After 34 years of X-ray astronomical observations, we approach the time when we will be able to explore AGN using line spectroscopy with newly developed technology and methods. In the beginning, X-rays from AGN were observed using proportional counters in a sort of photometric way, allowing us to determine that the continuum is a power law. This result suggested a predominance of non-thermal emission mechanisms in AGN. Rapid variability on time scales as short as 1000 seconds implied a small size for the X-ray emitting region, of order 1014cm, which is 3 orders of magnitude smaller than the optical emission-line regions.

The first detection of line emission from AGN was the Fe-K line by the GINGA satellite. The line energy was 6.4 keV and its equivalent width was about 150 eV in Seyfert 1 galaxies. Detection became possible by use of large-area, low-noise proportional counters. The Fe-K line emission is important in X-ray astronomy because iron is rather abundant and this line is isolated in energy from neighboring lines.

The Fe-K line profile has been examined with the CCD detectors on board ASCA. Many Seyfert 1 galaxies exhibit a broad-line profile of more than 1 keV width with an asymmetric tail on the low-energy side. This is explained as the fluorescence line from a relativistic accretion disk around a black hole, which is broadened by the Doppler motion and distorted by the strong gravitational field. The CCD detectors also revealed the absorption edges of warm material in the line of sight, which is ionized by the strong emission from AGN.

In the next decade, new spectrometers will be launched which can perform spectroscopy with E/dE > 100: AXAF in 1998, XMM in 1999, and Astro-E in 2000. Dispersive spectrometers on board AXAF and XMM will be powerful tools for low-energy lines, while the calorimeter on board Astro-E will examine the Fe-K line profile. Spectral resolution E/dE of several hundred will reveal the intensity ratio of satellite and resonance lines. This will give us physical parameters, such as the density and absolute size of surrounding matter. We hope that the structure of the nucleus will be more deeply understood using X-ray spectroscopy with new instruments, and we will come close to the level of optical spectroscopy, which has worked well in the study of the outskirts of AGN.

The recent observations of a highly ionized, X-ray absorbing gas in active galactic nuclei (AGN) suggest a new nuclear component, the so-called ‘warm absorber’. This gas is likely to be at a temperature of ~ 1–2 × 105 K and is most easily detected in the 0.5–10keV range, where several oxygen absorption edges are often observed.

This review describes the properties of warm absorbers and the relationto other nuclear components, such as the broad-line emitting gas. The stability of the gas is a key issue and analysis shows that it is likely to be thermally stable, at the above mentioned temperature. When successful models are compared to the data, they can be used to infer the column density, composition and level of ionization of the X-ray absorbing gas. They also show that, on top of the strong continuum absorption, the gas must emit X-ray lines that are at the limit of detection by present day X-ray instruments.

New calculations of X-ray emission lines emitted by ionized X-ray absorbers are shown and discussed. Various line equivalent widths are defined and examples are shown over a large range of column density and ionization parameter. The equivalent width of the strongest 0.5-5 keV lines is only a few tens of eV, but in cases of obscured X-ray source, like in Seyfert 2s, the lines are measured against the scattered and diffuse radiation with much larger equivalent widths. The X-ray absorbing and emitting gas is responsible also for a fraction of the observed flux of some UV emission lines. It is also the cause of the detection of several UV absorption lines. The calculations predict that some of those absorption lines, in particular 0 VI λ1035, are very sensitive warm-absorber tracers. Thus, analysis of the combined X-ray and UV properties is the best way to identify the location and properties of this gas.

Understanding the origin and properties of warm X-ray absorbers is a major challenge of AGN research. Several new ideas are briefly discussed, trying to relate the location, mass, and motion of this gas to what is known about other observed nuclear components.

This paper is a very short description of the Advanced X-ray Astronomy Facility (AXAF) and its spectral capabilities. A more detailed description can be found at the WWW location http://hea-www.harvard.edu/asc/SIN/SIN.html.

Evidence is presented for widespread relativistic effects in the central regions of active galactic nuclei (AGN). A sample of 18 Seyfert 1 galaxies observed by ASCA show iron Kα emission which is resolved, with mean full width at half maximum (FWHM) ~ 50,000 km s−1 for a Gaussian profile. However, many of the line profiles are asymmetric. A strong red wing is indicative of gravitational redshifts close to a central black hole, and accretion-disk models provide an excellent description of the data. Such observations probe the innermost regions of AGN, and arguably provide the best evidence yet obtained for the existence of supermassive black holes in the centers of active galaxies.

ASCA observations of four narrow-line Seyfert 1 galaxies are presented. Among the four sources, two show X-ray spectra consisting of soft and hard components. Rapid X-ray variability is observed in all four sources. We estimate the central black-hole mass of these sources and find indications that the apparent luminosities exceed the Eddington limit under some assumptions.

