The discovery of the afterglows of gamma-ray bursts (GRB) at X-ray, optical and radio wavelengths heralds a new era in the investigation of these objects. This provides a quantum jump in the type of observational information available, providing much more severe constraints on models, and probably represents the light at the end of the tunnel for this long-standing puzzle of astrophysics.

X-ray variability is a distinguishing property of Active Galactic Nuclei (AGN), and the energetics and time scales of the emission dictate that the X-rays must originate very close to the central engine. In this review I discuss two basic topics from AGN variability research. The first is the correlation of the variability time scale with the X-ray luminosity, and the second is the structure of the X-ray light curve. In each case, I first review the old results that have been known for approximately the last 10 years and then I discuss very new results which may force us to modify our ideas about the origin of AGN X-ray variability. Note that I am discussing the variability of non-blazar type AGN.

Active Galactic Nuclei (AGN) are thought to be powered by accretion onto a massive black hole. Understanding how gravitational energy freed by the infall of matter into the black hole is transferred to the radiating regions of the nucleus is one of the main challenges of AGN research. The question is made very complex by the presence of several cooling mechanisms with very diverse physical properties: We observe synchrotron radiation, thermal emission from hot dust, possibly thermal optical and ultraviolet emission and Comptonization processes in the X-ray domain. For each component the radiation is a signature of the cooling process rather than of the heating process. It is our hope that by observing the links and correlations between the emission of the different components we will be able to understand how they are interrelated and how they get their energy supply from the accretion process.

During the last years it has become evident that blazar class of AGN emit a lot of energy in the gamma-ray regime. It is generally thought that the non-thermal emission from blazars, observed from radio to GeV/TeV 7-rays, is radiation of very energetic particles via both synchrotron and Compton processes. This underlines the importance of high-energy emission for models of the blazar class of active galactic nuclei. Most of the gamma-bright AGN are variable, and flares with time scales as short as a day have been observed. This suggests that the site of gamma-ray emission is very compact and situated in a fluid which moves relativistically at a small angle towards the observer. It is generally assumed that the emission originates from the jet of these objects.

This paper is devoted to a discussion of the rapid variability observed in γ-ray blazars. We do this in the framework of a widely accepted scenario, according to which the blazar emission arises from a jet, i.e. a continuous flow of relativistic fluid, with perturbations occasionally superimposed. The jet is assumed to have a bulk Lorentz factor Γ ≫ 1, and length z and radius r such that r ã z/Γ.

The discovery of strong, variable gamma-ray sources identified with blazars by CGRO has motivated many theoretical investigations concerning the emission from relativistic jets, and several models of gamma-ray blazars have recently been developed (e.g., Dermer & Schlickeiser 1993; Bloom & Marscher 1993; Manneheim 1993; Sikora, et al. 1994; Blandford &; Levinson 1995). However, most of these efforts have been devoted to examine spectral properties of blazars using steady-state models, which are most suitable for exploring quiescent states. Although some progress in identifying the emission mechanisms in different types of blazars, and constraining the content and dynamics of relativistic jets on small scales has been made, it has became widely recognized that quantitative analysis of variability data is crucial for advancing our understanding of the nature of relativistic jets further. Despite recent observational efforts (e.g., Reich et al. 1993; Maraschi et al. 1994; Wagner 1996; Buckley et al. 1996; Takahashi et al. 1996; Mattox et al. 1997) to characterize the transient emission in blazars, only very few attempts have been made (see e.g., Romanova & Lovelace 1997) to model variability data using time dependent models.

Almost all nondegenerate stars have chromospheres and coronae. These hot outer layers are produced by mechanical heating. The heating mechanisms of chromospheres and coronae, classified as hydrodynamic and magnetic mechanisms, are reviewed here. Both types of mechanisms can be further subdivided on basis of the fluctuation frequency into acoustic and pulsational waves for hydrodynamic and into AC- and DC-mechanisms for magnetic heating. Intense heating is usually associated with the formation of very small spatial scales, which are difficult to observe. Yet, global stellar observations, because of the dependence of the mechanical energy generation on the basic stellar parameters (Teff, gravity, rotation, metallicity) can be extremely important to identify the dominant heating mechanisms.

The last few years have seen the development of solar wind models characterized by high coronal proton temperatures, low electron temperatures and low densities in the inner corona. In addition the emphasis on “additional” acceleration via Alfvén waves has been muted and been replaced by models where the acceleration is essentially complete within 1OR0. This work has, in part, been spurred by Ulysses observations of the high speed wind and by the expectations of the observations to be made by the various instruments on the SOHO satellite. In this paper we show how these new models differ from earlier ones.

Dependencies of solar wind parameters as a function of latitude are presented and discussed, based on measurements of the Ulysses mission during times of declining or minimum solar activity. The heliosphere is found to be structured into a band of slow solar wind at low latitudes and two large high-speed streams from the polar coronal holes. Only small trends of solar wind parameters with heliolatitude are found within these streams. A small but significant temperature difference is seen between the south and the north polar coronal hole.