Regions Of Active Galactic Nuclei Research Articles

2.9. Microlens mapping of disks in active galactic nuclei

Generally, it is believed that there is a supermassive black hole and a surrounding accretion disk in a central region of active galactic nuclei (AGN). However, it is quite difficult to obtain direct information about the center of AGN, because the accretion disk size is far too small to resolve.

Steps toward Determination of the Size and Structure of the Broad‐Line Region in Active Galactic Nuclei. XI. Intensive Monitoring of the Ultraviolet Spectrum of NGC 7469

From 1996 June 10 to July 29, the International Ultraviolet Explorer monitored the Seyfert 1 galaxy NGC 7469 continuously in an attempt to measure time delays between the continuum and emission-line fluxes. From the time delays, one can estimate the size of the region dominating the production of the UV emission lines in this source. We find the strong UV emission lines to respond to continuum variations with time delays of about 23-31 for Lyα, 27 for C IV λ1549, 19-24 for N V λ1240, 17-18 for Si IV λ1400, and 07-10 for He II λ1640. The most remarkable result, however, is the detection of apparent time delays between the different UV continuum bands. With respect to the UV continuum flux at 1315 A, the flux at 1485 A, 1740 A, and 1825 A lags with time delays of 021, 035, and 028, respectively. Determination of the significance of this detection is somewhat problematic since it depends on accurate estimation of the uncertainties in the lag measurements, which are difficult to assess. We attempt to estimate the uncertainties in the time delays through Monte Carlo simulations, and these yield estimates of ~007 for the 1 σ uncertainties in the interband continuum time delays. Possible explanations for the delays include the existence of a continuum-flux reprocessing region close to the central source and/or a contamination of the continuum flux with a very broad time-delayed emission feature such as the Balmer continuum or merged Fe II multiplets.

Spectral Dependence of the Broad Emission Line Region in Active Galactic Nuclei

We derive a theoretical relation between RBLR, the size of the broad emission line region (BLR) of active galactic nuclei (AGNs), and the observed soft X-ray luminosity and spectrum. We show that in addition to the well-known RBLR~L1/2 scaling, RBLR should depend also on the soft X-ray spectral slope, and we derive the expected relation between RBLR and the X-ray luminosity and spectral index. Applying this relation to calculate a predicted BLR radius for 10 AGNs with reverberation data, we show that including the dependence on the spectrum improves the agreement between the calculated BLR radius and the radius independently determined from reverberation mapping. Similarly, we evaluate an expression for the line width and show that including the dependence on the spectrum significantly improves the agreement between the calculated BLR velocity dispersion and the observed FWHM of the Hβ line. The theoretical expression for the line width also provides a physical explanation to the anticorrelation between the soft X-ray slope and the emission-line width observed in narrow-line Seyfert galaxies.

Steps toward Determination of the Size and Structure of the Broad‐Line Region in Active Galactic Nuclei. X. Variability of Fairall 9 from Optical Data

The results of an optical monitoring campaign on the active nucleus in the luminous Seyfert 1 galaxy Fairall 9 are presented. This campaign was undertaken in parallel with ultraviolet spectroscopic moni- toring with the IUE satellite which is described in a separate paper. The primary purpose of this program is to measure the response times (or lags II) of the emission lines to continuum variations and thus to extend the range in luminosity of active galactic nuclei (AGNs) for which such measurements have been made. The main conclusions of this work are as follows: 1. Continuum (at 5340 variations of amplitude D12% are detected on timescales as short as D20 Ae ) days. These variations are much larger than the typical uncertainties in the measurements, which are of order D2%. Over D94 days, a factor of 2 change in the nuclear continuum was observed. 2. The optical continuum light curve resembles that of the UV continuum, showing two events II of low-amplitude variations with a duration of D70 days and with no measurable lag between the UV and optical continuum light curves. The UV data show a third larger amplitude event that occurred after the optical monitoring had terminated and unfortunately went unobserved in the optical. 3. The Hb emission-line Nux also underwent signi-cant, low-amplitude (20%) variations. Cross- correlation analysis reveals that Hb lags behind the UV continuum by about 23 days, a value much smaller than what was previously suggested by earlier variability studies. However, this small lag is con- sistent with the lags for the UV lines during this campaign in the sense that the Hb lag is approximately 50% larger than that of Ly aj 1216, as it has been found for lower luminosity AGNs. 4. The Hb di†erence pro-le produced by subtracting the low-state from the high-state data can be described as a two-component structure with blue and red components of similar width (D2500 km s~1) and that appear to vary in phase. Subject headings: galaxies: active E galaxies: individual (Fairall 9) E galaxies: nuclei E galaxies: Seyfert

