9:15 SUTO, Y. (Solicited Paper)

Cosmological implications of galaxy clusters in X-ray, mm and submm bands

It is known that the cluster abundance puts a stringent constraint onthe mass fluctuation amplitude $\sigma_8$ at $8h^{-1}$Mpc scale andthe cosmological density parameter $\Omega_0$; in canonical cold darkmatter models, for instance, log $N$-log $S$ for clusters from theROSAT observations (Ebeling et al. 1997; Rosati et al. 1997) yield $\sigma_8=(0.54 \pm 0.02)\Omega_0^{-0.35-0.82\Omega_0+0.55\Omega_0^2}$ ($\lambda_0=1-\Omega_0$) and $\sigma_8=(0.54 \pm 0.02) \Omega_0^{-0.28-0.91\Omega_0+0.68\Omega_0^2}$ ($\lambda_0=0$), where $\lambda_0$ is the dimensionless cosmological constant (Kitayama \& Suto 1997). In this talk, I will discuss possibilities to break the degeneracy of $\sigma_8$ and $\Omega_0$ for viable cosmological models using the number counts at different wavelengths through the Sunyaev \& Zel'dovich effect and/or X-ray cluster abundances at higher redshifts (Kitayama, Sasaki \& Suto 1998). The former is of particular significance in relation to the future projects including the European {\it PLANCK} mission and the Japanese Large Millimeter and Submillimeter Array ({\it LMSA}) project. I also describe further several implications of the multi-band observations of galaxy clusters including the determination of the Hubble constant, gas density and temperature profiles and peculiar velocity field (Makino, Sasaki \& Suto 1998; Yoshikawa, Itoh \& Suto 1998).

9:45 MUSHOTZKY, R. (Solicited Paper)

X-ray probing of baryons and dark matter in groups and clusters

Recent x-ray observations of groups have revolutionized our understanding of the nature and distribution of the baryonic and dark matter mass in the local universe and the evolution of metals. These ASCA and Rosat data have shown that many poor groups, the "average" place in the universe, are dominated by dark matter and have a different abundance pattern in the intergalactic gas than that seen in rich clusters. The lower ratio of the mass of metals to starlight in groups compared to clusters indicates that either the groups have had much of their metals removed or that the process of metal formation in groups is significantly different than that in rich clusters. If the former is true it indicates that the intergalactic medium is metal enriched and if the latter then there have been strong variations in the efficiency of star formation on very large mass scales. A census of baryons in the local universe indicates that most of the visible baryons are in the hot intergalactic gas in groups. Integration of the mass function derived from x-ray imaging spectroscopy indicates that most of the total visible mass density in the local universe is also in groups with minimum contribution to the closure density of x-ray luminous groups and clusters of 10%.

10:15 OHASHI, T.; KIKUCHI, K.; MATSUSHITA, K.; YAMASAKI, N.Y. et al.

ASCA measurements of hot-gas properties in bright clusters of galaxies

The hot-gas properties of bright near-by clusters have been studied in detail from ASCA, and we will summarize important results and address their implications. In the center of cD clusters a significant concentration of the gravitational mass has been newly found for several systems, shown by a narrow concentration of the hot intergalactic medium. Large-scale gradients in metal abundances are detected from at least AWM7 and Perseus, and the spatial correlation between galaxies and mass density of metals gives strong constraint on the metal injection scenario in clusters. Temperature distribution in several poor clusters is found to be uniform with 10-20\% fluctuations, and suggest that some systems do not follow the universal gradient feature recently reported for high-temperature systems by Markevitch et al.

X-ray study of hot interstellar medium in early-type galaxies

We have analyzed ASCA data of about 30 early type galaxies, and studied their X-ray emitting ISM (InterStellar Medium) properties. Our study has been motivated by the apparently very low metallicity ISM, which cannot easily be reconciled with theoretical predictions. By carefully examining the abundance ratios and uncertainties in the Fe-L complex, we have concluded that the ISM abundances in X-ray luminous galaxies are in fact about 1 solar. Therefore, the severe discrepancy between the ISM and stellar abundance has been relaxed. The ISM metallicity of X-ray fainter galaxies are uncertain, but at least SNe Ia contribution to the ISM abundance is smaller than in the X-ray luminous ones.

We have also discovered that X-ray emissions from X-ray luminous galaxies are very extended, and expressed with two beta models of different angular scales. This means that the X-ray luminous ellipticals are central galaxies of some larger-scale potential structures. We show that presence/absence of such a larger-scale potential can consistently account for several unsolved problems with the ISM.

Thermal and non-thermal emission from clusters

The first results of BeppoSAX on clusters of galaxies are presented. BeppoSAX offers broad band spectroscopy in the 0.1--100\,keV band with in addition imaging spectroscopy (arcminute resolution) in the 0.1--10\,keV band.

The high sensitivity and well-behaved point-spread function at high energies allows a study of the spatial distribution of both the iron K$\alpha$ (6.7\,keV) and K$\beta$ (8\,keV) complex. In the Perseus cluster the K$\beta$/K$\alpha$ ratio is significantly larger then predicted by standard thermal emission codes. This ratio decreases significantly outwards, providing evidence for resonance scattering of a part of the iron K$\alpha$ line. Taking this into account the iron abundance in the core of the Perseus cluster is nearly twice as large as previously thought.

Also in other clusters evidence for resonance scattering is found. In A\,2199 there is evidence for a hard non-thermal tail in the spectrum. Spatially resolved spectroscopy with the MECS instrument shows spectral hardening in the outer parts of the cluster. Moreover the simultaneous PDS spectrum of this cluster shows a tail extending to at least 50\,keV. The hard component cannot be due to a population of point sources like AGN but must be due to a diffuse non-thermal component. Possible scenarios for this component are discussed.

