Abstract

The properties of the iron Kα line emitted by a photoionized accretion disc have been calculated for different source geometries.

The properties of the iron Kα fluorescence line emitted by an α-viscosity accretion disc illuminated by an external X-ray source have been calculated for different values of the disc accretion rate ṁ. The vertical ionization structure of the matter has been computed by using the numerical code described in. Two different source geometries have been studied: a point source located at 20 rg (=GM/c2) above the disc on its symmetry axis, and an extended source above the innermost part (r = 6 – 50rg) of the disc. Assuming α=0.1, a Schwarzschild black hole and a hard luminosity equal to the disc luminosity, we find that for large values of ṁ (≲ 0.2, in units of the critical value) the matter can be significantly ionized, and the iron line equivalent width can reach values as high as 250 eV for the point source, and up to about 400 eV for the extended source (while for neutral matter it is ∼150eV for a face-on disc). The line centroid energy, in the emitting rest frame, is significantly higher than 6.4 keV, the value for neutral iron. A further increase of ṁ (≲ 0.5) leads to a strong decrease of the line intensity, because the iron becomes fully stripped in the inner region of the disc.

Abstract

Long time scale ultraviolet light curves of blazars and Seyfert 1s both show very strong continuum variations, but this similarity vanishes when short time scales, spectral variability and correlations between variability and luminosity are studied. For instance, blazars show much more rapid variations than Seyfert 1s. Also, the spectra of Seyfert 1s harden as the source brightens, while blazars show little spectral variability. Third, the most luminous blazars tend to be the most strongly variable, while for Seyfert 1s, the strongest variations are seen in the least luminous sources. These differences suggest that in spite of some overall similarities, the observed emission from blazars and Seyfert 1s have different physical origins. These results are consistent with models which hold that the ultraviolet emission from blazars is incoherent synchrotron emission from a jet, while that from Seyfert 1s is dominated by thermal emission from an accretion disk.

Background

In the 13 years since its launch, IUE has obtained over 5000 ultraviolet spectra of active galactic nuclei (AGN). In this paper, we use ∼2500 spectra of 16 objects to survey the ultraviolet variability properties of Seyfert 1s (defined to include quasars as well; [2]) and blazars (BL Lacs and OVV quasars; [6]).

Long and Short Time Scale Variability

Although it is a common prejudice that blazars are more strongly variable than Seyfert 1s, the long term variability properties of the two types of object are actually very difficult to distinguish.

Abstract

Gamma ray emission from extragalactic sources is interpreted as the Doppler boosted annihilation and Inverse Compton radiation from a relativistic electron-positron beam in the frame of the two-flow model. In the case of 3C279, the high luminosity and the rapid variability of gamma ray emission suggest a relativistically moving source, but even so the compactness cannot be smaller than unity at light week scale with a reasonable Doppler factor. This supports the two-flow model of extragalactic radio sources, where the small scale emission comes from a relativistic electron-positron beam, heated by a MHD jet responsible for the large-scale (kpc) radio structures.

Introduction

The GRO satellite has detected intense gamma ray emission from several Active Galactic Nuclei and quasars. Remarkably, all of them are associated with a flat spectrum radio source, whose radio spectral index αr is smaller than 0.5, and half of them exhibit known or probable superluminal motions (the others have not been observed at different epochs in VLBI). Just like the commonly invoked Doppler beaming amplification of radio emission, the high γ-ray luminosity suggests also that the emitting source is moving relativistically.

For 3C279 in particular, the spectrum reported by Hermsen & al. show a maximum emission per logarithmic energy interval around 10 MeV, with a photon spectral index of approximately 1.5 below the turn-over frequency and approximately 2 above it. A rapid flare has been observed with an increase of the luminosity by a factor 5 on a time scale of 2 days.