We have observed a sample of Seyfert 2 galaxies with ASCA. The targets were selected based on their [O III] λ5007 flux, the existence of polarized broad lines, and previous X-ray detection. The nuclear luminosities of these galaxies range from 1042 to 1044 ergs s−1 in the 2–10 keV band. Obscured nuclei were found in all galaxies in our sample, with absorption column densities distributed around a value of 1023.5 cm−2.

The cold iron Kα line was clearly detected in half of our sample, and emission from He-like and H-like Fe Kα was detected from two galaxies. These lines are explained by fluorescent lines from thick cold matter and warm/hot thin matter. Highly ionized lines originating in starburst activity were also detected from NGC 1068.

Through X-ray observations with ASCA, low-luminosity active galactic nuclei have been found in at least seven near-by spiral galaxies. Some of them exhibit very intense, and possibly broad, Fe-K emission lines. Their time variability is relatively insignificant, in contrast to lowluminosity Seyfert galaxies.

ASCA observations have revealed the presence of low-luminosity AGN in ~10 LINERs as a hard point sources at the nucleus. The X-ray continuum shapes (photon indices Γ ≈ 1.8) are very similar to those of Seyfert galaxies (Makishima et al. 1997, Serlemitsos et al. 1996). An iron emission line is observed from heavily absorbed low-luminosity AGNs (Makishima et al. 1997), while M81 is the only object among those with small intrinsic absorption from which an iron line detected (Ishisaki et al. 1996) on account of limited photon statistics for other objects.

We summed up the ASCA spectra of 5 LINERs which host low-luminosity AGN of low intrinsic absorption to search for an iron emission line. We used 7 observations of 5 objects (NGC 1097, NGC 3310, NGC 3998, NGC 4450, and NGC 4594) to make a composite spectrum. All the objects have very similar X-ray characteristics (spectral slope, intrinsic absorption, no short time-scale variability; Iyomoto et al. 1996, Serlemitsos et al. 1996).

Irradiated accretion disks around massive black holes are expected to produce part of the line spectrum of AGN, but most of the disk emission must be thermal, observed at UV wavelengths. The two emission components, lines and continuum, are fitted by a unique accretion-disk model that gives the mass of the black hole and the inclination of the disk. The distribution of the disk inclination in a complete sample of Seyfert 1 galaxies suggests that their nuclei are orientation-selected, affected by strong absorption at low disk latitudes. The black-hole masses in the same sample confirm the long-standing non-linearity between M and L for AGN and the non-causal relationship between nearby Seyfert 1 galaxies and distant quasars (i.e., pure luminosity evolution is ruled out).

Irradiated accretion disks are also combined with the relativistic jet model in order to constrain the orientation and the Lorentz factor of 14 superluminal radio sources. At least for a few objects, the line and the radio data are inconsistent with both models, unless a new parameter (jet bending, a second emission-line component, etc.), is also involved. Despite this inconsistency and the ambiguous evidence for combined disk and jet fits in the remaining superluminal sources, a successful merger of these two models might address questions about the nature of AGN and also constrain the Hubble constant.

The most definitive evidence for an accretion disk is provided by the discovery of megamasers (Claussen et al. 1984) which appear to be located in a molecular torus around the nucleus of the mildly active galaxy NGC 4258. Confirmation of Keplerian rotation speed was obtained from radio interferometer (Nakai et al. 1993) and VLBI observations (Greenhill, this volume). Based on the correlation between the spatial locations and radial velocities of the masers, Moran et al. (1995) deduced that the masers are located in a disk at radii R = 0.13–0.26 pc around a black hole with a mass M ≈ 3.5 × 107 M ⊙ (Watson & Wallin 1994, Maoz 1995). A limit on the thickness (H < 2.5 × 10−3 R) is deduced from the velocity dispersion of the maser sources. The mass-diffusion time scale is estimated to be τd > 1015 α−1 s, where α is the viscosity parameter. An efficient angular-momentum transfer mechanism (α > 0.1) is needed for τd ≈ 108 yr, which is the disk evolution time scale inferred from the correlation between interacting galaxies and intense AGN activities (MacKenty 1989, Hernquist 1989). A relatively large value of a is also required to account for the accretion rate needed to power the X-ray flux of NGC 4258.

We determine the variation with radius of the physical parameters (scale height, central density, and central temperature) of a stationary self-gravitating, magnetized accretion disk at radii larger than 102 R G . Our results are relevant in the context of line and continuum emission from the outer region of the disk. We have also studied the influence of self-gravitation and magnetic fields over typical radii, and have shown that the value of R BB is larger than that computed by Collin- Souffrin & Dumont by a factor of about 4.7.

We study the effects of different shock conditions upon shock properties, such as their location and strength. We find that it is very important to include the effects of radiative energy loss in the shock front.