Finally the BeppoSAX data on other clusters of galaxies are reviewed, among these some intriguing results on two distant clusters.

X-ray constraints on cluster magnetic fields

Formation of localized strongly magnetized regions in galaxies andgalaxy clusters

Formation of localized strongly magnetized regions in galaxies andgalaxy clusters Through three-dimensional global MHD simulations, we show that localized strongly magnetized magnetic flux tubes are created in galaxies and clusters of galaxies. Magnetic reconnection taking place inside the flux tube or by the interaction between the flux tube and external magnetic fields heat up the plasma typically up to 10KeV. The magnetic flux tubes can also constrict the hot X-ray emitting plasma. We carried out simulations of (1) dynamical evolution of magnetic fields in differentailly rotating disks and (2) amplification of magnetic fields through the rotation and motions of individual galaxies in clusters of galaxies. In either case, we find that even though the mean magnetic energy is smaller than the thermal energy ($\beta=P_{gas}/ P_{mag} \ge 1$), magnetic pressure dominated, low-$\beta$ ($\beta \le 1$) filaments appear and remain in low-$\beta$ state for time scale longer than the dynamical time-scale.

The first detection of X-rays from a nova shell: the shell of nova Persei 1901 (GK Per)

Thermal emission from SNRs and plasma diagnostics

We present here the results of SNRs showing thin thermal emission. The X-ray spectra from SNRs showing thin thermal emission components give us a good diagnostics of the plasma. Generally, they cannot be fitted by using model spectra in collisional ionization equilibrium. Some of them show gradients both in the temperature and in the abundance. The X-ray spectrum of young SNRs, like Kepler's SNR, can be well fitted by the superposition of the fore shock component and the reverse shock component, both of which show gradients in various parameters: temperature, density, ionization parameter etc. Rosat observation on the Vela SNR revealed that there were several debris running over the shell which was formed by the strong shock transferred in the surrounding medium. They appear in cusp shapes. Asca confirmed the high metal abundance with abundance anomaly in the debris suggesting that they were the fossil of the progenitor star. The Cygnus Loop, an older SNR, also showed the gradients, the shell regions show low metal abundance and low temperature while the core region shows high metal abundance and high temperature. Based on the gradients of various SNRs, we will also report the mixture evolution of the hot gas inside SNRs.

ASCA observation of two SNRS and NEI analysises

We present nonequilibrium ionization(NEI) analysises of X-ray spectral data from ASCA for the SNRs Kes79 and W49B.The spectrum of Kes79 is well described by a two NEI component model with low temperature about 0.5Kev and high temperature about 2.0Kev.The abundances of Mg,Si,S are all above cosmic.We attribute the low temperature component to the reverse shock with less $n_{e}$t but larger emission measure and larger heavy element abundances,and attribute the high temperature component to the blast wave with larger $n_{e}$t but less emission measure and less heavy element abundances.The center-semi-ring -brighten morphology of Kes79 is also discussed.We prefer that Kes79 is the remnant of a massive star.The spectra of W49B are well described by a NEI model(Chevalier) with temperature about 1.1Kev and the abundances of Al,Si,S,Ar ,Ca,Fe,Ni are all over abundant of 4.5-7.5 times.The derived temperature is smaller than previous value.We also get the age of W49B from the spectra fitting.It's about 2000yr.We suggest the possibility that W49B may be the candidate remnant of the ancient SN occurred in BC4,AD389 or AD588. The spectra of W49B also show a weak emission in about 5.56Kev. We think it may be the emission line of Cr,so W49B may be the first SNR which has Cr line in X-ray band.Combined the images of SIS with the images of Einstein HRI,ROSAT HRI,the "two-core" morphology of W49B is also discussed.The progenito r of W49B is also discussed.

X-ray spectroscopy of stellar coronal sources with BeppoSAX

The {\it Beppo}SAX satellite is providing new important results on stellar coronae thanks to its unique capabilities. The LECS and MECS instruments on board have obtained excellent spectra over the range 0.1 to 10 keV for a number of relatively bright coronal sources, including RS CVn binaries, dMe flare stars and very young, rapidly rotating stars. These data allow derivation of physical quantities such as temperatures, emission measure distributions and elemental abundances during both quiescent and flaring conditions. The most significant result obtained by {\it Beppo}SAX has been the detection of hard ($>$20 keV) X-ray emission from flares on Algol, UX Arietis and AB Doradus. This is the first time that hard X-ray emission has been detected in normal stars other than the Sun and is a unique result made possible by the broadband response and high sensitivity of {\it Beppo}SAX. The nature of this emission (either thermal or non-thermal), and whether it occurs only during flares or also during relatively quiescent periods after major flares, are some of the key questions that need to be addressed. We will report on these new exciting results with emphasis on our own observations of UX Arietis and AB Doradus, and on plans for future observations of HR1099 and other active stars.