Abstract

We present and discuss the results of long-slit spectroscopic observations of the extended emission line regions (EELR) in NGC4388, a Seyfert 2 galaxy in which extended, off-nuclear broad Hα emission (FWZI ≈ 4000 km s−1) has been reported (Shields & Filippenko 1988). These features have been interpreted as scattered radiation from a Seyfert 1 nucleus that is obscured along our line of sight. Our spectroscopic observations cover a large fraction of the inner part of the EELR, including some of the positions where the presence of broad lines has been claimed. Broad wings in the Hα + [NII] λλ6548, 6583 complex are also present in our data but they can be explained by the superposition of several narrow components. However, we cannot, at present, exclude the possibility that an intrinsically broad component to Hα exists at some locations. The implications of our results for unified models of Seyfert galaxies are briefly discussed.

Introduction

NGC4388 is a high-inclination Seyfert 2 galaxy with bright EELR (Colina et al. 1987, Pogge 1988, Corbin et al. 1988). Radio observations reveal a double-peaked source close to the apparent optical nucleus and more extended emission aligned roughly perpendicular to the galaxy disk (Stone et al. 1988, Hummel & Saikia 1991). The axis of the cone-like, high-excitation gas is almost perpendicular to the galaxy disk and close to that of the extended radio emission (Pogge 1988, Corbin et al. 1988), suggesting that the EELR are photoionized by nuclear radiation which escapes preferentially along and around the radio axis.

Abstract

Although claimed to possess a Seyfert nucleus, NGC 1808 reveals radio properties, line widths and ratios that are consistent with a few, albeit powerful, SNRs. There is as yet no compeling evidence for Seyfert activity, and this galaxy should be reclassified as a starburst galaxy (it shows many similarities to NGC 253 and M82).

Introduction

NGC 1808 is a ‘hotspot’ or Sersic–Pastoriza galaxy at a distance of 16.4 Mpc (1″ = 80 pc). Although this galaxy shows optical hotspots, these hotspots have largely disappeared at near–infrared wavelengths. It is a highly inclined and dusty Sbc spiral, with evidence for a burst of star formation in the circumnuclear region about 5 × 107 yrs ago. Two pieces of evidence suggested that NGC 1808 habours a Seyfert nucleus:

• High resolution spectra revealed broad lines (Veron–Cetty & Veron 1985).

• The 6cm radio luminosity of the compact nucleus is 500 times that of the most luminous Galactic supernova remnant (SNR), suggesting a non–stellar nucleus (Saikia et al. 1990).

Line Ratios

We have obtained a red spectrum of NGC 1808 (see Forbes, Boisson & Ward 1992 for details) containing the [SIII] lines at 9069 and 9532Å. Fig. 1 shows a diagnostic diagram based on the sulphur line ratios. We also show the mixing curve between the location of HII regions and Galactic SNRs. This line ratio diagram (and those for [OI]6300Å, [NII]6583Å, [SII]6717+6731Å) are consistent with a high abundance HII region and a ∼ 20% contribution from SNRs, i.e. no Seyfert nucleus is required to explain the line ratios.

Abstract

The two persistent soft gamma-ray sources in the galactic centre region are black hole candidates of stellar mass with comptonized accretion disks that radiate 1037−38 erg s−1. They appear as microquasar stellar remnants from which emanate double-sided radio jets that extend over distances of a few parsecs.

The galactic centre in soft γ-rays

Contrary to the standard X-ray band (below 20keV), where many sources have been detected at less than 5° from the galactic centre, at higher energies (35-500 keV) the field is dominated by only two persistent sources: 1E1740.7-2942 and GRS1758-258 located respectively at ∼ 50′ and ∼ 5° from Sgr A (see Fig. 1). In the 30-500 keV band these sources radiate a few ×1037 erg s−1, near the Eddington limit of collapsed objects of stellar mass. Although persistent, they are time variable. Since no γ-ray source has been detected from Sgr A, if there is a super-massive black hole at the dynamic centre of the Galaxy, at present it is in sepulchral silence.

