HEAO-1 and BBXRT non-source X-ray observations

A more appropriate title for this talk would have been “Measurements of Large Scale Structure from X-ray Background Fluctuations”. While it has long been recognized that the X-ray Background (XRB) is primarily of a cosmological origin (with z < a few), it has recently become apparent that surface brightness fluctuations in the surveys of the XRB can be used to trace the distribution of matter in much the same way as complete catalogs of individual objects. The distance which is probed is related to the angular resolution of the detector; for the HEAO-1 A2 experiment, which provides the best all-sky data base for the XRB in the 2–20 keV band, the effective depth is a few 100 Mpc.     

Simultaneous COMPTON GRO and optical monitoring of AGN

During two 14-day periods in January 1992 and March 1992, several AGN which have been in the wide field of view of COMPTEL and EGRET at that time were monitored optically for variations in intensity and polarization. We report on first results of this correlated optical and CGRO observations.     

High energy emission from AGN

The recent detection of radio loud quasars with a large high energy γ-ray luminosity is discussed, in the framework of non-thermal synchrotron self Compton models. These observations are strong evidence of relativistic beaming of the radiation, due to the absence of strong photon-photon opacity. If γ-rays are self Compton emission, these sources can be the first observed example of a Compton catastrophe, resulting from the dominance of the radiation over the magnetic energy density. Inhomogeneous and relativistic jet models can in this case explain the observed spectrum from the far infrared to the γ-rays. If, on the other hand, the magnetic energy density is dominant, then the γ-rays must be due to direct synchrotron emission, implying very large particle energies. Pure synchrotron models which take into account electron-positron pair production are then discussed. Multifrequency simultaneous observations are a powerful test of the models.     

Catalogue of ionized emission line spectra in obscured AGN

High-energy, high-resolution in the spatial and energy domains is crucial to derive the geometrical distribution and the physical properties of gas and dust surrounding Active Galactic Nuclei (AGN). These are, in turn, basic ingredients of any serious attempt to build an AGN structure model. We present in this paper CIELO-AGN, the first catalogue of ionized emission lines detected in the soft X-ray spectra of a sample of 69 nearby obscured AGN. This catalogue has been implemented according to an IVOA data model, and can be accessed through VO-compatible applications.     

High-density and high-temperature molecular gas around the AGN in M51

We investigated the physical properties of molecular gas in the nuclear region of M51 (Seyfert 2). We obtained an aperture synthesis ¹³CO(J = 1 − 0) image using the Nobeyama Millimeter Array (NMA), and compared it with NMA ¹²CO(J = 1 − 0) and HCN(J = 1 − 0) maps at similar spatial resolutions. Within a radius of 180 pc from the center, the ¹³CO(1 − 0) integrated intensity was found to be 3 times weaker than that of HCN(1 − 0). Large-Velocity-Gradient (LVG) calculations suggest that the observed high HCN(1 − 0)/¹³CO(1 − 0) intensity ratio would arise from dense (n_H2 ∼ 10⁵ cm⁻³) and hot (T_kin ≳ 300 K) molecular clouds in the nuclear molecular disk. We also observed in the ¹²CO(1 − 0), (3 − 2), ¹³CO(1 − 0), and (3 − 2) lines using the Nobeyama 45m and JCMT 15m telescopes. We detected weak ¹³CO lines as well as strong ¹²CO lines. The LVG calculations assuming a two-component model suggest that there is a large amount of low-density (n_H2 ∼ 3 − 6 × 10² cm⁻³), low-temperature (T_kin ∼ 20 – 50 K) gas, and a small amount of high-density (n_H2 ≳ 10⁴ cm⁻³), high-temperature (T_kin ≳ 500 K) gas. The existence of the high-density and high-temperature component, although having a quite small beam filling factor, supports the aperture synthesis observation results mentioned above. Since this dense, hot gas is located in the nuclear molecular disk around the Active Galactic Nucleus (AGN), it may be heated by the strong X-ray radiation and/or by the shock induced by the radio jet.     

On ultra-high energy cosmic rays: Origin in AGN jets and transport in expanding universe

The cosmic ray source spectrum produced by AGN (active galactic nucleus) jets is calculated. A distinctive feature of these calculations is the account for the jet distribution on kinetic energy. The expected cosmic ray spectrum at the Earth is determined with the use of a simple numerical code which takes into account interactions of ultra-high energy protons and nuclei with the background radiation in an expanding universe.     