X-ray study of isolated neutron stars (Solicited Paper)

Observational details on the measure of the cyclotron line properties as a function of pulse phase will also be reported for some of the pulsars in the sample. The search for single neutron stars at X-ray energies has brought to light three different classes of objects: (1) In total 30 rotation-powered could be detected whose emission is mainly non-thermal. The X-ray luminosities of these objects scale with the rotational energy loss $L_x (0.1-2.4\; \mbox{keV}) \sim 10^{-3}\;\dot{E}$. Three of these obje1cts show an additional soft thermal component, roughly consistent with photospheric emission from a cooling neutron star. (2) Three point sources in supernova remnants have been found whose spectra and luminosities are roughly consistent with photospheric emission as well. The temperatures and luminosities of all six candidates for photospheric emission are in coarse agreement with the prediction of standard neutron star cooling models. (3) In the third category are three field objects showing very soft X-ray emission and extremely faint optical counterparts. They are candidates for neutron stars accreting matter from the interstellar medium.

Particle acceleration in SNRS

Non thermal X-rays have been detected from shell-like SNRs, SN1006, G347.5+0.5, IC443, G156.2+5.6 and Cas A. Furthermore the ASCA Galactic Survey Projects revealed some extended sources with possibly non-thermal emission on the Galactic plane. Thus much evidence for synchrotron emission from the shell of SNRs, which supports the Fermi acceleration of electrons to extremely high energies, has been stimulating. The answer to the question how high energy can electrons be accelerated under the competing process of energy loss and escape, is largely related to the physical parameters of the shell, particularly on the magnetic field.

The detection of TeV gamma-rays from SN1006 provides further evidence for an extremely high energy particles. Coupled to the synchrotron X-rays, TeV gamma ray can be uniquely attributed to the 3K photons with Inverse Compton scattering. Then we can estimate electron energy distribution and magnetic field. Together with the age and scale of the SNR shell, we can discuss realistic acceleration scenario of electrons, hence protons, to an extreme relativistic energies. These are highly related to a long standing problem of the cosmic ray origin and acceleration.

Non-thermal features in borad band X-ray spectra of galactic supernova remants

One of the most salient features of the BeppoSAX X-ray satellite is its broad photon energy coverage from about 0.1 to 300 keV. In this paper we focus on the broad band X-ray spectra of supernova remnants with emphasis on the hard X-ray tails, since this has recently become a topic of great interest. In particular the wide band spectrum of the young remnant Cassiopeia A (Cas A) will be discussed (exposure $\sim$ 100ks). This spectrum of Cas A reveals the presence of a hard tail up to at least 80~keV, which is at variance with the generally accepted thermal origin of Cas A's X-ray emission. We shall discuss the implications of this hard tail on the thermal model and place it in the context of X-ray emission from other supernova remnants, notably SN1006, RCW 86 and Tycho. Finally we will come with stringent limits on the emission from radio-active $^{44}$Ti at 68 and 78~keV, the gamma-ray signature of which was found by GRO-Comptel at 1.157~MeV.

TeV gamma ray results and CANGAROO project

The CANGAROO Collaboration has searched for TeV gamma ray emission from a number of objects, such as pulsars, supernova remnants and active galactic nuclei, with a ground-based imaging \v Cerenkov telescope located in South Australia. Several galactic sources of synchrotron X-rays have been found to be TeV gamma ray emitters of inverse Compton radiation, allowing constraints to be put on the energy of progenitor electrons and the strength of magnetic field in pulsar nebula and supernova remnant. A summary of TeV gamma ray observations by CANGAROO is presented as well as the current status and efforts towards improving the sensitivity of TeV gamma ray detectors.

Radio to X-ray to gamma-ray emission from shell-type supernova

remnants

If supernova remnants (SNRs) produce galactic cosmic rays by diffusive shock acceleration, the energetic particles produce gamma-rays and lower energy photons via interactions with the ambient plasma. We present results from a non-linear shock structure and acceleration model which includes the dynamical influence of the accelerated ions on the shocked plasma and the injection of thermal electrons. We predict ion (proton and helium) and electron distributions that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron emission, yielding broad-band photon spectra from radio frequencies to gamma-ray energies. These spectra relate X-ray emission to all other wavelengths. Our emission spectra are consistent with Whipple's TeV upper limits on those EGRET unidentified sources that have SNR associations, and suggest that sources in low density regions will be $\gamma$-ray dim at GeV energies. Our study connects shock wave plasma properties with the $e/p$ ratio and, therefore, the X-ray to $\gamma$-ray ratio and affords new diagnostic capabilities for probing shell-type SNRs and diffusive acceleration.

Jet interaction of SS 433 with the ambient medium

Broad Fe-K lines from Seyfert galaxies

ASCA X-ray spectra of many Seyfert galaxies show that the iron K fluorescence line is broad. In the best observed cases it is also skewed to lower energies. This profile is indicative of matter in an accretion disk at a few Schwarzschild radii around the central black hole. These and new results from long observations with ASCA and RXTE will be discussed and compared with the profile expected from matter around a spinning black hole and within the marginally stable orbit of a non-spinning one. Finally, prospects for future instruments will be outlined.

The Fe K-alpha line profile in reflection dominated spectra of active galactic nuclei

We present the combined results of detailed transfer calculations and Monte Carl o simulations showing the importance of multiple reflections and Compton scatter ings on/in a photoionized plasma for the interpretation of the iron K line compl ex observed in many AGN. Assuming a high covering factor of the reprocessing plasma, the mean profile of Seyfert 1 galaxies can be reproduced without invoking gravitational or transvers e Doppler shifts if the constraint on the position of the peak is accounted for by either an outflow velocity or a contribution to the line from more weakly ion ized material. The ``amplification'' of the primary radiation in closed spherical geometry favo rs higher ionization than deduced from the observed primary. Compton scattering deeply affects the multiple-reflected spectra in the 1-50 keV band and the determination of the continuum is complicated on the blue side by recombination continua on highly ionized iron, and on the red side by the influe nce of the edges of other elements. High ionization leads to a significantly broadened line, without producing a str ong hard energy tail around 20 keV. The expected variability of the line profile in the context of this model will also be presented.