The telescope SIGMA on board GRANAT detected a powerful burst around 420 keV from the strongest γ-ray source 1E1740.7-2942 which is interpreted as the redshifted (probably gravitationally) 511 keV line from the annihilation of e+e− pairs. Since this Einstein source can produce and annihilate 10 billion (1010) tonnes of positrons in just one second, it is now known as the “Great Annihilator”. No annihilation line has yet been detected from GRS1758-258, the second strongest soft γ-ray source in that region.

Introduction

Several authors have proposed that one of the mechanisms for fuelling active galactic nuclei (AGN) may involve the presence of a stellar bar in the galaxy (Shlosman et al., 1989 and references therein). The bar potential produces an overall instability that drives the ambient gas of the disk inwards, forming a gaseous disk sorrounding the active nucleus at a radius of several hundred parsecs. Under certain circumstances the gas can dissipate its angular momentum and fall into the centre. Numerical simulations show that the movement of the gas towards the nucleus would be preferentially along the stellar bar in two arms (Athanassoula 1992). The shocked material in the flow would be delineated by dust lanes that, in regions close to the nucleus might be seen as a ring–like structure or “inner spiral arms”. We present here evidence of such a structure surrounding the nucleus in the Seyfert 1 galaxy NGC4151.

Results

Broad–band images of the nucleus of NGC4151 were obtained in the U,B,V,I and Z filters with the 1m JKT telescope in La Palma. The images were calibrated in magnitudes and then subtracted in pairs to produce a series of colour maps. The B and V images are clearly contaminated by extended emission lines. However, a line–free colour map was obtained by combining a continuum image obtained by Pérez et al. (1989) in a filter centred at 5700Å, with the I-band image.

Due to the combination of high radiation densities in the BLR and the high column densities of clouds which are optically thick to the Lyman continuum, many lines emitted from the BLR are likely to be optically thick, and hence will be emitted anisotropically. Ferland et al. (1992) discussed the case where the amount of anisotropy is constant for all clouds, but in reality the anisotropy will vary for a spatially extended BLR, depending on the local physical conditions. To study this effect, we have computed several models for spherical BLR's populated by an ensemble of spherical clouds in pressure balance with an intercloud medium, using our BLR modelling code, PROSYN (Goad, O'Brien & Gondhalekar 1993), which utilizes the photoionization code CLOUDY (Ferland 1991). Here we show some of the results for a constant pressure BLR model, where the cloud density (N) and column density (Ncol) are constant with radius (r), and hence the ionization parameter (U) varies as r−2. The model was normalized to have N = 1010 cm−3, Ncol = 1023.75 cm−2 and U = 10−2 at a radius of 10 light-days. The inner and outer radii were set at 0.3 and 56.2 light-days respectively, giving a range in log U from 1.0 to −3.5.

The total line emissivity εtotl = εin + εout, where εin is the emissivity of the inward cloud face (towards the ionizing continuum source) and εout is the outward emissivity (away from the ionizing continuum source).

Abstract

Double radio sources provide perhaps the most graphic evidence for anisotropy in AGN, and observations of their large-scale structure can potentially give us a direct means of measuring the emission axis of the AGN itself. Attempts to unify radio sources using orientation and doppler boosting arguments alone have been relatively successful, however recent discoveries of correlated asymmetries on the kpc- to Mpc-scale in the radio, optical and IR may pose serious problems for such models.

Introduction

At first sight a paper about the large-scale (kpc – Mpc) structure in radio galaxies appears to have little direct relevance to the nature of the compact object in AGN. If we are considering evidence for anisotropy however then radio sources provide the most graphic evidence for such emission from the AGN – for example 4C74.26 (Fig. 1) shows all the principal features: an essentially linear structure with a one-sided jet linking the core with the southern lobe, the alignment of core and hotspots and a one-sided VLBI jet aligned with the large-scale jet. Potentially, radio sources could provide us with one of the best ways of determining the orientation of the emission axis of the AGN, however the recent discovery of large-scale correlated asymmetries, may not only make this a difficult task, but may also provide strong evidence against unified models of AGN phenomena in terms of orientation dependencies.

