Interpreting broad emission-line variations I : Factors influencing the emission-line response

We investigate the sensitivity of the measured broad emission-line responsivity dlog f_line/dlog f_cont to continuum variations in the context of straw-man BLR geometries of varying size with fixed BLR boundaries, and for which the intrinsic emission-line responsivity is known a priori. We find for a generic emission-line that the measured responsivity, delay and maximum of the cross-correlation function are correlated for characteristic continuum variability timescales T_char less than the maximum delay for that line tau_max(line) for a particular choice of BLR geometry and observer orientation. The above correlations are manifestations of geometric dilution arising from reverberation effects within the spatially extended BLR. When present, geometric dilution reduces the measured responsivity, delay and maximum of the cross-correlation function. We also find that the measured responsivity and delay show a strong dependence on light-curve duration, with shorter campaigns resulting in smaller than expected values, and only a weak dependence on sampling rate. The observed strong negative correlation between continuum level and line responsivity found in previous studies cannot be explained by differences in the sampling pattern, light-curve duration or in terms of purely geometrical effects. To explain this and to satisfy the observed positive correlation between continuum luminosity and BLR size in an individual source, the responsivity-weighted radius must increase with increasing continuum luminosity. For a BLR with fixed inner and outer boundaries this requires radial surface emissivity distributions which deviate significantly from a simple power-law, and in such a way that the intrinsic emission-line responsivity increases toward larger BLR radii, in line with photoionisation calculations.Comment: 21 pages, 17 figures, accepted for publication in MNRAS July 201

The broad emission-line region: the confluence of the outer accretion disc with the inner edge of the dusty torus

(Abridged) We investigate the observational characteristics of BLR geometries in which the BLR clouds bridge the gap, both in distance and scale height, between the outer accretion disc and the hot dust, forming an effective surface of a "bowl". The gas dynamics are dominated by gravity, and we include the effects of transverse Doppler shift, gravitational redshift and scale-height dependent macro-turbulence. Our simple model reproduces many of the phenomena observed in broad emission-line variability studies, including (i) the absence of response in the core of the optical recombination lines on short timescales, (ii) the enhanced red-wing response on short timescales, (iii) differences between the measured delays for the HILs and LILs, and (iv) identifies turbulence as a means of producing Lorentzian profiles (esp. for LILs) in low inclination systems, and for suppressing significant continuum--emission-line delays between the line wings and line core (esp. in LILs). A key motivation of this work was to reveal the physical underpinnings of the reported measurements of SMBH masses and their uncertainties. We find that SMBH masses derived from measurements of the fwhm of the mean and rms profiles show the closest correspondence between the emission lines in a single object, even though the emission line fwhm is a more biased mass indicator with respect to inclination. The predicted large discrepancies in the SMBH mass estimates between emission lines at low inclination, as derived using the line dispersion, we suggest may be used as a means of identifying near face-on systems. Our general results do not depend on specific choices in the simplifying assumptions, but are in fact generic properties of BLR geometries with axial symmetry that span a substantial range in radially-increasing scale height supported by turbulence, which then merge into the inner dusty TOR.Comment: 29 pages, 23 figures and 1 tabl

Atomic Data for Astrophysics. II. New Analytic Fits for Photoionization Cross Sections of Atoms and Ions

We present a complete set of analytic fits to the non-relativistic photoionization cross sections for the ground states of atoms and ions of elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization thresholds, the fits are based on the Opacity Project theoretical cross sections interpolated and smoothed over resonances. At higher energies, the fits reproduce calculated Hartree-Dirac-Slater photoionization cross sections.Comment: 24 pages including Postscript figures and tables, uses aaspp4.sty, accepted for publication in Astrophysical Journal. Misprint in Eq.(1) is correcte

Locally Optimally-Emitting Clouds and the Variable Broad Emission Line Spectrum of NGC 5548

