Since the discovery of fading X-rays from Gamma-Ray Bursts (GRBs) with BeppoSAX (Piro et al. 1997, Costa et al. 1997), world-wide follow-up observations in optical band have achieved the fruitful results. The case of GRB 970228, there was an optical transient, coincides with the BeppoSAX position and faded (Paradijs et al. 1997, Sahu et al. 1997). These optical observations also confirmed the extended component, which was associated with the optical transient. The new transient are fading with a power-law function in time and the later observation of HST confirmed the extended emission is stable (Fruchter et al. 1997). This extended object seems to be a distant galaxy and strongly suggests to be the host.

The formation of Large Scale Structures (LSS) in the universe was first studied at optical wavelengths as the galaxy spatial distribution appeared to be far from homogeneous. Considerable effort has been invested in semi-analytical approaches and in numerical simulations (DM + hot gas) to explain the observed structures, given some set of initial conditions and using additional constraints provided by the COBE results. It is now clear however, that these two extreme data set are not sufficient to discriminate between the possible remaining cosmological scenarios. It is thus timely to investigate LSS at a much higher redshift than the present survey limits both in the optical and in other wavebands. In this context, the X-ray band will certainly become a hot field with the advent of the XMM observatory. The next section briefly summarizes what is known about LSS from optical wavelengths and simulations. Sect. 3 reviews the particular points that can be addressed in the X-ray band. Last section presents realistic prospects for mapping LSS with XMM.

Galaxy observers argue for the dark halo surrounding each galaxy, while cluster investigators discuss the dominance of dark matter (DM) in the intra-cluster space. Then, what is the relation between the cluster DM and the DM associated with member galaxies of the cluster?

Diffuse EUV emission has been detected in five clusters of galaxies: Virgo(Lieu et al., 1996a), Coma (Lieu et al., 1996b), Abell 1795, Abell 2199, and Abell 4038. These results were 'obtained with the Deep Survey Telescope of the Extreme Ultraviolet Explorer (Bowyer and Malina, 1991) using a Lex/B filter which covers the 60 to 170 A band. The extent of this diffuse emission from these clusters r_anges from 20' to 40' in diameter. The statistical significance of these results varies from 8 to 50 standard deviations. Some EUV emission would be expected fromthe low-energy tail of the wellstudied X-ray cluster emission, However, the EUV emission detected is far greater than the expected emission from the X-ray-emitting gas. Marginal sigatures of this “soft excess” are often present in the lowest energy resolution band of the ROSAT PSPC where it is typically less than a 20% effect. In the case of Abell 2199, no low energy excess is present in the ROSAT data. In the data taken with EUVE, however, the excesses range from 70% to 600%.

The hot gas in clusters of galaxies emits thermal bremsstrahlung emission that can be probed directly through measurements in the X-ray band. Another probe of this gas comes from its effect on the cosmic microwave background radiation (CMBR): the hot cluster electrons inverse Compton scatter the CMBR photons and thereby distort the background radiation from its blackbody spectral form. Although this, the Sunyaev-Zel'dovich (SZ) effect, is quite small, heroic efforts during the 1980's resulted in its detection in three moderately distant clusters of galaxies: A665, A2218, and CL 0016+16. It is well known that one of the purposes of conducting such measurements is to determine the Hubble constant. The technique has generated considerable interest because it is independent of all other rungs of the cosmic distance ladder and is effective over a wide range of redshifts: ~0.02 to ~1.

A definite prediction from recent N-body/hydro simulations of the structure formation of the universe is the presence of a diffuse hot intergalactic medium (HIGM; e.g., Ostriker & Cen 1996). The filamentary structure of the today's universe, as seen in various galaxies surveys, is thought to be a result of the gravitational collapse of materials from a more-or-less uniform and isotropic early universe. During the collapse, shock-heating can naturally raise gas temperature to a range of 105 – 107 K. Feedbacks from stars may also be an important heating source and may chemically enrich the HIGM. The understanding of the heating and chemical enrichment of the IGM is critical for studying the structure and evolution of clusters of galaxies, which are nearly virialized systems (e.g., Kaiser 1991; David, Jones, & Forman 1996). Most importantly, the HIGM may explain much of the missing baryon content required by the Big Bang nucleosynthesis theories (e.g., Copi, Schramm, & Turner 1995); the total visible mass in galaxies and in the hot intracluster medium together is known to account for ≲ 10% of the baryon content (e.g., Persic & Salucci 1992).

