Supersoft X-ray sources

More than 100 supersoft X-ray sources (SSS) are reported in 20 external galaxies, the Magellanic Clouds (MCs) and our Galaxy. The effective temperatures of the brighter SSS are 20–100 eV. SSS with luminosities below ≈3 × 10³⁸ erg s⁻¹ are consistent with accreting white dwarfs (WDs) with steady nuclear burning or post-novae. Optical identifications exist for SSS in our Galaxy and the MCs (including orbital period determinations) and for SSS in M31 (with novae and symbiotic stars, SySs). High resolution X-ray spectra of the brightest SSS in our Galaxy and the MCs reveal the existence of spectral features due to high gravity WDs. Timing studies in X-rays (combined with the optical) of the stable nuclear burning phase in steady nuclear burning sources and in post-novae allow to constrain the mass accretion rate onto and the mass of the nuclear burning WD. The nature of a few SSS with luminosities 10³⁹ erg s⁻¹ remains unclear.     

Superclustering of galaxies and X-ray sources

An attempt is made to compare optical properties of Abell clusters with X-ray sources.     

Galactic outflows and the pollution of the galactic environment by supernovae

We here explore the effects of the SN explosions into the halo of star-forming galaxies like the Milky Way. Successive randomly distributed and clustered SNe explosions cause the formation of hot superbubbles that drive either fountains or galactic winds above the galactic disk, depending on the amount and concentration of energy that is injected by the SNe. In a galactic fountain, the ejected gas is re-captured by the gravitational potential and falls back onto the disk. From 3D non-equilibrium radiative cooling hydrodynamical simulations of these fountains, we find that they may reach altitudes up to about 5 kpc in the halo and thus allow for the formation of the so called intermediate-velocity-clouds (IVCs) which are often observed in the halos of disk galaxies. The high-velocity-clouds that are also observed but at higher altitudes (of up to 12 kpc) require another mechanism to explain their production. We argue that they could be formed either by the capture of gas from the intergalactic medium and/or by the action of magnetic fields that are carried to the halo with the gas in the fountains. Due to angular momentum losses to the halo, we find that the fountain material falls back to smaller radii and is not largely spread over the galactic disk. Instead, the SNe ejecta fall nearby the region where the fountain was produced, a result which is consistent with recent chemical models of the galaxy. The fall back material leads to the formation of new generations of molecular clouds and to supersonic turbulence feedback in the disk.     

The X-ray evolution of galaxies: implications for future X-ray observatories

The X-ray evolution of the luminosity of normal galaxies is primarily driven by the evolution of their X-ray binary populations. The imprints left by a cosmological evolution of the star formation rate (SFR) will cause the average X-ray luminosity of galaxies to appear higher in the redshift range 1–3. As reported by White and Ghosh [ApJ, 504 (1998) L31] the profile of X-ray luminosity with redshift can both serve as a diagnostic probe of the SFR profile and constrain evolutionary models for X-ray binaries. In order to observe the high redshift (z>3) universe in the X-ray band, it is necessary to avoid confusion from foreground field galaxies. We report on the predictions of these models of the X-ray flux expected from galaxies and the implications for the telescope parameters of future deep universe X-ray observatories.     

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.     

Constraints on a hadronic model for unidentified off-plane galactic gamma-ray sources

Recently the H.E.S.S. collaboration announced the detection of an unidentified gamma-ray source with an off-set from the galactic plane of 3.5°: HESS J1507-622. If the distance of the object is larger than about one kpc it would be physically located outside the galactic disk. The density profile of the ISM perpendicular to the galactic plane, which acts as target material for hadronic gamma-ray production, drops quite fast with increasing distance. This fact places distance dependent constraints on the energetics and properties of off-plane gamma-ray sources like HESS J1507-622 if a hadronic origin of the gamma-ray emission is assumed. For the case of this source it is found that there seems to be no simple way to link this object to the remnant of a stellar explosions.     