On Nulling Interferometers and the Line-emitting Regions of Active Galactic Nuclei

The nulling interferometers proposed to study planets around other stars are generally well suited for studying small-scale structures surrounding other bright pointlike objects, such as the nuclei of active galaxies. Conventional interferometric techniques will produce useful maps of optical/IR line and continuum emission in active galaxies on scales of 10 mas, but similar studies of broad-line regions will require baselines longer than those currently envisaged. Nevertheless, nulling interferometers currently under development will be able to constrain quasar velocity fields on ~1 mas scales, as long as they are equipped with spectrographs capable of resolving lines several hundred km s-1 wide. This Letter describes how analyses of line emission leaking through the edges of the null in such an instrument can reveal the size, shape, and velocity field of nebular gas on the outskirts of a quasar broad-line region. If this technique proves effective, it could potentially be used to measure the mass function of quasar black holes throughout the universe.

Limits from rapid TeV variability of Mrk 421

The extreme variability event in the TeV emission of Mrk 421, recently reported by the Whipple team, imposes the tightest limits on the typical size of the TeV emitting regions in active galactic nuclei (AGN). We examine the consequences that this imposes on the bulk Lorentz factor of the emitting plasma and on the radiation fields present in the central region of this active nucleus. No strong evidence is found for extreme Lorentz factors. However, energetics arguments suggest that any accretion in Mrk 421 has to take place at low rates, compatible with an advection-dominated regime.

Physical Conditions of the Coronal Line Region in Seyfert Galaxies

The launch of the Infrared Space Observatory and new atomic data have opened a window for the study of high-ionization gas in active galactic nuclei (AGNs). We present the results of a large number of photoionization simulations of the region in AGNs, employing new atomic data from the Opacity and Iron Projects. Our grid of line emission spans 8 orders of magnitude in gas density and 14 orders of magnitude in ionizing flux in an effort to identify the optimal conditions in which these lines form. We show that coronal lines form at distances from just outside the broad-line region to ~400L1/243.5 pc, in gas with ionization parameter -2.0 log U(H) 0.75, corresponding to gas densities of 102-108.5 cm-3, with electron temperatures ~12,000-150,000 K. A large range of distances from the central source implies significant line width variation among the coronal lines. We identify several line ratios that could be used to measure relative abundances, and we use these to show that the coronal line gas is likely to be dust free.

Steps toward Determination of the Size and Structure of the Broad‐Line Region in Active Galactic Nuclei. IX. Ultraviolet Observations of Fairall 9

An 8 month monitoring campaign on the Seyfert 1 galaxy Fairall 9 has been conducted with the International Ultraviolet Explorer in an attempt to obtain reliable estimates of continuum-continuum and continuum-emission-line delays for a high-luminosity active galactic nucleus (AGN). While the results of this campaign are more ambiguous than those of previous monitoring campaigns on lower luminosity sources, we find general agreement with the earlier results: (1) there is no measurable lag between ultraviolet continuum bands, and (2) the measured emission-line time lags are very short. It is especially notable that the Ly? + N V emission-line lag is about 1 order of magnitude smaller than determined from a previous campaign by Clavel, Wamsteker, & Glass (1989) when Fairall 9 was in a more luminous state. In other well-monitored sources, specifically NGC 5548 and NGC 3783, the highest ionization lines are found to respond to continuum variations more rapidly than the lower ionization lines, which suggests a radially ionization-stratified broad-line region. In this case, the results are less certain, since none of the emission-line lags are very well determined. The best-determined emission line lag is Ly? + N V, for which we find that the centroid of the continuum-emission-line cross-correlation function is ?cent ? 14-20 days. We measure a lag ?cent 4 days for He II ?1640; this result is consistent with the ionization-stratification pattern seen in lower luminosity sources, but the relatively large uncertainties in the emission-line lags measured here cannot rule out similar lags for Ly? + N V and He II ?1640 at a high level of significance. We are unable to determine a reliable lag for C IV ?1550, but we note that the profiles of the variable parts of Ly? and C IV ?1550 are not the same, which does not support the hypothesis that the strongest variations in these two lines arise in the same region.

