BBXRT observations of supernova remnants

The Broad Band X-Ray Telescope (BBXRT) was designed to perform sensitive, moderate resolution spectroscopy of cosmic X-ray sources in the 0.3–10 keV band from the Space Shuttle. During its nine-day flight in December, 1990, the BBXRT observed a variety of supernova remnants and related objects. We present results from some of these observations, emphasizing the ability of the BBXRT to perform spatially-resolved spectroscopy. The improved spectral resolution and efficiency over previous instruments makes possible measurements of previously undetectable lines, and the broad bandpass allows simultaneous measurements of lines from oxygen through iron.     

BBXRT and Ginga observations of AGN

The X-ray properties of Active Galactic Nuclei (AGN) support recent theories which unify the Seyfert 1 nuclei with the Seyfert 2's and radio-quiet quasars. In these objects the underlying spectrum is strongly distorted by the effects of reflection from the accretion flow and by absorption in partially ionised material. These obscure any intrinsic changes in the spectrum, making it difficult (though not impossible) to constrain the nature of the emission process. Conversely, there is no evidence for either of these spectral distortions in the radio-loud AGN, supporting the hypothesis that the X-rays are dominated by beamed emission from the relativistic jet.     

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.     

Hard X-ray observations of Cygnus X-1 with the MISO telescope

The black hole candidate Cygnus X-1 was observed in the hard X ray - soft γ ray energy range by the MISO telescope on two different occasions: in September 1979 and May 1980. We have measured two hard X-ray states of the source: in 1979 the observed spectrum confirms the superlow state measured in the same period by the HEAO-3 satellite, while in 1980 the MISO X-ray data are consistent with the so called low state of Cygnus X-1. In both occasions, no γ -ray excess has been observed above 200 KeV.     

X-ray observations of black-hole accretion disks

Our current theoretical and observational understandings of the accretion disks around Galactic black-holes are reviewed. Historically, a simple phenomenological accretion disk model has been used to interpret X-ray observations. Although such a phenomenological interpretation is still useful, high quality X-ray data from contemporary instruments allow us to test more realistic accretion disk models. In a simple and ideal case, the standard optically thick accretion disk model is successful to explain observations, such that the inner disk radius is constant at three times the Schwarzschild radius over large luminosity variations. However, when disk luminosity is close to or exceeds the Eddington luminosity, the standard disk model breaks, and we have to consider the “slim disk” solution in which radial energy advection is dominant. Recent observations of Ultra-luminous X-ray sources (ULXs), which may not be explained by the standard disk model, strongly suggest the slim disk solution. We compare theoretical X-ray spectra from the slim disk with observed X-ray spectra of ULXs. We have found that the slim disk model is successful to explain ULX spectra, in terms of the massive stellar black-holes with several tens of solar mass and the super-Eddington mass accretion rates. In order to explain the large luminosities (>10⁴⁰ ergs s⁻¹) of ULXs, “intermediate black-holes” (>100M) are not required. Slim disks around massive stellar black-holes of up to several tens of solar mass would naturally explain the observed properties of ULXs.     

Einstein X-ray observations of cataclysmic variables

Observations with the imaging X-ray detectors on the Einstein Observatory have led to a large increase in the number of low luminosity X-ray sources known to be associated with cataclysmic variable stars (CVs). The high sensitivity of the Einstein instrumentation has permitted study of their short timescale variability and spectra. The data are adding significantly to our knowledge of the accretion process in cataclysmic variables and forcing some revision in our ideas concerning the origin of the optical variability in these stars.     

SIGMA observations of hard X-ray and soft gamma-ray emission from X-ray binaries

After more than two years of operation, the imaging γ-ray SIGMA telescope has accumulated several days of observation toward well known X-ray binaries. Four bright sources falling in this category have been detected so far: The pulsar GX 1+4 near the center of our galaxy, the stellar wind accreting system 4U 1700-377, and the black hole candidates Cygnus X-1 and GX 339-4. Moreover, SIGMA have observed three transients sources, which turned out to be also hard X-ray sources : The burster KS 1731-260, Tra X-1, and the Musca Nova. The properties of these systems in the SIGMA domain will be reviewed and a spectral distinction between black holes and neutron stars will be sketched.     

X-ray observations from the space shuttle

The flight of the University of Birmingham X-ray Telescope, which took place between 29th July and 6 August 1985 is reviewed. Despite the competing demands of twelve other investigations on-board, it was found possible to plan an observing programme for the XRT which enabled a good (43%) utilisation of the total operating time to be achieved, and then to carry out this programme with high efficiency. An important element here was the inclusion within the XRT of a pointing mount, which enabled it to point with a degree of independence of the Orbiter vehicle. The background of spurious events in the detector, due mainly to energetic particles, was found to be low and well-behaved, except for occasional events which are not easy to distinguish from X-ray bursts. The spectral intensity of unrejected background varies from about 2.10⁻³/cm²sec keV at 8 keV to 5.10⁻⁴/cm² sec keV at 25 keV.     