Probing general relativity effects near stellar mass black holes with X-ray observations

We review recent progress in probing general relativity effects near stellar mas s black holes in X-ray binaries. From the observed multicolor blackbody spectra in several dyn amically established black hole X-ray binaries (BHXBs) with the compact object in excess of 3 solar masses, we have estimated the angular momentum of these black holes (Zhang, Cui and Chen 1997, ApJ, 482, L155). The angular momentum of the black hole in the superluminal jet source GROJ1655-40 is about 0.93 of the maximum value allowed in the Kerr Metric . The black holes in several other radio quiet BHXBs all appear to have values consistent with zero for their angular momentum. We propose that Cyg X-1 contains a modestly spi nning black hole surrounded by a retrograde accretion disk in its regular hard/low sta te. The soft state transition is caused by a temporary flip-over of the accretion disk to a p rograde mode, thus causing the inner disk boundary much closer to the black hole horizon, givi ng rise to the enhanced soft X-ray emission. We also propose that many of the BHXBs without a soft X-ray component in their energy spectra are in the retrograde con figuration permanently. We further studied the high frequency quasi-periodic oscillations (QPOs) observed in the two galactic superluminal jet sources GROJ1655-40 (300 Hz) and GRS1915+105 (67 Hz) and concluded that these QPOs are most likely due to the pre cession of the inner accretion disk region caused by the general relativistic frame-drag ging effect (Cui, Zhang and Chen 1998, ApJ, 492, L53). The consistency between the spectrosc opic and timing studies of the black hole in GRS1915+105 suggest that it also contain s a maximal Kerr black hole with a mass around 30 times the Sun, possibly the most m assive one among all known stellar mass black holes. The absence of similar high freque ncy QPOs in the X-ray light curves of other BHXBs also suggest that they contain slowly or non-spinning black holes, in agreement with the conclusion we arrived from indep endent spectroscopic observations.

We also studied the evolution of black hole angular momentum in BHXBs (Chen, Cui and Zhang 1998, in preparation). Accretion can in principle change the angular momen tum of a black hole. However, our study shows that for all known BHXBs, the evolutio n of their angular momentum can be practically ignored. This is because for low-mass BHXBs, they are all transients and significant angular momentum transfer only occurs during their very rare outbursts, since their quiescent state accretion disk is most likely advect ion dominated so that very little angular momentum transfer to the black hole takes place. For these high mass (persistent) BHXBs, their life-times are too short to transfer sufficient a ngular momentum to the black hole. Therefore the black hole angular momentum distribution we obs erve today is very close to that when these black holes were born. This means that a black hole may be born as either a Schwarzchild or a Kerr black hole and that both retrograde and prograde systems should exist.

EUV and X-ray properties of AGN found in the broad band simultanteous observations

Radiation from AGN extends over broad wavebands from radio through gamma-ray. At the shortest wavelength side, X-rays are produced by inverse-Compton process between high energy electrons and soft photons. On the other hand, the optical-UV band is dominated by the thermal radiation from the accretion disk around the central black hole. We have performed real simultaneous observations of the bright Seyfert I galaxy NGC 5548 with EUVE and ASCA in 1996 for about 10 days. In both bands, intensity varied by a factor of two in the timescale of days. However, a rapid dip of 40 \% decrease was discovered with a time scale of 20000 sec only in the EUV data. There was no X-ray variation more than 5 \%. Though UV has been thought to be radiated from the hot accretion disk illuminated by X-rays from the central source, this UV dip suggests the reversed process (seed photons) where the variation of the UV seed photons could be smeared to produce inverse Compton scattered X-ray photons. Our approach with the simultaneous 'broad band' observations is crucial to investigate the emission mechanisms and structure of AGN.

High energy continua of BHCs and Seyferts

I will review X-ray and soft $\gamma$-ray spectra of BHCs in both soft and hard state and of Seyferts. The spectra of BHCs in the hard state and of Seyferts are very similar to each other and contain components due to thermal Comptonization and Compton reflection, as well as soft X-ray excesses. The spectra of BHCs in the soft state contain a strong disk blackbody component and a high energy tail. The tail appears to be mostly due to nonthermal Compton scattering with an addtional component due to Compton reflection. Physical interpretation of these results involves hot and cold accretion disks, outflows, and coronae.

X-ray emissions from Seyfert 2 galaxies having polarized broad lines

We observed four Seyfert 2 galaxies having polarized broad lines, Mkn 1210, Mkn477, NGC 7212, and Was 49b. All of the galaxies were detected with the X-ray satellite ASCA. In the 2--10keV band, we found heavily absorbed X-ray emission from these galaxies. The absorbing column density is about 10$^{23}$ cm$^{-2}$. The X-ray luminosity ranges from 10$^{42}$ to 10$^{43}$ erg s$^{-1}$ in the 2 -- 10 keV band. Since Ginga and ASCA PV observations revealed obscured X-ray emissions from other high polarization Seyfert 2 galaxies(e.g. NGC1068), our results suggested a good correlation between X-ray observation and spectropolarimetry.

We also detected the soft X-ray emissions, and found that the flux ratio to the obscured hard component was about 10$\%$, which is larger than those for Seyfert 2 galaxies for which existence of polarized broad lines have not been reported. We will discuss about the origin of the soft component.