ASCAObservations of Seyfert 1 Galaxies. II. Relativistic Iron Kα Emission

We present evidence for widespread relativistic effects in the central regions of active galactic nuclei. In a sample of 18 Seyfert 1 galaxies observed by ASCA, 14 show an iron K? line that is resolved, with mean width ?K? = 0.43 ? 0.12 keV for a Gaussian profile (full width at half-maximum, FWHM ~ 50,000 km s-1). 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. The peak energy of the line is 6.4 keV, which indicates that it arises by fluorescence in near-neutral material. Our fits imply a low inclination for the disk in these Seyfert 1 galaxies, with a mean of 30?, consistent with orientation-dependent unification schemes. Differences in the line profiles from source to source imply slight variations in geometry, which cannot be accounted for solely by inclination. In most cases, we require that the line emission arises from a range of radii. Although a small contribution to the emission from a region other than the disk is not ruled out, it is not generally required and has little effect on our conclusions regarding the disk line. Our data are fit equally well with rotating (Kerr) and nonrotating (Schwarzschild) black hole models. We find a mean spectral index in the 3-10 keV range of ?3-10 = 1.91 ? 0.07 after accounting for the effects of reflection. Such observations probe the innermost regions of active galactic nuclei and arguably provide the best evidence yet obtained for the existence of supermassive black holes in the centers of active galaxies.

Dense, thin clouds and reprocessed radiation in the central regions of active galactic nuclei

The primary radiation generated in the central continuum-forming region of Active Galactic Nuclei can be reprocessed by very dense, small-scale clouds that are optically-thin to Thomson scattering. In spite of the extreme conditions expected to prevail in this innermost, central environment, the radiative clouds can survive and maintain cool temperatures relative to the ambient emitting region by means of magnetic confinement. Motivated by these ideas, we present a detailed quantitative study of such clouds, explicitly describing the physical properties they can attain under thermal and radiative equilibrium conditions. We also discuss the thermal stability of the gas in comparison to that of other reprocessing material thought to reside at larger distances from the central source. We construct a model to predict the emergent spectra from a source region containing dense clouds which absorb and reemit the primary radiation generated therein. Our predicted spectra show the following two important results: (i) the reprocessed flux emitted at optical/UV energies is insufficient to account for the blue bump component in the observed spectra; and (ii) the amount of line radiation that is emitted is at least comparable to (and in many cases dominates) the continuum radiation. The lines are extremely broad and tend to accumulate in the extreme ultraviolet, where they form a peak much more prominent than that which is observed in the optical/UV. This result is supported by current observations, which indicate that the spectral energy distribution of radio-quiet AGN may indeed reach a maximum in the EUV band.

Velocity Diagnostics of the Broad Emission-Line Region of Active Galactic Nuclei

AbstractWe present a brief review of emission-line velocity differences, and describe an ongoing project to determine the driving mechanisms responsible. We conclude with a brief outline of the use of velocity differences as probes of the conditions in the nuclear region of AGNs.

Modelling the Polarization from Accretion Disks

AbstractThe 3D transfer equation for polarized radiation is solved for several different model configurations. In the case of Rayleigh scattering the phasematrix is decomposed into a sum of matrix products to allow the use of operator splitting iteration schemes. For 30 slabs with constant extinction matrix and constant emission the emerging polarized fiuxe are calculated. We find that the optical depth and the scattering parameter can be fixed from observations even if only the fluxes are known. Specifying the extinction matrix our models can be applied to the inner regions of active galactic nuclei.

Relativistic Iron Kα Emission in Seyfert Galaxies

AbstractEvidence 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.

The Disk Wind Model of the Broad Line Region of Active Galactic Nuclei

AbstractIn a fashion similar to winds in O stars, the AGN disk winds may be responsible for the broad, blue-shifted absorption of resonance lines which characterize the Broad Absorption Line QSOs. The dynamical features of the disk wind may also account for many of the properties of the broad emission lines of AGN. Although the velocity field of the material responsible for the line emission is primarily that of the accretion disk, the presence of an accelerated radial wind influences the line formation so that single-peaked profiles are generic. Further, the recent reverberation mapping response of NGC 5548 is also qualitatively explained by these same radiative transfer effects.