Coronal X-ray temperatures from Einstein and EXOSAT observations

Spectral analysis of coronal X-ray emission from stars observed with both the Einstein and EXOSAT Observatories is presented. Using computer codes developed by Raymond and Smith /1/ and Landini and Fossi /2/ to calculate the X-ray emission from optically thin plasma in collisional equilibrium we find that the derived coronal parameters depend only rather insensitively on the details of the calculated theoretical X-ray spectrum and demonstrate how both the Einstein Observatory IPC spectra and the EXOSAT LE filter ratios can be naturally and simultaneously explained by assuming an underlying continuous emission measure distribution as is the case in the solar corona.     

X-ray and gamma-ray observations of a white-light flare

HEAO-1 observed hard radiations (X- and gamma-rays) from a major solar flare on 11 July 1978. The observations showed gamma-ray line and continuum emission extending to the highest energy observed. The lines are identified with the 2.2 MeV line of deuterium formation and the 4.4 MeV line of inelastic scattering on ¹²C, both previously observed in the flares of August 1972 [1]. The 11 July flare was identified as a white-light flare by observations at Debrecen [2]. It thus provides the first opportunity for a detailed examination of white-light flare theories that depend upon proton heating of the photosphere. The line strength over a four-minute integration at 2.2 MeV was 1.00 ± 0.29 ph(cm² sec)⁻¹, and the gamma-ray emission (excluding the 2.2 MeV line which was appreciably delayed) lagged by less than 20 sec approximately after the hard X-ray and microwave fluxes. We conclude that the “second-stage” acceleration of high-energy solar particles must commence promptly after the impulsive phase.     

Einstein observations of high luminosity X-ray binaries

Recent observational advances in the study of high luminosity x-ray binaries have permitted investigation of the interaction of the outgoing x-radiation with the accreting matter surrounding the compact object. In two sources, 4U1822-37 and 4U2129+47, extended EINSTEIN coverage has led to the detection of partial x-ray eclipses, which indicate that the x-ray emitting regions must be extended in size. These have been interpreted as evidence for a large Compton-thick corona produced by evaporation of cool material off the surface of an accretion disk. In three other sources, 4U1915-05, 4U1624-49, and Cygnus X-2, evidence has been found for short x-ray absorption dips which are likely to be associated with obscuration by cool dense matter at the outer edge of the disk. In 4U1915-05, these dips are strictly periodic and determine the binary period for the system. In Cygnus X-2, the dips appear to be quasiperiodic, while in 4U1624-49, insufficient coverage has prevented clarification of the temporal properties of the absorption.For the brightest cosmic x-ray source, Scorpius X-1, the EINSTEIN objective grating spectrometer has provided high resolution spectra (λ/Δλ ～50) in the wavelength range 40-10 Å. The spectra reveal absorption features due to intervening helium, nitrogen, and oxygen. The implied nitrogen and oxygen abundances are anomalous and suggest that the absorbing material is intrinsic accreting matter which has been transferred from the surface of an evolved companion. Constraints on the inclination of the system then imply that this cool dense material must be well out of the orbital plane of the binary.     

Hard X-ray observations of the Crab Pulsar

The Crab was observed in a balloon flight from Palestine/Texas on 9/28/81 at hard X-ray energies (20–200 keV). The light curve is significantly sharper than reported previously for this energy range. The pulse-averaged as well as the interpulse spectra show breaks in our energy-range. The variation of spectral index across the pulse has an amplitude similar to that found at lower energies by OSO-8 and larger than reported by HEAO-1 A4 at hard X-rays. For a sharp emission line at 77 keV a 99% upper limit of 1.0*10⁻³ photons/ cm² sec can be placed, a factor of 4 lower than line fluxes reported previously. Pulse-shape fits to the optical, X-ray, hard X-ray and gamma ray light-curves reveal a consistent picture of the origin of the interpulse and off-pulse emission, the breaks in the spectra and the variation of spectral index, providing arguments against a thermal component and also a polar cap emission model for NP0532.     

Einstein X-ray observations of clusters of galaxies

The X-ray imaging capability of the Einstein Observatory has provided new observational material in many branches of astrophysics. In this contribution we will review the implications of the X-ray observations for the classification of clusters, the formation of SO galaxies, and the interaction of the galaxies with the intracluster gas.     