The X-ray background in the energy range above 2 keV is highly uniform except for an excess component along the Galactic plane. The excess along the plane is considered to be associated with our Galaxy, whereas the rest of the emission is believed to be of extragalactic origin. In this paper, the X-ray background at high Galactic latitude is discussed and is designated as the CXB (cosmic X-ray background) to distinguish it from the Galactic origin.

An overview of spatially resolved observations of solar radio bursts obtained by the Siberian Solar Radio Telescope at 5.8 GHz during the last ten years reveals the occurrence of different classes of burst emission defined by their source localization characteristics. Four major classes of bursts according to the source position relative to sunspots, the source size and structure, and the source height, could be tentatively distinguished and compared with burst spectral characteristics as well as with soft X-ray emission oberved by YOHKOH. These findings are in favour of a magnetic origin of the underlying flare process.

In this study, we perform 2.5-dimensional MHD simulations and clarify the role of perpendicular magnetic fields (which are perpendicular to the 2D plane) in a preflare current sheet of solar flares. At the first stage, a current sheet formed within a coronal magnetic structure is filled with the perpendicular fields (force-free structure). Then this sheet begins to be dissipated through the tearing instability under a uniform resistivity. As the instability proceeds, the distribution of the perpendicular fields vary in such a way that most of them gather around O-point (magnetic island) instead of X-point. Therefore, the magnetic pressure of these fields weaken in the vicinity of X-point so that they no longer suppress the inflows toward this point. These flows then make the current sheet thinner and thinner, which implies that the current density around X-point becomes high enough to cause an anomalous resistivity whose value is much larger than that of the normal collisional resistivity. In this way, the transition from a uniform resistivity to a locally-enhanced one occurs, which can make the violent energy release observed in solar flares.

Resent observations suggest that, during solar flares, plasmoids are injected into the interplanetary medium (Stewart et al., 1982). It has also been pointed out that solar wind irregularities modeled as plasmoids are penetrated into the magnetosphere (Lemaire, 1977). These plasmoid injections are considered to be an important process because they transfer mass, momentum, and energy into such magnetized plasma regions. Our objective is to investigate the dynamics of a plasmoid, which is injected into a magnetized plasma region and to reveal mechanisms to transfer them. To achieve this, we carried out three-dimensional magnetohydrodynamic (MHD) simulations.

Flare scenario, based on Yohkoh observations and magnetic reconnection (MR) hypotesis, has been elaborated recently (Tsuneta, 1996; Shibata, 1997). Using theoretical prediction that the fast MR results in a high-speed outflows from reconnection site, the reconnection downflow has been supposed to collide with the top of underlying magnetic loop, producing superhot plasma and high-energy particles.

Two-dimensional magnetohydrodynamic simulation of a solar flare is performed using a newly developed MHD code including nonlinear anisotropic heat conduction effect (Fig. 1; Yokoyama & Shibata 1997a). The numerical simulation starts with a vertical current sheet which is line-tied at one end to a dense chromosphere. The flare energy is released by the magnetic reconnection mechanism stimulated initially by the resistivity perturbation in the corona. The released thermal energy is transported into the chromosphere by heat conduction and drives chromospheric evaporation. Owing to the heat conduction effect, the adiabatic slow-mode MHD shocks emanated from the neutral point are dissociated into conduction fronts and isothermal shocks (Yokoyama & Shibata 1997b). Temperature and derived soft X-ray distributions are similar to the cusp-like structure of long-duration-event (LDE) flares observed by the soft X-ray telescope aboard Yohkoh satellite. On the other hand density and radio maps show a simple loop configuration which is consistent with the observation with Nobeyama Radio Heliograph. Two interesting new features are found. One is a pair of high density humps on the evaporated plasma loops formed at the collision site between the reconnection jet and the evaporation flow. The other is the loop-top blob behind the fast-mode MHD shock.

We present new X-ray, EUV, and radio observations of the coronal emission from the RS CVn binaries HR1099 (V711 Tau) and σ2 CrB. RS CVn systems possess coronae that display extreme activity levels and frequent flaring. Our observations provide multiwavelength records of coronal variability and flaring over more than two binary orbits for each system. While the EUV and X-ray spectra show the flare response of the thermal plasma, the radio data give the corresponding information for the nonthermal electron population. The HR1099 data contain one of the most energetic flare outbursts observed from HR1099 that lasts for over 3 days. Coronal flaring is common in these systems and, in fact, is the normal condition.

