Although most powerful extended radio quasars have lobes of radio emission on opposite sides of the nuclear component, a significant number appear to have extended emission on only one side. We have observed ∼40 such sources (mostly from the compilation of Kapahi 1981) with the VLA, to confirm their classification and study their properties.

One of the best studied results of the X-ray observations of Active Galactic Nuclei is the statistical correlation which has been observed between the X-ray and optical luminosities in these objects. In this paper I will review the present situation of the analysis of such correlation, focusing my attention on the large number of topics which are more or less directly linked to it.

We find that the inconsistencies discussed by many authors between the expected and observed number of X-ray selected quasars cannot be attributed to incompleteness of the Medium Sensitivity Survey (MSS). We are presently expanding the survey, this effort will produce a very large sample of X-ray selected quasars. Preliminary results are presented.

The relationship between low energy X-ray emission and radio emission in radio quasars is considered. A survey of early results is presented and the current status summarized. Some remarks are made on the simultaneous correlation of X-ray emission with optical and radio emission.

We describe a program aimed at characterizing the X-ray emission of high redshift QSOs. We have obtained slitless spectra of 50 high galactic latitude fields previously imaged at very high levels of sensitivity by the Einstein Observatory, generally for original goals unrelated to QSOs. Our survey, covering ∼ 17 deg2 of sky to limiting magnitude Bcont ∼ 21, has yielded ∼ 400 previously uncatalogued QSO candidates, each with sensitive new X-ray information available. About 100 of these objects, constituting a “high confidence” set of QSOs, chiefly in the redshift range 1.7 < z < 3 and thus complementary to previous samples with X-ray data, are used to derive the X-ray properties of high redshift QSOs. Even at these most sensitive available X-ray flux levels, only about 25% of the objects are positively detected in X-rays; thus extensive attention has been given to proper treatment of the upper-limit information. We find a mean optical-to-X-ray slope parameter for the sample of . Our results are combined with those of previous surveys to estimate the fraction of the diffuse X-ray background radiation due to QSOs. QSOs are capable of supplying the majority of the radiation, but the chief contribution comes from an annulus of intermediate redshift, moderate luminosity objects.

We present results of studies carried out with the imaging instruments on the Einstein Observatory. We summarize a statistical analysis of the X-ray properties of optically selected, radio quiet quasars including nine new high redshift quasars detected in two deep X-ray surveys. We find that the X-ray to optical luminosity ratio of optically selected quasars decreases with increasing optical luminosity. It depends only weakly, if at all, on redshift. However, the distribution function does not properly account for the properties of the X-ray selected Medium Sensitivity Survey sample (MSS). We note that part of the discrepancy could be due to the presence of red, low luminosity quasars in the MSS but not in the optically selected samples. We also summarize some results from a detailed study of the X-ray properties of 64 Seyfert galaxies. None of the spectral fits performed for the brightest 20 required unusually steep spectra, although in many cases the spectral indices were not well constrained. Of the ten objects with good measurements of the absorbing column density, three showed excesses above the galactic value while the remaining seven gave excess columns generally less than 2 × 1020 cm−2 and consistent with zero. Variability studies of the full Seyfert sample showed three objects to be variable on timescales of a few hours. One of these is the Seyfert II Mkn 78.

We present Einstein spectra for 30 QSOs which indicate a large diversity in their soft x-ray slopes, unlike the uniform slopes found for AGN higher x-ray energies. The slope is related to radio properties, x-ray luminosity or possibly redshift, but we cannot definitively distinguish these possibilities.

Our statistical study finds that the X-ray luminosity, lx for an average QSO of given optical luminosity, lo is an increasing function of radio luminosity, lr. An average QSO of given lo and lr has higher lx if the radio spectrum is flat than if it is steep. The correlation between lx, lo and lr appears to be improved if lr refers only to core emission, lrcore. Assuming an average radio-loud QSO is a radio-quiet QSO with a separate component which produces lrcore and Ixcore, lxcore/lrcore is larger if the core is that of a QSO with extended radio structure than if that of a compact flat-spectrum QSO.

The Einstein Observatory Medium Sensitivity Survey contains four sources identified with BL Lac objects. The radio and optical properties of these objects have been described in Stocke et al. (1985).

The bright BL Lac object PKS 2155-304 (mv=13; z=0.117) was observed with EXOSAT at five epochs (1983 Oct. 31, Nov. 30, 1984 Nov. 6, 7 and 11), for a total of about 30 hours of exposure time. Here we present data and results obtained with the Low Energy (LE) telescopes, in the band 0.05–2. keV, and with the Medium Energy (ME) argon proportional counters in the range 1.–6. keV.