IUE and Einstein observations of supernovae

In this paper IUE observations of two type II and three type I Supernovae (see Table I and Table II) are reviewed. The type II events were also observed by Einstein and SN 1980k was detected in the soft X-ray energy band. Combining UV spectra available of the same events one has the elements to construct a physical picture which accounts for all observations. For the type II events it appears that the progenitor star was a massive supergiant which underwent a severe mass loss before becoming Supernova. For the type I events there is neither X-ray nor, for the time being, radio detection. The high similarity of the optical and UV spectra for the three events indicates that type I Supernovae are the end products of one and the same phenomenon.     

RHESSI and radio imaging observations of microflares

We present an analysis of five microflares, three observed simultaneously by RHESSI in hard X-rays and Nobeyama RadioHeliograph (NoRH) in microwaves (17 GHz) and two observed by RHESSI and Nancay RadioHeliograph (NRH) at metric wavelengths (150–450 MHz). Since we have no radio imaging telescopes simultaneously operating at microwave and meter wavelengths in the same time zone, we are obliged to use a different set of metric events in contrast to that used for comparison with the two radio wavelengths. We are interested in using the locations and other imaging characteristics of the events from both RHESSI and radio observations instead of just temporal correlation. So we have used the Nancay (France) metric radioheliograph at 150–450 MHz for this purpose. Here we describe the properties of five events – three in microwaves and two at metric wavelengths. We discuss the brightness temperatures, emission measures and the hard X-ray spectral properties of these microevents. One sees small (mini) flaring loops clearly in NoRH and RHESSI images. The microwave emission often seems to come from the RHESSI foot points (for higher energies), and from the entire small (mini) flaring loop (for lower energies).The RHESSI microflares seem to be associated in position with metric type III bursts. Frequently, the hard X-ray spectrum of the microwave associated RHESSI microflares can be fit by a thermal component at low energies (∼3–12 keV) and a nonthermal component at higher energies (∼12–20 keV).     

COMPTEL observations of AGNs

During the first part of the COMPTON Gamma Ray Observatory sky survey, COMPTEL has detected the quasars 3C273 and 3C279 and the radio galaxy Centaurus A. This paper summarizes the preliminary findings and gives an upper limit on the MeV flux of the Seyfert galaxy NGC4151.     

X-ray observations of Vela-X

Exosat ME observations of the Vela-X region are presented. It is found that the 2–10 keV emission is divided into two components. One is associated with the rulsar but is probably extended by 10–20 arc minutes, the other is associated with the Vela-X radio nebula and is probably more extended ($\text{? 1}\text{degree}$). The Vela-X component is softer than the pulsar component. No pulsed emission is observed.     

Voyager photopolarimeter observations of Saturn and Titan

The Voyager 2 photopolarimeter experiment observed the intensity and polarization of scattered sunlight from the atmospheres of Saturn and Titan in the near-UV at 2640 Å and in the near-IR at 7500 Å. Measurements of Saturn's limb brightening and polarization at several phase angles up to 70° indicate that a significant optical depth of UV absorbers are present in the top 100 mbar of Saturn's atmosphere in the Equatorial Zone and north polar region, and possibly at other latitudes as well. UV absorbers are prominent in polar regions, suggesting that charged particle precipitation from the magnetosphere may be important in their formation.The whole-body polarization of Titan is strongly positive in both the UV and near IR. If spherical particles are responsible for the polarization, no single size distribution or refractive index can account for the polarization at both wavelengths. The model atmosphere proposed by Tomasko and Smith [1], characterized by a gradient in particle size with altitude, seems capable of explaining the Voyager observations. If non-spherical particles predominate, the Voyager observations place important constraints on their scattering properties.     

Modelling and observations of photospheric magnetic helicity

Mounting observational evidence of the emergence of twisted magnetic flux tubes through the photosphere have now been published. Such flux tubes, formed by the solar dynamo and transported through the convection zone, eventually reach the solar atmosphere. Their accumulation in the solar corona leads to flares and coronal mass ejections. Since reconnections occur during the evolution of the flux tubes, the concepts of twist and magnetic stress become inappropriate. Magnetic helicity, as a well preserved quantity, in particular in plasma with high magnetic Reynolds number, is a more suitable physical quantity to use, even if reconnection is involved.     

ROSAT observations of galaxy clusters and cosmological implications

Clusters of galaxies are filled with a hot teneous plasma which is emitting soft X-rays over very extended regions. The ROSAT observatory with its high resolution and high sensitivity X-ray telescope allows to image nearby clusters in great detail and is therefore ideally suited to study the structure and morphology of galaxy clusters. In particular the distribution of the intracluster gas can be mapped, some information on the temperature structure of the gas can be obtained, the gravitational mass can be infered, and the dynamical state of the clusters can be investigated. Results of such studies for some nearby clusters as Virgo, Perseus and Coma are presented. Gas mass fractions of the total gravitational mass of the cluster in excess of 10 to 15% are infered from X-ray observations where the cluster emission can be traced out to an Abell radius (3 Mpc). If these values are taken to be representative for the matter composition in the Universe it has interesting consequences for cosmology as for example for the nucleosythesis models and the mean density of the Universe.     

