The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is dedicated to the fine spectroscopy (ΔE: 2 keV FWHM @ 1 MeV) and fine imaging (angular resolution: 12' FWHM) of celestial gamma-ray sources in the energy range 15 keV to 10 MeV. INTEGRAL was selected in 1993 as the next ESA medium-size scientific mission (M2) to be launched in 2001. ESA has the overall spacecraft and mission resposibilities, Russia will provide a PROTON launcher and launch facilities, and NASA will provide ground station support through the Deep Space Network. The scientific payload complement and the INTEGRAL Science Data Centre (ISDC) will be provided by large collaborations led by Principal Investigators (PI).

Imaging X-ray observations have resolved X-ray sources in many nearby ‘normal’ galaxies. These sources are associated with different galaxian component, including: bulge and globular clusters(LMXB, e.g. in M31); the disk/arm component (LMXB, HMXB, SNR, SN, ISM shells); and low-luminosity AGN (see reviews in Fabbiano 1989; 1995; 1996). A number of these sources have X-ray luminosities in excess of the Eddington luminosity for 1 M⊙ accreting object (~ 1.3 × 1038ergs s−1), and have been called Super-Eddington sources. Nearly 100 Super-Eddington sources have been reported so far, based on Einstein and ROSAT images (see Fabbiano 1989; 1995; 1996; Makishima 1994). The number of these sources increases with the maximum distance of the galaxy sample considered (e.g. Read et al 97). This is in part due to confusion, but it is also true that considering the larger sample of galaxies one obtains in a larger volume of space the amount of these relatively rare sources would increase as well.

Accreting magnetic white dwarfs are usually found as component stars in Magnetic Cataclysmic Variables (MCVs), in which a white dwarf with B = 105-8 G accepts mass from a late type (secondary) star via Roche Lobe overflow. Matter from the secondary is funneled by the magnetic field and concentrates on the magnetic pole(s) of the white dwarf. Since the accretion flow becomes highly supersonic, a standing shock wave is formed close to the white dwarf. The temperature of the plasma at the shock front reflects the gravitational potential and can be denoted as a function of the mass (M) and the radius (R) of the white dwarf as:

Note here that the height of the shock is expected to be within 10% of the white dwarf radius, and hence neglected here.

Massive X-ray binary pulsars have often evolved early-type companion stars which emanate strong stellar winds. X-rays emitted from the accreting neutron star irradiate and ionize the surrounding stellar wind, thus forming a photoionized sphere surrounding the neutron star. The photoionization structure of matter surrounding the neutron star was calculated by Hatchett and McCray (1977) and McCray et al. (1984), for Cen X-3 and Vela X-1 respectively.

The study of X-ray binaries has made great progress with the advent in the past few years of a number of very capable X-ray astronomy missions. These are reviewed, for example, by Bradt et al 1992, and a set of recent relevant papers in Makino and Mitsuda, 1997. For example, ASCA has allowed a significant increase in sensitivity and spectral resolution in 0.5-10 keV X-rays (Tanaka et al 1994). Many recent Compton/GRO results on X-ray binaries are reviewed in the proceedings of the Second Compton Symposium (Fichtel et al 1994). Another source of recent results from analysis of data from several satellite missions is the proceedings of the Evolution of X-ray Binaries (Holt & Day, 1994). In this short paper, the emphasis is on guiding the reader to some relevant literature.

Millisecond time-scales are natural for some neutron star and black hole processes, although possibly difficult to observe. The Rossi X-Ray Timing Explorer (RXTE) has found that for the neutron stars in low-mass X-ray binaries (LMXB) there are flux oscillations at high frequencies, with large amplitudes. Z sources and bursters tend to exhibit oscillations in the range 300-1200 Hz. Persistent emission may exhibit one or both of two features. In bursts from different bursters, a nearly coherent pulsation is seen, which may be the rotation period of the neutron star. For some the frequency equals the difference between the two higher frequencies, suggesting a beat frequency model, but in others it is twice the difference. The sources span two orders of magnitude in accretion rate, yet the properties are similar. The similar maximum frequencies suggests that it corresponds to the Kepler orbit frequency at the minimum stable orbit or the neutron star surface, either of which would determine the neutron star masses, radii and equation of state. Theories of accretion onto black holes predict a quasi-periodic oscillation (QPO) related to the inner accretion disk. The two microquasar black hole candidates (BHCs) have exhibited candidates for this or related frequencies.

The bursts from GRO J1744-28 are due to accretion instabilities as is the case for type II bursts in the Rapid Burster. Both sources are transient Low-Mass X-ray Binaries, and they both exhibit unusual quasi-periodic-oscillations in their persistent X-ray flux following several (not all) of the type II bursts. There are important differences too. GRO J1744-28 is an X-ray pulsar; the Rapid Burster is not. In addition, the pattern of bursts and the burst peak luminosities are very different for the two sources. Time intervals between the rapidly repetitive bursts in the Rapid Burster can be as short as 10 sec, in 1744-28 they are as short as 200 sec. The peak luminosities of the bursts from GRO J1744-28 can exceed the Eddington luminosity (for assumed isotropic emission) by one to two orders of magnitude. The QPO centroid frequencies (see above) differ by an order of magnitude (~0.04 Hz for the Rapid Burster, and 0.3 Hz for GRO J1744-28). The difference in behavior p obably lies in the difference in the magnetic dipole field strength of the accreting neutron stars (for GRO J1744-28 it is almost certainly much higher than for the Rapid Burster). It remains puzzling, why GRO J1744-28 and the Rapid Burster are the only known sources which exhibit rapidly repetitive type II bursts.

