We have analyzed the X-ray spectra of all BL Lacs observed (as pointed or serendipitous sources) by ROSAT. Spectral indices were obtained from the hardness ratios given in the WGA catalogue, a large list of X-ray sources generated from all the ROSAT PSPC pointed observations. The selection of the objects was done by cross-correlating the first revision of the WGA catalogue with our recent catalogue of BL Lacs. This resulted in 163 observations of 85 distinct BL Lacs, which correspond to about half the confirmed BL Lacs presently known. This represents the largest number of BL Lacs for which homogeneous X-ray spectral information is available and the largest BL Lac sample ever studied at X-ray frequencies.

The number of BL Lacs discovered in the last 20 years is very small (≃ 200) if compared to that of quasars. Owing to their featureless optical spectrum, BL Lacs have been discovered mainly in radio (RBL) and X-ray surveys (XBL). The limited statistics available prevents a detailed study of the properties of BL Lacs and, in general, all the conclusions based on the present data sets are affected by large uncertainties. Yet, current results are intriguing, for instance RBL and XBL are found to have a cosmological evolution that differs not only in magnitude but most of all in sign (e.g. Stickel et al., 1991 and Wolter et al., 1994 and reference therein). Therefore, we have initiated a project aimed at discovering a significant number of new BL Lac objects (≃ 100), exploiting the fact that they are both radio and X-ray emitters and in particular the fact that they occupy a well defined region in the αox–αro plane. The method we use rests on the expertise grown during the construction of the EMSS sample (Gioia et al. 1990, Stocke et al. 1991) and is described in detail in Wolter et al. (1995).

Twenty six Radio Emitting X-ray Sources (REXs) have been observed in two observing runs as part of a project aimed at selecting a new and large sample of BL Lac objects (see Wolter et al. this conference). Sources were selected by cross-correlating ROSAT-PSPC public images against VLA radio surveys. Here we present results from the first observations of 14 sources conducted in April 1995 with a B&C spectrograph in longslit mode using a CCD-Tektronix TK-1024 AB detector at the 2.1m San Pedro Martir telescope (Mexico). Table 1 shows the data derived for the 8 confirmed REXs. X-ray fluxes are in units of erg cm–2 sec–1 in the 0.1-2.4 keV band, radio fluxes are in mJy at 5 GHz, X-ray luminosities are in erg sec–1 in the energy range 0.1-2.4 keV and radio luminosities are in erg sec–1 at 5 GHz. We assumed Ho = 50 km sec–1 Mpc–1 and qo = 0. A more detailed description of individual sources will be published elsewhere.

X-ray spectral properties of radio galaxies are reviewed in comparison with those of Seyfert galaxies, mainly based on results from Japanese X-ray astronomy satellites, Ginga and ASCA.

The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory is an imaging high-energy telescope with sensitivity from approximately 20 MeV to 30 GeV. EGRET has observed more than 129 sources during more than 4 years of operation. Among these sources, 51 have been identified with active galaxies. A common characteristic of the AGN sources is that they are all radio-loud, flat radio spectrum sources. Many of them are seen as superluminal radio sources as well. The gamma-ray emission characteristics of these sources are reviewed and some of the proposed emission models are discussed.

The conventional paradigm of active galactic nuclei (AGN) holds that they are powered by accretion onto a gravitational engine. In addition, the presence of Doopler-boosted radiation from jets and anisotropic obscuration can create large differences between the observed and intrinsic properties of AGN (Antonucci 1994). The “big picture” of extragalactic radio sources must include observations across the electromagnetic spectrum. Relativistic processes produce radiation with a very wide bandwidth. In addition, much of the energy from AGN is reprocessed, and the opacity is a function of wavelength. Finally, it turns out that only a small fraction of the bolometric luminosity of most AGN is emitted in the traditional radio and optical bands.

Forty AGN have been detected at high significance level by EGRET in the γ-ray band. Previous studies based on radio observations near or at the times of the EGRET detections suggest that there is a causal connection between individual events in these two wavebands. Here we examine the question of whether the cm-λ and the γ-ray activity are related.

In this paper we present first results of a statistical analysis of correlated variability behaviour of flat-spectrum radio quasars (FSRQ) on the basis of EGRET data and cm-wavelength monitoring data. We use EGRET observations obtained between April 1991 to September 1993 and multifrequency radio observations at 2.8cm, 6cm and 11cm taken with the 100-m-Effelsberg Telescope parallel to the CGRO observations. In the following discussion the observed FSRQ which have not been detected by EGRET yet, are referred to as ‘candidates’ in contrast to the detected ones, called ‘EGRET-sources’. The methods used in this paper are described in more detail in Mücke et al. (1996).

The observations by the Compton Gamma Ray Observatory (CGRO) have shown that highly variable and radio-loud quasars emit a significant fraction of their energy in the γ band. According to the Inverse Compton model, the γ-ray emission is due to upscattering of soft (IR-optical-UV) photons by high energy particles. Optical monitoring is thus of great value in providing information on the mechanisms that rule the production of the seed photons for the γ-ray radiation and on the γ-ray emission itself. In particular, detection of variability correlations between optical and γ-ray emissions would be a crucial test for the theoretical predictions.