So as to limit my discussion I will consider only high-power objects with P(178 MHz) ≥ 1025.5 W Hz sr−1.

Abstract

We present a [O III] λ5007 image of the nuclear region of the Seyfert 1 galaxy NGC 4151 obtained with the Planetary Camera on the Hubble Space Telescope. The [O III] image shows a striking biconical structure centred on the bright, unresolved central source. Simple geometric arguments place our line of sight outside the cone of ionizing radiation. Since we have a nearly unobstructed view of the UV continuum and broad-line region, an optically thick molecular torus cannot be the source of the collimation. Lower column density material visible in UV spectra is largely transparent at UV and optical wavelengths, but opaque beyond the Lyman limit. It can collimate the ionizing radiation field without obscuring our view of the central engine.

The simplest unified models of Seyfert 1 and Seyfert 2 galaxies invoke a dense, optically thick, molecular torus surrounding the central engine and broad-line region which serves as both a shield which prevents us from viewing the centres of Seyfert 2's and as the source of the collimation for the ionizing radiation. The difference between Seyfert 1's and Seyfert 2's is then due entirely to the opening angle of the torus and its orientation relative to our line of sight. The observations presented here and by Evans et al. present a direct challenge to this simple view.

Figure 1 shows the raw image obtained with the Planetary Camera on the Hubble Space Telescope through the F502N filter in a 1608 s exposure.

Abstract

Here is presented a statistical test for the accretion disc model. The effect of different inclinations of the disc to the line of sight is shown. Then the possibility that the ratio R of the radio core luminosity to the radio lobe luminosity is a good orientation indicator is investigated. It is shown that R. describes the intrinsic power of the central radio source.

A statistical test of the accretion disc model

The model adopted here is that, described by Collin and Dumont (1989), of an accretion disc illuminated by the central nonthermal source. The disc emits in its inner part the UV and optical continuum, and in its outer parts the low ionization lines (LIL; e.g., the Balmer lines) as proposed by Collin (1987). A second region emits the high ionization lines (HIL; e.g., Lα and CIV); it can have a spherical geometry. Both regions are illuminated by the central source. For the disc, illumination occurs through backscattering of the nonthermal radiation by a hot medium (halo). This model has been worked out by Dumont and Joly (1992).

Using the code of Dumont and Collin (1990) we have computed the line spectrum emitted by the illuminated disc as a function of the central source luminosity, the Eddington ratio (Lbol\LEdd) and the characteristics of the scattering halo (density structure and optical thickness).

Abstract

The γ-ray emission from three of the sources detected by the COMPTON Observatory is discussed in the framework of relativistic jets emitting via the synchrotron self-Compton mechanism. The physical conditions in the three sources are derived and compared.

One of the most exciting discoveries of the COMPTON Observatory is the observation of γ-ray emission from 16 blazars. The broad band continuum (radio to X-ray) from these objects can be understood on the basis of the synchrotron self-Compton process in an inhomogeneous jet of plasma moving with relativistic speed (e.g.). In this model the synchrotron emission usually extends up to the UV and possibly to the X-ray bands. It is therefore natural to expect γ-rays produced by first order self-Compton scattering. The strength of this component depends on the ratio between the radiation and magnetic energy densities, and on the optical depth to photon-photon absorption. The latter process can be avoided if the source is beamed, and independent evidence for beaming is indeed found from radio and X-ray observations.

Nevertheless the γ-ray intensity measured from 3C 279 was a great surprise, in that the γ-ray power exceeded that in all other bands. This requires the radiation energy density to be larger than the magnetic energy density, a situation usually called the “Compton catastrophe” since at first sight it gives rise to divergent Compton energy losses.