We study shock formation in a stationary, axisymmetric, adiabatic flow of a perfect fluid in the equatorial plane of a Kerr geometry. For such a flow, there exist two intrinsic constants of motion along a fluid world line, namely the specific total energy, E = −hu t , and the specific angular momentum, l = −u φ /u t , where the uμ ’s are the four velocity components, h is the specific enthalpy, i.e., h = (P + ε)/ρ, with P, ε, and ρ being the pressure, the mass-energy density, and the rest-mass density, respectively.

As shown in Fig. 1 (Fig. la is for a Schwarzschild black hole, i.e. the hole’s specific angular momentum a = 0; Fig. lb is for a rapid Kerr hole, i.e. a = 0.99M, where M is the black-hole mass, and prograde flows: and Fig. 1c is for a = 0.99M and retrograde flows), in the parameter space spanned by E and l there is a strictly defined region bounded by four lines: three characteristic functional curves l k (E), l max (E), and l min (E), and the vertical line E = 1. Only such a flow with parameters located within this region can have two physically realizable sonic points, the inner one r in , and the outer one r out . In between there is still one more, but unrealizable, sonic point, r mid . The region is divided by another characteristic functional curve l c (E) into two parts: in region I (= Ia + Ib) only τout is realized in a shock-free global solution (i.e., that joining the black-hole horizon to large distances), while in region II (= IIa + IIb) only r in is realized.

In this paper, we consider a compilation of 55 objects with known superluminal motions (SM), and whose flux density (X-ray, optical, radio), core dominance parameter (R), superluminal velocity, and radio Doppler factor (δ R ) are known. Our study shows that SM is consistent with the beaming model, and the relation

is reasonable. The statistical correlation between superluminal velocity and redshift is a result of selection and the statistical correlation between R and brightness temperature (T ob ) is actually a reflection of the correlations between δ, R, and T ob for objects with SM. Up to now, 59 objects have been reported to have SM, but for reasons discussed elsewhere (Vermeulen & Cohen 1994), only 55 are considered here.

We present preliminary results of two monitoring projects on AGNs and BL Lacs with four intermediate-band optical filters at Lick Observatory, South African Astronomical Observatory, and Siding Spring Observatory. We applied differential photometry between the target and non-variable field stars to derive the variability amplitudes. All the AGNs varied in all wavebands, with amplitudes from 30% to 90%. The variations in shorter wavelength filters are always larger than in longer wavelength filters. We did not observe significant continuum slope changes. Continuum variations of BL Lacs up to 0.2 mag in 20 minutes were observed. Interestingly, we found a time lag of 6–8 minutes between two seemingly correlated variations observed in the intermediate U band with the 1-m telescope and in broad V band with the 24-in. telescope at SSO.

The bright Seyfert 1 galaxy NGC 4151 was monitored intensively by IUE, ROSAT, ASCA, CGRO, and ground-based telescopes for 10 days in 1993 December. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths (see Fig. la). The strongest variations were seen in medium energy (~1.5keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA = 6%) variations, uncorrelated with those at lower energies, were seen at soft γ-ray energies of ~100keV. No significant variability was seen in softer (0.1–1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased within increasing wavelength, from 9% to 1% between 1275 Å and 6900 Å.

This paper assesses what we have learned about the structure of AGN broad-line region (BLR), with emphasis on the work other than reverberation studies in order to complement other recent reviews. Basic photoionization models are briefly described. Current models of the BLR and some of the observations that might distinguish among them are discussed.

We describe two ongoing studies of QSO broad emissionline regions (BELRs). The first employs the N V λ1240/He II λ1640 and N V/C IV λ1549 line ratios as diagnostics of QSO metallicities. Hamann & Ferland and Ferland et al. showed that many observed N V ratios require enhanced N abundances and Z > Z ⊙. Here we present new measurements of large line ratios at redshifts z > 4, which indicate super-solar abundances within ~1 Gyr of the Big Bang (for q 0 ≈ 0.5). We also note that the N V line is relatively stronger in more luminous QSOs, in contrast to the well-known Baldwin effect in Lyα, C IV, and O VI λ1034. This unusual behavior in N V could be due to a luminosity-metallicity correlation among QSOs that is coupled to a mass-metallicity relation in their host galaxies.

Our second study involves Ne VIII λ774 as a probe of highly ionized gas. We show that a broad emission feature near 774 Å is common in QSOs. Photoionization models indicate that Ne VIII is the most likely identification for this feature. The models also indicate that the Ne VIII emitting gas covers > 40% of the continuum source, has a total hydrogen column density of N H > 1022 cm−2 (for solar abundances) and an ionization parameter of U > 5 (for a nominal QSO continuum shape). This gas would be an X-ray “warm73x201D; absorber — with O VII–O VIII bound-free edges — if it lies along our line-of-sight to the X-ray continuum source.