ROSAT observations of X-ray sources in globular clusters

We review the first observations of globular clusters obtained with the X-ray telescope on board of the ROSAT satellite. In the All-Sky Survey, all known bright sources and two new transient sources were detected. In addition a super-soft source was found in NGC5272. Concerning the dim sources, the survey suggests that those outside the core of ω Cen are not related to the cluster. The survey further improved by one or two magnitudes on many upper limits previously obtained by the HEAO-1 satellite. Pointed observations have improved the positional accuracy of a number of sources, and added to the number of known dim sources. In 47 Tuc, the known central source is resolved into 4 separate sources, which we argue are all soft X-ray transients. A comparison with observations of the old open cluster M67 leads us to suggest that some of the dim sources may be conglomerates of a large number of RS Can Ven type binaries.     

SIGMA/GRANAT observations of hard X-ray emission from type I X-ray bursters

Unlike black hole candidate systems, accreting neutron stars seem to encounter appreciable difficulties in emitting strong hard X-ray fluxes. However, in the catalogue of the hard X-ray sources detected by SIGMA, three sources are associated with type I X-ray bursters. In this paper, we review the present status of the SIGMA observations of these three X-ray burst sources, namely X 1724-308 in the globular cluster Terzan II, KS 1731-260, and GX 354+0.     

XMM-Newton observations of faint X-ray sources in globular clusters

Using the new generation of X-ray observatories, we are now beginning to identify populations of close binaries in globular clusters, previously elusive in the optical domain because of the high stellar density. These binaries are thought to be, at least in part, responsible for delaying the inevitable core collapse of globular clusters and their identification is therefore essential in understanding the evolution of globular clusters, as well as being valuable in the study of the binaries themselves. Here, we present observations made with XMM-Newton of six globular clusters, in which we have identified neutron star low mass X-ray binaries and their descendants (millisecond pulsars), cataclysmic variables and other types of binaries. We discuss not only the characteristics of these binaries, but also their formation and evolution in globular clusters and their use in tracing the dynamical history of these clusters.     

X-ray observations of high-z radio loud/quiet quasars

It is widely recognized that a knowledge of the X-ray properties of high-z radio loud/quiet quasars (RLQs/RQQs) would have important consequences for probing the conditions of the early Universe, studying the evolution of quasars, and assessing quasar contributions to the CXB. In distant active galactic nuclei (AGN) we can detect only the quasars having high luminosities or collimated beams with current X-ray astronomy satellites. We have also detected more than ten RLQs above z=2 with ASCA. We have observed 8 high z RQQs above z=2, but have not detected the higher z RQQs, above z=2.5. A considerable fraction of the high z RLQs show excess absorption over Galactic column densities, while most high z RQQs do not indicate excess absorption, although the samples are small. The X-ray photon index of RLQs is smaller than that of RQQs. The two RLQs above z=4 are very flat, suggesting that they are blazar-like AGN. A long term observation of RQQ, H1400#10 (z=2.078) did not indicate an Fe Kα emission line, giving a weaker upper limit than a typical line intensity of Seyfert 1 galaxies.     

X-ray and radio observations of the galaxy cluster 3C 129

The galaxy cluster 3C 129 contains two radio galaxies, the prototypical head tail radio galaxy 3C 129 and the weaker radio galaxy 3C 129.1. The tail of the first radio galaxy extends over more than 15′ across the sky. In this paper, we report on Chandra spectroscopy observations of the galaxy cluster, complemented by new and archival radio data taken with the Very Large Array (VLA) at 0.33, 1.4, 5, and 8 GHz and by HI-observations performed with the dominion radio astrophysical observatory (DRAO). We describe the Chandra results on the properties of the Intra-Cluster Medium (ICM) and discuss extended X-ray emission detected from the host galaxy of the radio galaxy 3C 129.1 and from the inner jet of the radio galaxy 3C 129. Finally, we report on the results of an ICM/radio plasma pressure balance study along the tail of the radio galaxy 3C 129.     

XMM-Newton observations of low luminosity Be/X-ray candidates

A small number of early Be stars exhibit X-ray luminosities intermediate between those typical of early type stars and those radiated by Be/X-ray binaries in the quiescent state. We report on XMM-Newton observations of two such Be stars, HD 161103 and SAO 49725 which were originally discovered in a systematic cross-correlation between the ROSAT all-sky survey and SIMBAD. The new observations confirm the X-ray luminosity detected by ROSAT (L_X  10³² erg s⁻¹) and the hardness of their X-ray spectra (thin thermal with kT  8–10 keV or power law with photon index of 1.7) which are both unusual for normal early type stars. We discuss the possible origin of this excess X-ray emission in the light of the models proposed for γ-Cas, magnetic disc-star interaction or accretion onto a compact companion object, neutron star or white dwarf, and compare the properties of these two sources with those of the new massive systems discovered in the XMM- Newton/SSC survey of the Galactic plane.