We monitored the γ-ray loud BL Lac object S5 0716+714 between November 1994 and April 1995 (J.D. 2449671–2449838), from the Observatories of Torino, Perugia and Roma. The source was always active in the R and B bands, showing a particularly high optical state during the period of the CGRO pointing (February 14–28, 1995). The lightcurve in the R band is presented in Fig. 1. The complete set of observations is discussed in Ghisellini et al. (1995).

The current data on the highest energy cosmic rays (UHECRs) is discussed and an understanding of the origin of these particles is reviewed. New and proposed facilities for measurement of UHECRs, neutrinos and γ-rays can interestingly and significantly constrain the physics of the source origin. Cosmic magnetic field strengths are the most uncertain physical parameter.

Since the Albuquerque IAU meeting, most of the advances in the understanding of radio sources in clusters has come from combining radio observations with x-ray and optical data. The x-ray images from Einstein and ROSAT have been particularly important because they have allowed us to see the external medium with which the radio sources co-exist and interact. I will cover three examples of such work in this review.

So far, the number of so-called giant radio galaxies (GRGs) is small. We define them as radio sources with linear sizes larger than 1 Mpc (H0 = 75 km s–1 Mpc–1). On-going low-frequency surveys may come up with many more candidates for this species. Their very existence may ultimately be connected with their environment. For instance, for a source like 3C236, which exhibits a linear and undisturbed structure over some 4 Mpc, the surrounding medium must have a low density. GRGs may thus serve to probe large volumes of the most tenuous intergalactic medium (IGM), e.g. via depolarization studies.

The extended radio structures or lobes found in edge-brightened radio galaxies represent interactions with the ambient medium over the source lifetimes. Their study probes the temporal evolution in the radio sources and the properties of the ambient medium encountered at different locations. Previous studies (Leahy & Williams, 1984; Leahy et al., 1989) involved radio galaxies with sizes ≃ 400 kpc, and revealed radio morphologies with a variety of off-axis distortions that could be interpreted as due to different ways in which the lobes interact with the galactic halos. Such studies are lacking for radio galaxies having Megaparsec sizes, which extend to distances well beyond the observed galactic halos and are suspected of evolving in a different regime (Baldwin, 1982). Towards learning the evolution of radio galaxies in this size regime, we have carried out a study of a complete sample of Mpc-size radio galaxies; details are presented in Subrahmanyan, Saripalli & Hunstead (to appear in MNRAS, 1996).

Giant Radio Galaxies are the largest radio sources associated with galaxies. They have linear sizes exceeding 1 Mpc (e.g. Saripalli et al., 1986). They used to be known only at redshifts below 0.2, and it has been argued that this was not only due to a selection effect, but that there is a real cutoff in the space distribution of these objects (e.g. Gopal-Krishna & Wiita, 1987). Recently, a small number of giants have been found at redshifts largely exceeding 0.3, indicating that such a cutoff does not exist (e.g. Cotter et al., 1995).

In the framework of our high-frequency survey of giant radio galaxies with the Effelsberg 100-m telescope (Klein et al., 1994; Saripalli et al., 1995) we have obtained radio continuum maps of NGC6251, a source of 1.5 Mpc size (H0 = 75km s–1Mpc–1). Together with low-frequency WSRT observations (Willis & O'Dea, 1990), these measurements form a unique data base which for the first time allows thorough studies of the spectral index over a large frequency range. Theoretical models of particle ageing have been fitted to the spectrum to determine particle ages and other relevant physical parameters. Because of the immense size of NGC6251 these numbers provide information about the physics of the surrounding intergalactic medium.

Powerful extended radio sources are observed out to relatively large redshift. They may be used to study the properties and redshift evolution of the gaseous environment in the vicinity of each source, the active galactic nucleus (AGN), and the source size. This information may then be used to study and constrain cosmological and cosmogonical models. It is interesting to note that the rate of change of quantities with redshift allows constraints to be placed on global cosmological parameters and on models of structure formation and evolution that are completely independent of those inferred using the cosmic microwave background or local dynamical studies, and thus provide an important complement to these studies. The method does require that we understand the physics of the sources well enough to account for intrinsic source evolution. The physics of powerful extended radio sources that propagate supersonically appears to be relatively straight-forward. We have used the radio properties of the sources to deduce the ambient gas density, the beam power of the AGN, the characteristic time a particular AGN is on, and a characteristic source size that allows the sources to be used to probe global cosmological parameters. We plan to use the radio properties of the sources to deduce the ambient gas temperature, which will be combined with the ambient gas density to constrain cosmogonical models, that is, models of structure formation and evolution.

The most powerful radio sources in the local Universe are found in giant elliptical galaxies. Looking back to a redshift of 0.5 (≈ half the age of the Universe for ω = 1), we see that these host galaxies are increasingly found in moderately rich clusters. This fact gives us hope that radio sources can be used as tracers of high density environments at high redshift. By exploiting radio source samples selected over a wide range in luminosity (Blundell et al., these proceedings), we will also be able to test whether the luminosities of radio sources are correlated with their environments.

In the unified schemes of AGNs BL Lac objects are believed to be Fanaroff-Riley (FR) type I radio galaxies with a relativistic jet aligned to the observer's line of sight (e.g. Urry & Padovani 1995). Kollgaard et al. (1992) and Owen et al. (1995) suggest some FR II sources can also be BL Lac parents. Clearly, isotropic properties such as the galaxy environment of both beamed and unbeamed objects should be identical.