter (HPGSPC, 3-120 keV, Manzo et al. 1997) and Phoswich detector system (PDS, 15-300 keV, Frontera et al. 1997). All the NFI are coaligned and are normally operated simultaneously. In addition, the payload includes two wide field cameras (WFC, 2-30 keV; Jager et al. 1997) which observe in directions perpendicular to the NFI. These allow the detection of X-ray transient phenomena, as well as long-term variability studies.

The past three decades have seen an explosion in high-energy astrophysics. We have found X-ray astronomy to be an indispensable tool in understanding our Universe. The discipline has become mature, and future X-ray observatories must be more highly specialized. High-resolution spectroscopic imaging in the band above 2 keV, systematically exploited by ASCA, has led to much new astrophysical knowledge. ASCA has also been playing a particularly important role in studying sources hidden behind dense material. Astro-E, the successor of ASCA, is scheduled for launch in the year 2000 by the Institute of Space and Astronautical Science (ISAS) with its newly developed M-V rocket (Ogawara & Inoue 1997). Our new Astro-E observatory features high energy resolution and high sensitivity over the broad energy range 0.5 keV to 600 keV. The general emphasis of the observatory is to provide large collecting areas at higher energies, with angular resolution good enough to avoid the confusion limit. This paper provides a brief description of the performance of the Astro-E instruments.

AXAF has been a long time, 20 years, in coming. This has made AXAF so overfamiliar that it is hard to see how revolutionary it still is. Some even describe AXAF as the end of the line in X-ray technology; a never to be repeated venture to high resolution. This view is wrong. To see why we need to see where X-ray astrophysics is going.

All Sky Surveys are very useful for several reasons: Firstly, their unbiased view provides the potential for new discoveries. Secondly, they yield large unbiased samples of a variety of objects which can be used for statistical investigations. And thirdly, the unlimited field of view of telescopic sky surveys allows to study extended objects and large scale structures.