In the ROSAT all-sky survey 11 HgMn stars were detected as soft X-ray emitters (Berghöfer, Schmitt & Cassinelli 1996). Prior to ROSAT, X-ray observations with the Einstein Observatory had suggested that stars in the spectral range B5-A7 are devoid of X-ray emission. Since there is no X-ray emitting mechanism available for these stars (also not for HgMn stars), the usual argument in the case of an X-ray detected star of this spectral type is the existence of an unseen low-mass companion which is responsible for the X-ray emission. However, this hypothesis is not easily testable. Based on high resolution X-ray images taken with the ROSAT HRI, Berghöfer & Schmitt (1994) showed that known visual late-type companions can be disregarded in this context. In almost all cases studied so far (including two HgMn stars in our sample) the X-ray emission is associated with the primary B star.

Highly blue or red shifted broad emission components of H I lines have been noticed in some recent spectra of the symbiotic star CH Cyg. Their intensities, profiles, and displacements from the narrow component have changed with time scale of days. For example, a blue-shifted (-1545 km s−1) intense emission component and a red-shifted weak (+757 km s−1) emission were seen on 1994 September 9. One day after, the blue-shift and red-shift became −2076 km s−1 and +1285 km s−1, respectively.

Observations with the Einstein Observatory indicated that stellar X-ray activity diminishes in clusters older than 70 Myr (Pleiades). ROSAT observations of older clusters also support this result (see Caillault 1995 and references therein). The timescales over which young stars diminish in X-ray luminosity depends on spectral type (Randich et al. 1996), leading to the conclusion that X-ray activity in late type PMS depends on age and stellar mass. F and G-stars approach the main sequence much faster and the diminishing rates of X-ray activity from F to M stars start to differ considerably. Kastner et al. (1997) observed that the mean of the ratio Lx/Lbol for K and M dwarf stars increases monotonically for low-mass stars from the very early T Tauri stage through the age of the Pleiades cluster, reflecting the contraction and spin-up of such stars during pre-main sequence evolution. This ratio then decreases towards middle aged stars, as late-type main sequence stars spin down. Here we extend this result by including more distant clusters that are younger overall than those considered by Kastner et al. and also including earlier spectral types.

Study of X-ray coronae from late-type stars with moderate resolution X-ray spectroscopy with ASCA, has led to the characterization of temperatures and measurements of elemental abundances in their coronae. Several RS CVn and Algol-type binary systems, and single late-type stars have been observed. We present here the results obtained from X-ray spectroscopy of recently observed single F-G-K-M type dwarfs. The sample observed with ASCA contains αCen (Mewe et al. 1997), π1UMa (Drake et al. 1994), YY Gem, Speedy Mic, GJ 890 (Singh et al. 1997), EK Dra, HN Peg, κ1Cet (Guedel et al. 1997), AB Dor (Mewe et al. 1996) and HD 35850 (Tagliaferri et al. 1997).

X-rays from early-type stars are emitted by the corona or the stellar wind. The materials in the surface layer of early-type stars are not contaminated by nuclear reactions in the stellar inside. Therefore, abundance study of the early-type stars provides us an information of the abundances of the original gas. However, the X-ray observations indicate low-metallicity, which is about 0.3 times of cosmic abundances. This fact raises the problem on the cosmic abundances.

We analyze X-ray archive data of the W UMa-type binaries 44i Boo and VW Cep, taken with ASCA on 1994 May 10 and 1993 November 5, respectively. By analyzing the light curve of VW Cep, we find a long-duration flare of ≈ 7.5 hrs with the peak luminosity of 1.2 × 1030 ergs s−1 (0.4-3.0 keV) for the assumed distance of 23.2 pc. We also find appreciable flux variations from the light curve of 44i Boo, and the variations are erratic and are not orbital phase dependent. From the spectral analysis of both data, we see that the spectra could be reproduced by the variable abundance plasma model with a combination of two different temperatures, kT = 0.64 – 0.65 keV and kT = 1.85 – 1.91 keV.

The region south of the reflection nebula NGC1333 in Perseus is an active star forming region including numerous Herbig-Haro objects and at least 5 protostar candidates with molecular outflows and far-infrared emission. It has been actively studied in various wave bands (e.g. Aspin et al 1994 and references therein). We observed this region with ASCA with the primary objective to detect X-rays from the protostars embedded deep in the molecular cloud.