The BeppoSAX view of Compton-thick Seyfert 2 galaxies

We present BeppoSAX results on a couple of Compton-thick Seyfert 2 galaxies: NGC1068 and Circinus Galaxy. Data of the former are presented from two pointings, separated by about one year. Unprecedented high energy sensitivity (3 $\sigma$ detection of a mCrab source above 20~keV in 50 ks) has allowed to put firm constraints on the nature of the reprocessing ongoing in the nuclear environment. A ``cold'' reprocesser ({\it e.g.} the molecular torus which is supposed to surround the Seyfert 1-like nucleus according to the unified scenario) is strongly favoured in NGC1068. In the meanwhile, the broadband (0.1-200~keV) coverage and good LECS energy resolution below 1~keV have allowed to study in detail the soft X--ray spectrum. The first deep (exposure time $\sim 150 \ ks$) measure of the huge ($EW \sim 1 \ keV$) iron line in Circinus after its recent discovery is also presented and discussed.

The broad band spectra of Seyfert 2 galaxies:

BeppoSAX observations of a smaple of 5 galaxies

Recent observational constraints on the warm absorber in AGN

Among the most prominent observational signatures of the partly ionized component in active galactic nuclei (AGN), the so-called warm absorber, are spectral absorption features detected in the keV region. These features provide important diagnostics on the location and nature of the material surrounding the active nucleus. The Low Energy Concentrator Spectrometer (LECS) on board the BeppoSAX satellite operates in the range 0.1--10 keV. Because of its good low-energy resolution and effective area down to 0.1 keV it is a very efficient tool for the study of the complex spectral features due to warm absorbers, particularly when used in combination with the other narrow field instruments on BeppoSAX, sensitive at higher energies. We will present here new constraints on the physical state of the warm absorber in AGN brought by BeppoSAX/LECS observations as well as by recent results from other X-ray observatories.

The cosmic X-ray background and the identification of its constituents

In this presentation I review the measurements and understanding of the X-ray background (XRB). I describe the progress from ROSAT deep surveys and optical identifications of the faint X-ray source population, in particular the role that active galactic nuclei (AGNs) play as dominant contributors for the XRB. So far there is no need to postulate a hypothesized new population of X-ray

sources. The recent advances in the understanding of X-ray spectra, and in particular X-ray absorption, led to a population synthesis model which is so far the most promising to explain all observational constraints, however, with large uncertainties. AGN are strongly evolving and a new determination of the X-ray luminosity function indicates that there are many more active galactic nuclei at large redshifts than anticipated previously, producing a substantial fraction of the X-ray background. Only sensitive X-ray surveys in harder energy bands, will be able to unambiguously disentangle X-ray absorption and evolutioneffects and provide much tighter constraints for the origin of the XRB.

We have carried out the first systematic wide-area survey around a north Galactic pole region with the \ASCA\ satellite in the 0.7--10 keV energy band (Large Sky Survey; LSS). From the entire field of 7 deg$^2$ in three energy bands (0.7--2 keV, 2--10 keV, and 0.7--7 keV), we detected 104 sources in total with a sensitivity limit of $6 \times 10^{-14}$ erg s$^{-1}$ cm$^{-2}$ (2--10 keV). Derived Log $N$ - Log $S$ relation in the 2--10 keV band lies on the extrapolation from the previous results obtained with {\it Ginga} and {\it HEAO-1} with the Euclidean slope of $-3/2$. The average spectrum of the sources with fluxes less than $ 4 \times 10^{-13}$ erg s$^{-1}$ cm$^{-2}$ shows a photon index of 1.5$\pm$0.2 in the 2--10 keV band, suggesting that the ``spectral paradox'' is beginning to be solved. Our results imply that a population of sources with hard energy spectra, which is the key to solve the origin of the CXB, comes to dominate the X-ray sky above 2 keV.

The nature of the faintest X-ray sources

We outline the results of a deep ROSAT/ASCA X-ray survey, the aim of which was to determine the nature of the faintest X-ray sources and the origin of the Cosmic X-ray Background (XRB). While ordinary, broad-line QSOs can account for up to $50 \% $ of the XRB in the ROSAT passband, their X-ray spectra are too steep to explain the XRB at harder energies.

We present evidence that a population of emission-line galaxies are emerging at the faintest X-ray fluxes with harder X-ray spectra than ordinary QSOs. Despite recent claims by Hasinger et al, we argue that many of these remain real identifications. We present evidence from optical and infra-red spectroscopy which suggests that a hybrid explanation is required, consisting of obscured/Type 2 AGN surrounded by starburst activity. This model can explain the properties of these galaxies and perhaps the origin of the entire XRB.

I will present a progress report on the BeppoSAX Core Program project dealing with X-ray weak Seyfert 2 galaxies. The aim of the project is to increase the sample of Seyfert 2 galaxies with measured hard X-ray spectra, by adding progressively weaker objects within a well-defined optical sample. The hard X-ray information, together with the [OIII] and IR fluxes, the orientation of the host galaxy and the geometry of the ionization cone, should give the distribution of the absorbing column N$_H$, and assess the reliability of various proposed ``isotropic'' indicators of the luminosity. We have up to now a 100\% detection rate, and find systematically low X/[OIII] ratio, high N$_H$, and high Fe EW. While the first finding is a selection bias, the remaining two support strongly the unified scenario, and point to a fraction of X-ray thick sources higher than usually assumed.

Evolution of AGNs and a model of the X-ray background

New ROSAT surveys and optical identifications of the X-ray sources, with unprecedented completeness over a large range of fluxes, revealed a strong number evolution of luminous AGNs soft X-ray luminosity function, while weaker or no evolution for less luminous AGNs. These results are significantly different from previous results by a few groups, reporting a pure luminosity evolution as the best-fit model.