Recent EXOSAT observations of active galactic nuclei are presented. Unlike earlier X-ray satellites (all of which flew in low earth orbit), the deep orbit of EXOSAT allows long continuous observations of celestial X-ray sources, uninterrupted by earth occultation etc. We present the results of EXOSAT observations of several AGN which have been seen to vary rapidly (timescale 0.2–6 hours). We also consider the implications of rapid variability in AGN. For Seyfert galaxies and quasars, we find a highly significant correlation between the timescale of variability and their X-ray luminosity. They are not, howwever, bounded either by the (classical) Eddington limit nor by efficiency arguments. We sugest, rather, that the emitting plasma is dominated by electron-positron pairs.

The X-ray Observations of two BL Lac Objects H2155-304 and PKS 0548-322 made with HRI and MPC on the Einstein Observatory show intensity variations on time scale of hours in both the sources. X-ray spectra of the two BL Lacs are derived. Limits on the mass of the accreting compact objects are obtained from time scale and magnitude of variations. Implications of the results are briefly discussed.

A subtraction technique is applied to the X-ray image of 3C 273 in order to determine the location and other properties of emission associated with the jet.

“Standard” photoionization models of the broad line region (BLR) consist of numerous small optically thick clouds of identical ionization, density, temperature and optical depth, moving under the influence of the gravity and radiation field of a 107–8 M⊙ black hole and confined by a high temperature gas. Such models have provided a good description of observed line strengths and widths with only very minor modification. Although photoionization remains the most important heating mechanism in the BLR, new observations point to a wider range of physical conditions and to clues about the geometric and dynamic arrangement of the emitting gas. I want to highlight the observations that have led us to this view.

Optical spectra having moderately high resolution (~ 2 — 5 Å) are being used to study the profiles of narrow emission lines in active galactic nuclei (AGNs). It is often found that forbidden lines associated with high critical densities for collisional deexcitation are the broadest. A good example is [O III] λ4363 [ne(crit) ≈ 3 × 107 cm−3], whose width can be more than twice that of [O III] λ5007 [ne(crit) ≈ 8 × 105 cm−3]. The tight correlation between line width and ne(crit) implies that a much larger range of densities (∼ 102 — 107 cm−3) must be present among clouds in the narrow-line region than was previously believed. At times there almost appears to be a continuity between the narrow- and broad-line regions. In some objects the dense, high-velocity clouds are optically thick to ionizing radiation, since they emit [O I] λ6300 as well as species of much higher ionization (such as [Ne V] λ3426). These results help eliminate several difficulties in photoionization models of LINERs. It may also be possible to use the observed line widths as probes of the gravitational potential in AGNs.

Our knowledge of the physical conditions and of the structure of the broad line emissive region -the BLR- is reviewed. First we derive the model independent constraints on the different zones emitting the broad lines. Then we discuss photoionized models. In a first step the BLR is assumed to be made only of one type of cloud emitting all the broad lines. We show that this model is unable to explain the observed spectrum. We thus assume that the high and low ionization lines are emitted by different clouds, still photoionized. This assumption is also in contradiction with the observations and we are led to the idea of a large variety of emitting clouds. Finally the hypothesis of a purely radiative heating mechanism should also be questioned.

Observational and theoretical constraints on self-consistent gas dynamical models of the line emitting regions of QSOs and AGNs are reviewed. General equations governing the global flows are analysed and models for the formation and acceleration of the line emitting “clouds” are discussed.

Observational constraints on models for broad absorption line (BAL) QSOs are discussed. The picture which emerges is one in which the broad absorption line region (BALR) contributes to the broad emission line region (ELR) as a high ionization component. Depending on QSO luminosity, the limits on the distance of the BALR from the central source are 1–1000 pc. A highly metal enriched and disk-like geometry for the region is indicated. The similarity between the absorption in some radio quiet BAL QSOs and the associated complexes of absorption in some radio loud QSOs may indicate that the BALR outflow can affect the region surrounding a QSO out to distances in excess of several hundred kpc.

We investigate the constraints on dynamic models for the line-emitting regions in quasars and AGN. The parameters characterising the central energy source (Mass, efficiency, accretion rate) are calculated in terms of the physical conditions in the line emitting gas. In a large sample the central mass (calculated assuming the emission-line clouds are bound) is proportional to the continuum luminosity. We find typical values of L/LE∼10−2±0.5, e∼0.1-l%, and Ṁ/ṀE∼1–10.