A. Cavaliere

On syntheses of the X-ray background with power-law sources

We discuss if and under what conditions the combined emission from power-law sources can mimic the XRB spectrum in the range 3-50 keV measured with the A2 experiment on HEAO-1. We confirm that i good fit can be obtained, but the required spectral properties of component sources differ from those observed for local active galactic nuclei. Strong constraints are deduced for the low luminosity extension and for the evolution of such local objects. We show that any other class of sources significantly contributing to the X-ray background must be characterized by an energy spectral index y < 0.4, the mean index of the XRB (3-15 keV), and must exhibit steeper spectra at somewhat higher energies. Subject headings: galaxies: cluster of radiation mechanisms X-rays: sources X-rays: spectra

The Sunyaev‐Zeldovich effect as a probe of the galaxy formation process

The Sunyaev‐Zeldovich (SZ) effect has proven to be an extremely powerful tool to study the physical and evolutionary properties of rich clusters of galaxies. Upcoming SZ experiments, with their much improved sensitivity and angular resolution, will provide unique information also on phases of galaxy evolution characterized by the presence of large amounts of hot proto‐galactic gas. We present a preliminary analysis of the SZ signals that can be expected at the collapse of the proto‐galaxy when, according to the standard scenario, the gas is heated at its virial temperature, and during episodes of strong energy injections from the active nucleus. The contributions of such signals to excess power on arc‐minute scales recently found by CBI and BIMA experiments are briefly discussed.

Quasars in the Life of Astronomers

We are approaching the 50th anniversary of the discovery of quasars. Those old enough to have been cognizant of astronomy in 1962–1963 can remember the sense of excitement connected with this finding. There was talk of a major new constituent of the universe. The excitement of the discovery was palpable even to one of us (the most senior of the editors) who was then a high school teenager.

Quasars in the Cosmic Environment

We now consider the environment of quasars in the widest possible sense, from the circumnuclear regions to very large scales of hundreds of kiloparsecs. The circumgalactic environment of nearby quasars has been widely studied since the late 1960s in an attempt to test its influence on the triggering of nuclear activity. The underlying hypothesis is that gravitational perturbations might ease the infall of matter toward the nucleus, providing accretion material for the central black hole.

The Intracluster Plasma: probing its amount and thermal state in X‐rays and in μwaves

Rich clusters of galaxies shine in X‐rays since the large masses m of hot intracluster plasma (ICP) they contain radiate strong luminosities LX ∝ m2T1/2/R3 by thermal bremsstrahlung. The same ICP may be also probed in μwaves through the Sunyaev‐Zel’dovich (SZ) effect, whose intensity parameter y ∝ mT/R2 is a measure of the thermal energy content. But the small groups are found to be underluminous in X‐rays when compared to clusters; they emit far less than expected from the law LX ∝ T2 that would hold if m passively scaled with the dark mass. In other words, the ICP in groups is considerably underdense relative to the cluster value m/R3 ∼ 10−3 cm−3. This behavior may be traced back to substantial energy fed back into the ICP, when the baryons in the member galaxies condense into stars then exploding as supernovae, or accrete onto central supermassive black holes energizing active galactic nuclei (AGNs). Such energy inputs deplete the ICP density in the shallower potential wells by causing thermal outflow ...

Ultraviolet Italian Sky Surveyor (UVISS) on the International Space Station (ISS): study report

A Mission into Hot Phenomena in the Universe is proposed by means of a small telescope of 50 cm aperture accommodated on the International Space Station. Two operating modes are envisaged: 3 angstrom dispersion imaging spectroscopy in the 90 - 320 nm range (1st priority) or wide field (1 degree) medium bandwidth imaging in the same range but Ly-(alpha) (2nd priority). It will use a pointing platform attached to an Express Pallet Adapter available to the Italian Space Agency (ASI) more than 4 - 6 months per year. During a life time of 6 yr focal plane instruments may be changed when on-ground refurbishment occurs. With reasonable exposure times hot thermal sources as faint as V equals 19 - 2 can be observed in the spectroscopy mode at 110 nm and active chromospheres on cool stars as faint as V equals 15 at 250 nm can be monitored. Assessment of FUV imaging is underway, possibly providing observations of hot sources as faint as V equals 21 - 22. Nominal uplift to ISS is set in Autumn 2005.

The X-Ray Background 2–50 keV: A Superposition of Known Sources, or Evidence of a New Component?

The contribution of the canonical Quasars to the XRB 2–50 keV is not likely to exceed 50%. Nay, consideration of X-ray selection effects results in a further reduction of the value of sensible estimators for the intensity ratio Lx/Lo; in addition, recent counts of radioloud and radio-quiet Quasars definitely limit the number of faint sources. A “missing component” with definite spectral properties should comprise a sizeable fraction of the XRB.

Discrete Source Synthesis of the X-Ray Background

The recent deep X-ray surveys suggest that discrete sources comprise most, if not all, of the energy content of the background (cf. Giacconi, this volume). We have shown that, if sources have flat power law spectra and relatively sharp high-energy cutoffs, their combined emission can also mimic very accurately its extended (2 - 400 keV) spectral shape. A broad distribution of cutoff energies E c is required, in this case, to model the high-energy (E ≳ 20 - 40 keV) part: a power law envelope of distribution functions of various types of sources can be envisaged; alternatively, the well-known fact that most of the 2 − 10 keV background should be produced by low flux, low E c sources can suggest that E c ∝ (1 + z) −η . The quality of the fit turns out to be not very sensitive to the amount of number/luminosity evolution assumed, though the minimum χ 2 test slightly favours strongly evolving sources. In the case of differential luminosity ∝ E −γ and evolution ∝ (1 + z) α with 0 ≤ α ≤ 6, the best fits to pre HEAO-1 data are obtained for mean values of the spectral index γ ≈ 0.5 − 0.9 and for dispersions Δ γ ≃ 0.5 - 0.7. Rather wide ranges of values of γ and Δ γ are, however, still allowed; e.g., for α = 6, the allowed intervals are 0.2 ≤ γ ≤ 1.3, 0 ≤ Δγ ≤ 0.7.