Based on the new data, we construct a new baseline population synthesis model of the Cosmic X-ray background (XRB), which consists of unabsorbed and self-absorbed X-ray emitting AGNs with a luminosty-dependent evolution found in the new ROSAT surveys. The model is made consistent with various observational constarints avalable with ROSAT, ASCA and other surveys, e.g., the total spectrum of the XRB, the luminosity function, $\log N-\log S$ relations in the soft (0.5-2 keV) and hard (2-10 keV) bands, distribution of self-absorption column densities for a sample of X-ray selected AGNs. We also discuss the implications to auto-/cross- correlation properties of the XRB.

AGN populations, evolution and AXAF

A sample of persistent and transient X-ray pulsars was observed by BeppoSAX during the Science Verification Phase and the AO-1 program. The telescopes on board BeppoSAX offer an unprecedented broad band coverage >from 0.1 to 300 keV. The good sensitivity of the high energy instruments allowed a detailed investigation on the presence and on the properties of the cyclotron lines in the X-ray pulsar spectra. The capability of BeppoSAX to provide an accurate measurement of the underlying continuum below and above the cyclotron line energy proved to be an essential tool to avoid ambiguities in the cyclotron line detections and measurements.

The observed cyclotron lines, their implication on the available emission models and some insights into the complex and fascinating physics of plasma in relativistic magnetic fields will be discussed.

Observational details on the measure of the cyclotron line properties as a function of pulse phase will also be reported for some of the pulsars in the sample.

The BeppoSAX HELLAS survey: resolving the hard X-ray background

BeppoSAX founds about 20 sources per square degree in the band 5-10 keV at a flux limit of 7E-14 cgs. This accounts for 35 to 48 \% of the 5-10 keV XRB. The BeppoSAX High Energy LLarge Area Survey (HELLAS) covers this new 5-10~keV band and has cataloged 150 sources over 50~square degrees down to the above flux limit. The discovery of this population of hard sources, many of them detected "only" above 5 keV, supports the view that a major contribution to the hard X-ray background comes from heavily cutoff objects, as predicted in the AGNs synthesis models (e.g. Comastri et al. 1995, AA 296,1). We will present results on the number density and X-ray spectrum of the HELLAS ources. We will also report on the optical identification of the first 8 HELLAS 5-10 keV sources. We have found 3 normal blue quasars, 2 type 1.9 AGNs, 2 `red' quasars and 1 LINER. We then conclude that the hard X-ray background is mostly made by AGN. The fraction of AGN showing evidence of absorption in X-ra and optical (more than 50 %) is much higher than in ROSAT PSPC or ASCA-ROSAT surveys, providing for the first time strong experimental support to the scenario in which the bulk of the hard XRB is made by highly obscured AGN, emerging only at high energies.

Results from ASCA X-ray observations of high-redshift radio-quiet quasars

Here we report the X-ray results for a sample of 5 high-redshift radio-quiet quasars observed by ASCA. Differently from the high-z radio-loud quasars, the situation concerning the high-redshift radio-quiet quasars is far less clear. For two quasars of our sample, Q1101-264 (z=2.15) and C25.36 (z=2.30) we have obtained a typical X-ray spectrum with a spectral slope $\Gamma$ $\sim$ 1.8$\pm{0.2}$, whereas for WEE 83 (z=2.04), Q0040+0034 (z=2.0) and Q1352-2242 (z=2.0) the spectral slope seems flatter. Neither a reflection component nor an excess of absorption above the galactic value is required. A marginal ($\sim$ 95 \% confidence level) evidence of a {\rm Fe}$K\alpha$ emission line has been found in the spectrum of Q1101-264. Because of their weak detection, for the other three quasars of our original sample, Q1559+089 (z=2.27), Q1725+503 (z=2.10) and Q1726+504 (z=1.90), we have only flux upper limits. A comparison between the high-redshift radio-quiet quasars and the radio-loud ones (Cappi et al. 1997, ApJ 478, 492) is also presented.

Broad-band spectra of survey sources with ASCA and predictions for XMM

(i) The log(N) - log(S) number-flux relation for ASCA and GINGA sources has a factor of two higher normalization than the ROSAT relation, and this is accounted for by NXLG/Type II AGN. Unidentified ASCA sources are more likely to be NXLG/Type II AGN than Type I AGN.

(ii) Within the limitations of the statistics of the current source detections, the effective spectral index (2-10 keV) of the Type I AGN is $1.78\pm0.16$, steeper than the XRB spectral slope of $ 1.4 - 1.5$ in ASCA data (Gendreau et al. 1995) but consistent with the ROSAT spectra and the canonical value for Seyferts. The spectral slope of the NXLG/Type II AGN and unidentified sources may be harde r, however, $\sim 0.92\pm0.16$ and these may explain the overall XRB slope (Georgantopoulos et al. 1997).

(iii) Simulations are shown of the response of XMM to Type I AGN and NXLG/Type II Seyferts, and the contributions of these sources to the XRB are shown as a function of energy, for various values of their evolutionary parameters.