Astrophysical motivation for X-ray polarimetry

In this paper I will review the motivation for measuring polarization in the X-ray band from astrophysical sources. Emission models designed to reproduce X-ray spectra can be tested using polarization, and polarization detected in other wavelength bands makes clear predictions as to the X-ray polarization. Polarization is a powerful tool to infer geometrical properties of sources which are too small to be spatially resolved. At the same time, there has been recent progress in instrumentation which is likely to allow searches for X-ray polarization at levels significantly below what was possible for early detectors. This paper will review the history of X-ray polarimetry, discuss some experimental techniques and the scientific problems which can be addressed by future experiments.     

A hybrid X-ray imaging spectrometer for NeXT and the next generation X-ray satellite

We propose a new type of wide band X-ray imaging spectrometer as a focal plane detector of the super mirror onboard on future X-ray missions including post Astro-E2. This camera is realized by the hybrid of back illumination CCDs and a back supportless CCD for 0.05–10 keV band, and a Micro Pixel Gas Chamber detecting X-rays at 10–80 keV.     

Infrared radiation from compact X-ray sources

Observations of the X-ray binaries 4U 0115+634 and A 0535+262 performed in 1981 – 1982 revealed significant IR variabilities of these sources. 4U 0115+634 was observed twice, in the state close to X-ray activity, and in the state of low activity, thus changing from K = 7^m.75 to K > 9^m. A 0535+262 shows the temporal variations ranged from tens minutes to some days. The regular variations of IR brightness are suspected with teh period close to 104 s that corresponds to the period of axial rotation of neutron star in A 0535+262. The results obtained present arguments in favour of hypothesis that IR radiation is generated near the accreting neutron star and is possibly of maser nature.     

Nuclear activity in a volume-limited sample of nearby galaxies

We report on some of the most recent results from a multiwavelength study of a volume-limited sample of galaxies. This paper will concentrate on the detection of variability in NGC 4395, the least luminous known Seyfert galaxy, and the high resolution X-ray imaging of the sample.     

The SAX mission

SAX denotes the X-Ray Astronomy Satellite selected by the Italian National Space Plan for inclusion in the Science Programme. The purpose of SAX is to perform spectroscopic, spectral and time variability studies of celestial X-Ray sources in the energy band from 1 to 200 KeV. It is intended to continue and expand upon previous observations of such sources. The instrumentation consists of four X-Ray imaging concentrators sensitive from 1 to 10 KeV (one of them extending down to 0.1 KeV), one Gas Scintillation Proportional Counter sensitive from 3 to 120 KeV, a Sodium Iodide Scintillator Crystal in Phoswich configuration operating from 15 KeV to 200 KeV; these detectors are coaligned to a common pointing axis. Three Wide Field Cameras (2–30 KeV) with axis at 90° to that of the narrow field instruments complete the payload.The Satellite launch is foreseen for 1988, in a low altitude (500 Km), low inclination (12°) orbit.The SAX scientific programme is carried out by a Consortium of Italian Institutes, in cooperation with Institutes from Holland; a partecipation of the Space Science Department of ESA is also foreseen.     

Fluid instabilities around accreting X-ray sources

Some aspects of fluid instabilities occurring in the magnetospheres of accreting neutron stars are discussed. It is pointed out that (i) in the absence of strong differential rotation, the accreting plasma should be drawn out into spiralling, sheet-like structures, resulting in efficient mixing between the two media; (ii) the Rayleigh-Taylor instability also acts to limit the X-ray luminosity in super-critical sources; and (iii) magnetic shear has a strong stabilizing influence on Kelvin-Helmholtz modes, and its presence may allow substantial amounts of material to be supported around the magnetosphere.     

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.     

Hi content in cluster and supercluster galaxies

In this lecture I review part of the published and unpublished observational evidence showing that cluster spiral galaxies are hydrogen deficient. Always using observational data, it is argued that the deficiency is due to the interaction between the galaxies and the intergalactic medium. Furthermore it is shown that the stellar population of deficient galaxies has also been affected and a brief discussion is given of the mechanisms at work.