Clumpy Scattering Regions in Active Galactic Nuclei

AbstractSimple unified models predict that many Seyfert 2s should be highly polarized ( ≳ 50%), but such highly polarized Seyfert 2s are not observed. As one of the possibilities to explain this problem, the existence of clumpy scattering clouds with optically thick condensations in the nuclear regions is discussed.

Physical constraints on the sizes of dense clouds in the central magnetospheres of active galactic nuclei

The range of microphysical and global dynamical timescales in the central regions of Active Galactic Nuclei (AGN) is sufficiently wide to permit the existence of multiphase structure. In particular, very dense, cool clouds can coexist with a hot, magnetically-dominated medium and can thereby efficiently reprocess the continuum radiation generated in this primary source region. The strong dynamical forces in this central magnetosphere can give rise to extremely small clouds. Microphysical processes then determine whether such clouds can indeed survive, in spite of their extremely contrasting properties relative to the surrounding environment, for long enough to produce potentially observable thermal reprocessing signatures. We examine specific physical constraints on the thicknesses of such reprocessing clouds. Our results are plotted to show the range of conditions that is representative of the central regions of AGN. We find a parameter subspace in the extreme high density regime for which the effects of microphysical diffusion processes can be overcome and for which cool gas can maintain pressure equilibrium with the ambient magnetosphere.

Doppler Tomography of Relativistic Accretion Disks

Spectral lines from a source orbiting around a compact object are studied. Time variations of observed frequency and count rate due to motion of the source and gravitational lensing are considered. Gravitational field of the central object is described by the Kerr metric. It is shown that: (i) simultaneous temporal and frequency resolution enables us to restrict parameters of the model (inclination angle, position of the source, angular momentum of the black hole); (ii) techniques of image restoration, familiar from other fields of astronomy, can be applied to study inner regions of active galactic nuclei. This contribution is relevant for extremely variable X-ray sources with high parameter of efficiency, such as Seyfert 1 galaxy PHL 1092 observed by ROSAT.

Red Giant--Disk Encounters: Food for Quasars?

We explore the role that red giants might play in the central regions of Active Galactic Nuclei. Due to their large radii and the low binding energy of the stellar envelope, giants are vulnerable to envelope stripping from collisions with the accretion disk. Using hydrodynamic simulations we show that such collisions will typically deposit a substantial fraction of the envelope mass into the disk on each passage. Repeated encounters will then lead to the complete destruction of the star save for the dense core. We estimate the rate of fuel supply by this mechanism using simple models for the AGN disk and central stellar cluster. If the central stellar density is of order 10^7 solar masses per cubic pc, then stripping of giants could account for the activity of typical AGN provided that the accretion disk extends beyond 0.1 pc. For AGN with smaller disks, or clusters of lower central density, giant stripping could be an important source of gas enriched via stellar nucleosynthesis.

A dynamical model for the narrow-line region of active galactic nuclei

A dynamical model for the narrow-line region of active galactic nuclei

Light Curve of a Source Orbiting a Black Hole: A Fitting Formula

view Abstract Citations (10) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Light Curve of a Source Orbiting a Black Hole: A Fitting Formula Karas, V. Abstract A simple, analytic fitting formula for the photometric light curve of a source of light orbiting a black hole is presented. The formula is applicable for a source in a circular orbit with radius smaller than 45 gravitational radii from the black hole. This range of radii requires gravitational focusing of light rays and the Doppler effect to be taken into account with care. The fitting formula is therefore useful for modeling the X-ray variability of inner regions in active galactic nuclei. Publication: The Astrophysical Journal Pub Date: October 1996 DOI: 10.1086/177904 arXiv: arXiv:astro-ph/9605083 Bibcode: 1996ApJ...470..743K Keywords: ACCRETION; ACCRETION DISKS; BLACK HOLE PHYSICS; GALAXIES: ACTIVE; X-RAYS: GALAXIES; Astrophysics E-Print: 12 pages, requires aasms.sty, to appear in The Astrophysical Journal, Vol. 470 (October 20, 1996), figures available upon request from the Author, or at http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.htm full text sources arXiv | ADS |