Broad band X-ray spectrum of the burster 1E1724-3045 observed with RXTE

The X-ray burster 1E1724-3045 located in the globular cluster Terzan 2 is known as one of the persistent (though variable) hard X-ray sources of the Galactic center region, as shown by SIGMA. 1E1724-3045 was observed with the PCA and HEXTE experiments aboard the {\it Rossi X-ray Timing Explorer} (RXTE) on November 1996 for about 100 ksec. The unprecedented broad band spectral capability of RXTE enables for the first time to study simultaneously the X-ray (2-20 keV) and hard X-ray components (E$>$20 keV) of this source. In this paper, we report on the results of a detailed spectral analysis which combines both the PCA and HEXTE data. First we present the fitting of its 2-200 keV spectrum and compare the RXTE and SIGMA data. Second, we compare the spectral characteristics of 1E1724-3045 with those of black hole candidates (when observed in their low luminosity states) both in terms of luminosities and spectral shapes of the continuum spectrum.

High energy emission from active pulsars

Nonthermal emission is seen from some pulsars across much of the electromagnetic spectrum, implying acceleration of particles to high energies. The power for this process comes from the rotation of the magnetized neutron star. The luminosity of these pulsars peaks in the hard X-ray to hard gamma-ray bands. The light curves suggest particle acceleration and radiation from a hollow surface above one magnetic pole of the star. Multiwavelength observations have been used to construct and test models of high-energy pulsar radiation.

Broad-band study of non-thermal pulsed emission from

PSR J0218+4232 and PSR B1821-24

In a 100 ks ROSAT HRI observation we confirmed our earlier indication for pulsed X-ray emission from the binary ms-pulsar PSR J0218+4232. The pulsar light curve exhibits two narrow peaks, similar to that of the isolated ms-pulsar PSR B1821-24. This makes PSR J0218+4232 the fourth ms-pulsar for which to date modulation at the pulse-period is observed. Of these four, PSR J0437-4715 and PSR J2124-3358 show broad and smooth X-ray light curves and have luminosities in the ROSAT 0. 1-2.4 keV band about 3 orders of magnitude lower than PSR J0218+4232 and PSR B1821-24, which are the youngest (by 1-2 orders of magnitude) ms-pulsars detected s ofar.In an earlier publication we also reported the likely detection of PSR J0218+423 2 with EGRET at high-energy $\gamma$-rays. Further timing and spatial analyses strengthen this conclusion. Furthermore, also PSR B1821-24 is located close to an EGRET source. Broad band studies of these objects from the radio domain up to high-energy $\gamma$-rays are crucial in understanding the production of non-ther mal magnetospheric emission from these ms-pulsars for which the magnetic field strengths are orders of magnitude weaker than for normal rotational pulsars. We will present the experimental results from radio to $\gamma$-ray energies and discuss these in the context of the most recent models invoked for explaining the n on-thermal emission.

Galactic positron annihilation radiation

Positron annihilation radiation from the galactic center region was discovered 25 years ago. Several potential sources may contribute to this emission, including: (1) $\beta$$^{+}$-decay of nuclei produced in explosive objects (supernovae and novae); (2) $\gamma$-$\gamma$ pairproduction associated with the accretion of matter onto galactic black hole candidates; (3) positrons produced in pair cascades in neutron star magnetospheres; (4) $\beta$$^{+}$-decay of radioactive nuclei synthesized in Wolf-Rayet and AGB stars and carried in their stellar winds; and (5) positrons produced by interactions of cosmic rays with the interstellar medium. Early evidence for variability of the galactic center emission suggested that discrete sources dominated the emission. However, more recent SMM, CGRO and TGRS/WIND results indicate that the dominant emission is steady and of diffuse origin. There is evidence for transient positron production associated with galactic blackhole candidates, however the non-detection of such emission by CGRO/OSSE suggests this evidence is not compelling. The OSSE instrument on CGRO has provided the first maps of the emission, and the discovery of a surprising excess at positive galactic latitudes above the center of the Galaxy. The history of these observations will be covered, along with the latest data available from CGRO and TGRS. We will also discuss the implications of the observations on our understanding of galactic nucleosynthesis, galactic distributions of supernovae and novae, and the study of the interstellar medium.

ASCA measurements of field-particle energy distribution in radio lobes

The excellent hard X-ray sesitivity and imaging capability of ASCA allow us to detect inverse-Compton (IC) X-rays from lobes of radio galaxies. Comparing the IC X-ray flux with the synchrotron radio flux, we sorted out electron energy density and magnetic field intensity in the Fornax A radio lobes (Kaneda et al. 1995; Feigelson et al. 1995). Following the first crop, we conducted a series of ASCA observation of radio lobes, PKS~1343$-$601 (Centaurus B) and NGC 612. The bright X-rays from PKS~1343$-$601 strongly suggest particle domination in the lobes. Meanwhile, X-ray and radio image analysis shows an outward increase in the magnetic energy. This suggests a gradual compression of magnetic fields toward the edge of lobes. On the other hand, the X-ray image and spectrum observed from NGC 612 reveals a heavily absorbed low luminosity core emission with extended emission associated with the lobes. The extended source emits 3 $\times 10^{-8}$ Jy at 1 keV, which is rather close to the expected value assuming equipartition state, although we cannot exclude the possibility of particle dominant lobes.

Non thermal emission from relativistic jets in AGNs: advances from BeppoSAX observations

Recent advances in our understanding of the non thermal emission processes occurring in relativistic jets formed in Active Galactic Nucle will be reviewed. Particular emphasis will be given to simultaneous observations of spectra in the soft, medium and hard X-ray bands made possible by the capabilities of SAX as well as to simultaneous observations in other wavebands. In particular, results on three X-ray bright BL Lac objects, Mrk 501, Mrk 421 and PKS 2155-304, obsreved by SAX during AO1, simultaneously with the Compton Gamma-Ray Observatory and with ground based TeV telescopes will be presented. The theoretical implications will be briefly discussed.