Pulsars provide useful diagnostics of isolated neutron stars, because the timing information allows many physical parameters to be derived. This brief review describes some of the X-ray and gamma-ray properties of pulsars.

The suggestion that the shocks of supernova remnants (SNR's) are cosmic ray acceleration sites dates back more than 40 years. While observations of nonthermal radio emission from SNR shells indicate the ubiquity of GeV cosmic ray production, there is still theoretical debate about whether SNR shocks accelerate particles up to the well-known “knee” in the primary cosmic ray spectrum at ~3,000 TeV. Recent X-ray observations of SN1006 and other SNR's may have provided the missing observational link between SNR shocks and high energy cosmic ray acceleration. We discuss these observations and their interpretation, and summarize our ongoing efforts to find evidence from X-ray observations of cosmic ray acceleration in the shells of other SNR's.

In spite of the recent progress of high energy gamma-ray astronomy, there still remains quite unclear and important problem about the origin of cosmic rays. Supernova remnants (SNRs) are the favoured site for cosmic rays up to 1016 eV, as they satisfy the requirements such as an energy input rate. But direct supporting evidence is sparse. Recently intense non-thermal X-ray emission from the rims of the Type Ia SNR SN1006 (G327.6+14.6) has been observed by ASCA (Koyama et al. 1995)and ROSAT (Willingale et al. 1996), which is considered, by attributing the emission to synchrotron radiation, to be strong evidence of shock acceleration of high energy electrons up to ~100 TeV. If so, TeV gamma rays would also be expected from inverse Compton scattering (IC) of low energy photons (mostly attributable to the 2.7 K cosmic background photons) by these electrons. By assuming the magnetic field strength (B) in the emission region of the SNR, several theorists (Pohl 1996; Mastichiadis 1996; Mastichiadis & de Jager 1996; Yoshida & Yanagita 1997) calculated the expected spectra of TeV gamma rays using the observed radio/X-ray spectra. Observation of TeV gamma rays would thus provide not only the further direct evidence of the existence of very high energy electrons but also the another important information such as the strength of the magnetic field and diffusion coefficient of the shock acceleration. With this motivation, SN1006 was observed by the CANGAROO imaging air Cerenkov telescope in 1996 March and June, also 1997 March and April.

The current status of very high energy gamma ray astronomy (in ~ 1 TeV region) is described by using as example results of CANGAROO (Collaboration of Australia and Nippon for a GAmma Ray Observatory in the Outback). Gamma rays at TeV energies, emitted through inverse Compton effect of electrons or π0 decay from proton interaction, provide direct evidence on “hot” non-thermal processes of the Universe, as well as environmental features, such as the strength of magnetic field in the emission region, for the non-thermal processes.

So far, seven X-ray sources have been identified to be radio-jet sources. We present a review of the observations of IE 1740-2942, GRS 1758-258, GRS 1915+105, GRO J1655-40, SS 433, Cir X-1, Cyg X-3.

Structure of the accretion disk is compared between the soft and hard states of Cyg X-1 using the ASCA data. Large uncertainty of the disk parameters in hard state prevent us from drawing clear conclusion, but the data are consistent with a factor of 3 larger (optically thick) inner disk boundary in the hard state than in the soft state.

X-ray spectroscopy of the broad iron line has revealed some relativistic effects caused by strong gravity about a black hole in active galactic nuclei (AGN). Recent results from ASCA observations of AGNs are reviewed.

Several IRAS galaxies have been detected at X-rays, with a variety of satellite observatories. About half of these are classified optically as Seyfert galaxies. Among those not (convincingly) classified as AGN, many have X-ray luminosities for which stellar evolution products offer convenient explanations. Some non-active IRAS galaxies display anomalously high levels of X-ray emission for which several conceivable origins are investigated: optical misclassification, X-ray misidentification, hidden AGN, incidental activity, starburst, environmental sources. X-ray spectral studies and temporal variations constitute important tools for further investigation, for instance to assess the strength of a starburst or to establish signatures of an active core.

One of the most exciting recent developments in astrophysics is the detection of members of the blazar class of AGN at ~1 GeV by the EGRET instrument aboard the Compton Observatory. Although a weak detection of 3C 273 was obtained by the COS B mission, the very strong detection of 3C 279 by EGRET (Hartman et al. 1992) two months after launch was not anticipated.

BL-Lac objects and optically violent variable quasars (OVVs), called together blazars, are characterized by rapid time variability, strong optical polarization, superluminal expansion and strong gamma-ray emission. Such properties are understood in the framework of a relativistic jet emanated from the central powerhouse. Blazars are considered to be objects for which the direction of the jet is very close to the line of sight.

Gamma-ray bursts (GRBs) will be recorded as one of the outstanding new phenomena discovered in astronomy this century. About once per day, a burst of gamma rays appears from a random direction on the sky. Often, the burst outshines all other sources of gamma-rays in the sky, combined. This paper reviews some of the key observed phenomenon of bursts in the hard x-ray/gamma-ray region, as observed with the BATSE experiment on the Compton Gamma Ray Observatory. The observed time profiles, spectral properties and durations of gamma-ray bursts cover a wide range. Recent breakthroughs in the observation of gamma-ray burst counterparts and afterglows in other wavelength regions have marked the beginning of a new era in gamma-ray burst research. Those observations are described in following papers in these proceedings.

The nature of Gamma-Ray Burst (GRB) has been the object of many investigations but their origin has remained a mistery primarily for the difficulties in finding a counterpart. This difficulty derived from the intrinsically poor positioning capability of available GRB detectors.

Gamma-Ray Burst (GRB) research has been recently revitalized with exciting new results that have effectively started a new era in this thirty year old field, marked by the launch of the Italian-Dutch satellite BeppoSAX on April 1996.