IUE observations of cataclysmic variables

The four years of IUE operation have revealed that the ultraviolet region of the spectrum contributes a dominant share of the emerging energy from cataclysmic variables and provides important clues to the physical nature of these systems. The implications of the continuum flux distributions and line spectra for the determination of the accretion rates and mass loss rates are considered.     

Einstein observations of supernova remnants

Recent observations of SNRs have produced X-ray images with resolution comparable to that routinely achieved at optical wavelengths. There has also been a great improvement in the quality of X-ray spectra. Since most of the energy radiated by SNRs appears as X-rays, these new data are crucial to studies of SNRs, the interaction of SNRs with the interstellar medium, and the SN explosion itself. Images show a variety of shapes ranging from shel-like remnants to those dominated by the influence of central objects which appear both as point sources and as centers of diffuse activity. Once the temperature and spatial distribution of X-ray emitting material is known, the mass of ejected material and the energy release of the SN explosion can sometimes be calculated. X-ray images and spectra of several remnants are shown, and some quantitative results are given.     

Einstein observations of hot stars

Surveys with instruments on the $\text{Einstein Observatory}$ have shown that essentially all 0 and B main sequence stars are X-ray sources as are many, if not all, 0B supergiants and Wolf-Rayet stars. The X-ray luminosities are sufficient to explain broad lines from the superionization stages seen in the UV spectra of the stars. High energy resolution spectra from the Solid State Spectrometer are shown to place severe constraints on various models for the location of the X-ray sources in the outer atmospheres of the stars. Coronal and embedded shock models for the X-ray emission are discussed and each is found to have some problems in explaining the X-ray emission of 0B stars. X-ray line emission of Si XIII and S XV in ? Ori is discussed and interpreted as arising from magnetically confined loops.     

COMPTEL observations of gamma-ray bursts

The COMPTEL experiment on GRO images 0.7 – 30 MeV celestial gamma-radiation that falls within its 1 steradian field of view. During the first fifteen months in orbit, preliminary localizations from BATSE triggers indicated that about 1 in 6 cosmic events could have fallen within COMPTEL's field of view. We summarize work on the brightest of these gamma-ray bursts and present new position constraints for GRB 911118 and GRB 920622.     

EUV observations of Comet Halley

The observation of EUV emissions of comet Halley and its plasma-gas environment by means of rocket- or satellite-borne resonance absorption cell spectrophotometer devices is planned. The technical outlay of the payload, the estimated EUV intensities, and the scientific objectives of this mission are presented. Due to complete suppression of the geocoronal He I emissions by He I resonance absorption cells, a quantitative identification of the cometary object in the He I 58.4 nm line is possible, if the He/H abundance ratio in the evaporating cometary matter is higher than 4.0 E-4.     

IUE observations of supernova remnants

Ultraviolet spectra of supernova remnants obtained with the IUE satellite provide unique information concerning the shock conditions and elemental abundances in the optically bright filaments. High temperature species such as N V provide diagnostics for shock velocities above 100 km s^?1, and strong lines of carbon and silicon in the IUE spectral range make it possible to study the destruction of refractory grains in shocked interstellar gas. Observations of a non-radiative shock at the edge of the Cygnus Loop provide constraints on the physics of the shock front itself. Most of the very young remnants whose optical spectra show anomalous elemental abundances are too highly reddened for IUE observations, but extensive observations of the Crab Nebula and a spectrum of the supernova remnant in NGC 4449 yield carbon to oxygen ratios from which the mass of the progenitor may be estimated.     

IUE observations of cool stars

A summary of IUE results concerning late-type stars is presented. Observations show that high-temperature outer atmospheres, as indicated by N V, C IV emission at T ≈ 10⁵K, are generally present only in high-gravity (log g ? 2) stars. Objects with high-temperature emission tend not to exhibit cool circumstellar shells, and vice versa, although there are several transition objects, the hybrid atmosphere stars, which combine C IV emission with cool winds. Ultraviolet emission from stellar transition regions correlates well with chromospheric and X-ray emission. Transition-region line ratios indicate that many stars have differential emission measure distributions similar to the Sun's. Ultraviolet observations also give indications of important dynamical effects in low-gravity stars. Density diagnostics indicate extended chromospheres for some red giants and supergiants. In addition, the large widths of lines of high temperature ions in several luminous stars indicate supersonic motions.