Abstract

We present the results of a monitoring campaign of the low luminosity Seyfert 1 NGC 4593, at X-ray, ultraviolet, optical and near IR frequencies. The rapid and large amplitude fluctuations detected set interesting upper limits to the sizes of the various emitting regions. We investigate the properties of the nuclear energy distribution, its pattern of variability and the relationship between the emission processes at different frequencies. We also study the variations of the broad emission lines in relation to those of the continuum. The implications of our data are discussed. It is found that they are consistent with a very hot accretion disk illuminated by the hard X-ray source.

Observations

Repeated observations of the nucleus of NGC 4593 have been performed from the near IR to X-rays. Though the temporal density of our observations is poor, they have the advantage of a wide wavelength coverage. Besides, NGC 4593 turned out to be highly variable at all frequencies.

Results and implications

The strong and rapid variations detected in all wavebands imply that the continuum source is unusually compact: ≤ 1.1 lt-hr in the hard X-rays, ≤ 3 lt-hrs in the soft X-rays, 70 lt-hrs at 1450 Å, and 37 lt-days at 2.5 microns. The latter corresponds roughly to the dust evaporation radius (∼ 33 lt-d, Barvainis, 1987) in such a low luminosity source. Such an IR behaviour is reminiscent to that of Fairall 9, a more luminous Seyfert 1, (Clavel et al., 1989).

Abstract

Accretion disks are the preferred central engine for AGN, but theoretical progress has long been hampered by the unknown nature of the angular momentum transport mechanism. An obstacle preventing the general acceptance of turbulent disk theories has been the intransigent stability of thin Keplerian disks to hydrodynamic perturbations. This difficulty is overcome by the result discussed here, that a weak magnetic field renders a disk locally unstable whenever the angular velocity decreases outward. The salient and remarkable feature that distinguishes this instability is that the maximal growth rate (which is of order the angular velocity) is independent of the strength of the field, even as the latter vanishes. The instability can be derived straightforwardly from the equations of orbital mechanics and a spring-like magnetic interaction. We list several key questions regarding the instability's implications for realistic disks, and review the results from two-dimensional numerical simulations.

Introduction

Many of the scenarios created to explain the spectra from Active Galactic Nuclei (AGN) involve, either implicitly or explicitly, the release of gravitational energy through accretion. The classical paper of Lynden-Bell (1969) was the first to suggest that the heart of an active nucleus consisted of an accretion disk surrounding a supermassive black hole. The appeal of this model (and its direct descendents) has always been one of energetics: no other proposed mechanism is as efficient and as compact. Gravitational potential energy release is hard to beat as an energy production mechanism.

Abstract

From spectropolarimetric observations of the galaxy, IRAS 110548-1131 we report strong, broad Hα emission (FWHM 7600 km s−1) in the polarized flux spectrum. This suggests that IRAS 110548-1131 has an obscured broad line region, whose radiation is scattered into our-line-of-sight by scatterers outside the obscured region.

Introduction

Since the discovery by Antonucci & Miller (1985) and its confirmation by Bailey et al. (1988) that the type II Seyfert galaxy NGC 1068 has broad hydrogen Balmer lines in it's polarized flux spectrum, and therefore has a Seyfert I type nucleus, it has been possible to construct a physical model in which the two types of Seyfert are in fact the same. Whether an object is seen as a type I or II depends upon the orientation of the galaxy and obscuration of the broad line region, probably in the form of a dusty torus, although the universality of such a model is an open question. Radiation from the BLR can escape along the axis of the torus and then be scattered into the line-of-sight by electrons and/or dust. The scattered component of these lines should then be observable in polarized flux.

In recent years there has been considerable interest in those galaxies identified from the IRAS survey which are very luminous in the far infrared region. Some evidence that highly luminous IRAS galaxies contain obscured QSO nuclei comes from the work of Hough et al.