X-ray observations of TEV blazars with ASCA

We present results of the ASCA observation of TeV blazars. The discovery of the TeV emissions from three nearby BL Lac objects has given us an opportunity to study the origin of the electromagnetic radiation endending upto 10$^{12}$ eV (TeV). In addition to the multiband spectrum, the relationship of the variability patterns obtained from one band to another provides information as to the relationship of the physical processes responsible for the emission in these bands. The 1994 and 1995 multi-frequency observations of Mkn421 resulted in the detection of a simultaneous keV/TeV flare by ASCA and the Whipple Observatory. This suggests that both keV and TeV spectral regimes are produced by the same, most energetic end of the electron population, radiating via the synchrotron process in the keV, and Compt on (most likely SSC) process in TeV band. In this presentation, we discuss the implication of the spectral evolution observed during the flare and the correlation between X-rays and TeV gamma-rays. The first result of a continuous 7.5 day observation of Mkn421 with ASCA, as a part of multiband campaign in April 1998, will also be reported on behalf of the Mkn421 collaboration.

X-ray variability in BL Lac objects

The X-ray spectra of BL Lac objects are classified into two categories. The one is those of smoothly connected from radio to X-rays, which is thought to be synchrotron radiation. The other is of flatter X-ray spectra than those in optical and UV bands, which examples are BL Lacertae and OJ 287. X-rays >from bright BL Lac objects such as Mkn 421 and PKS 2155-304 exhibit correlated flux variation with spectral index and soft lag. The spectra become flatter with increasing flux and the variation of soft X-rays delay from that of hard X-rays. It is shown that these chacteristic properties can be reproduced by solving time dependent transport equation of synchrotron-emitting high energy electrons. The index-flux correlation and soft lag can be quantitatively interpreted by synchrotron cooling and magnetic field strength are derived. However, it should be noted that these can also be interpreted by energy dependent diffusive escape of electrons from radiating region. The general solution of the time dependent equation make it possible to distinguish from each other by applying this model to observed light curves. The electron injection functions can be determined too, which make enable to discuss acceleration of electrons in the sources. The sources with flatter X-ray spectra are also discussed in connection with synchrotron sources.

Cyclotron line studies of X-ray pulsars (Solicited Paper)

A sample of persistent and transient X-ray pulsars was observed by BeppoSAX during the Science Verification Phase and the AO-1 program. The telescopes on board BeppoSAX offer an unprecedented broad band coverage >from 0.1 to 300 keV. The good sensitivity of the high energy instruments allowed a detailed investigation on the presence and on the properties of the cyclotron lines in the X-ray pulsar spectra. The capability of BeppoSAX to provide an accurate measurement of the underlying continuum below and above the cyclotron line energy proved to be an essential tool to avoid ambiguities in the cyclotron line detections and measurements.

Magnetic fields of binary X-ray pulsars

We systematically analyzed the spectra of X-ray binary pulsars observed with GINGA. We detected cyclotron structures from 12 pulsars, about a half of the observed 23 sources. The magnetic fields were obtained in the range of 4x10^11 - 5x10^12 G, whose distribution was shown for the first time. It is similar to that of radio pulsars. The resonance energy was observed to change with the luminosity. The double harmonic cyclotron structures of 4U 0115+63 in 1990 changed to a single structure in 1991 as the luminosity decreased to 1/6. The resonance energy increased by 40 % at the same time. If this change is caused by the height change of the scattering region in a dipole magnetic field, it is as much as about 1.1 km. Such changes of the resonance energy with luminosity from 5 pulsars are successfully explained by the accretion column height model.

Studies of binary sources with the Indian X-ray Astronomy Instrument

An Indian X-ray Astronomy Experiment (IXAE) consisting of 3 multi-wire proportional counters with field of view of $2.3^\circ \times 2.3^\circ$ and an effective area of 1200 cm$^2$ onboard the Indian satellite IRS-P3 has been in operation since the middle of 1996. Observations of several X-ray binaries including Cyg X-1, GRS 1915+105, Cyg X-2, 4U1907+09, Cir X-1 etc have been made in 5 months of observing time made available so far for this instrument. Many new and interesting results on several binary sources have been obtained from these observations. These include : (a) Rapid variability of Cyg X-1 in both the `soft' and the `hard' states from the power density spectra obtained for the two states, (b) Detection of a unique kind of quasi-regular X-ray bursts from the black hole candidate GRS 1915+105 which have slow rise and fast decay. The rapid decay may be indicative of the disappearance of the accreting matter behind the event horizon of the black hole in GRS 1915+105, (c) Pulsation characteristics of 4U1907+09, (d) Chaotic variability of Cir X-1. Details of these results and their implications for the models of these sources will be discussed in this paper.

A wide field X-ray survey of the galactic bulge region

A long monitoring campaign on the Galactic Bulge region with the Wide Field Camera's have been carried out with BeppoSAX. Various new X-ray transients have been found and will be discussed. In addition, the large Field of View of $40^{o}\times 40^{o}$, full width zero response, makes the intrument very suitable to detect type-I X-ray bursts in X-ray binaries. These bursts identify the compact object as a neutron star. This way a significant increase in the total number of neutron-star Low Mass X-ray Binaries have been found, including (1) new transients, (2) sources previously thought to contain black holes, (3) sources previously thought to be a High Mass X-ray Binary, (4) sources in globular clusters. I will discuss the status of this on going research and the implications it has (or will have in the near future) on the number distribution of X-ray binaries containing Neutron Stars versus Black Holes.