[abridged] We test the ``locally optimally-emitting clouds'' (LOC) model against the extensive spectroscopic data base of the Seyfert~1, NGC 5548. The time-averaged, integrated-light UV broad emission line spectrum from the 1993 global geometric parameters: the outer radius, the index controlling the radial cloud covering fraction of the continuum source, and the integrated cloud covering fraction. We make an {\em ad~hoc} selection from the range of successful models, and for a simple spherical BLR geometry we simulate the emission line light curves for the 1989 {\em IUE} and 1993 {\em HST} campaigns, using the respective observed UV continuum light curves as drivers. We find good agreement between the predicted and observed light curves and lags --- a demonstration of the LOC picture's viability as a means to understanding the BLR environment. Finally, we discuss the next step in developing the LOC picture which involves the marriage of echo-mapping techniques with spectral simulation grids such as those presented here, using the constraints provided by a high quality, temporally well-sampled spectroscopic data set

Is the slope of the intrinsic Baldwin effect constant?

We investigate the relationship between emission-line strength and continuum luminosity in the best-studied nearby Seyfert 1 galaxy NGC5548. Our analysis of 13 years of ground-based optical monitoring data reveals significant year-to-year variations in the observed H-beta emission-line response in this source. More specifically, we confirm the result of Gilbert and Peterson (2003) of a non-linear relationship between the continuum and H-beta emission-line fluxes. Furthermore, we show that the slope of this relation is not constant, but rather decreases as the continuum flux increases. Both effects are consistent with photoionisation model predictions of a luminosity-dependent response in this line.Comment: 9 pages, 6 figures, accepted for publication in MNRA

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. VIII. An intensive HST, IUE, and ground-based study of NGC 5548

We present the data and initial results from a combined Hubble Space Telescope (HST)/IUE/ground-based spectroscopic monitoring campaign on the Seyfert 1 galaxy NGC 5548 that was undertaken in order to address questions that require both higher temporal resolution and higher signal-to-noise ratios than were obtained in our previous multiwavelength monitoring of this galaxy in 1988-1989. IUE spectra were obtained once every 2 days for a period of 74 days beginning on 1993 March 14. During the last 39 days of this campaign, spectroscopic observations were also made with the HST Faint Object Spectrograph (FOS) on a daily basis. Ground-based observations, consisting of 165 optical spectra and 77 photometric observations (both CCD imaging and aperture photometry), are reported for the period 1992 October-1993 September, although many of the data are concentrated around the time of the satellite-based program. These data constitute a fifth year of intensive optical monitoring of this galaxy. In this contribution we describe the acquisition and reduction of all of the satellite and ground-based data obtained in this program. We describe in detail various photometric problems with the FOS and explain how we identified and corrected for various anomalies

Charge Transfer between Neutral Atoms and Highly Ionized Species: Implications for \u3cem\u3eISO\u3c/em\u3e Observations

We estimate rate coefficients for charge transfer between neutral hydrogen and helium and moderately to highly ionized heavy elements. Although charge transfer does not have much influence on hot collisionally ionized plasmas, its effects on photoionized plasmas can be profound. We present several photoionization models that illustrate the significant effect of charge transfer on the far-infrared lines detected by ISO

Locally Optimally Emitting Clouds and the Origin of Quasar Emission Lines

The similarity of quasar line spectra has been taken as an indication that the emission line clouds have preferred parameters, suggesting that the environment is subject to a fine tuning process. We show here that the observed spectrum is a natural consequence of powerful selection effects. We computed a large grid of photoionization models covering the widest possible range of cloud gas density and distance from the central continuum source. For each line only a narrow range of density and distance from the continuum source results in maximum reprocessing efficiency, corresponding to ``locally optimally-emitting clouds'' (LOC). These parameters depend on the ionization and excitation potentials of the line, and its thermalization density. The mean QSO line spectrum can be reproduced by simply adding together the full family of clouds, with an appropriate covering fraction distribution. The observed quasar spectrum is a natural consequence of the ability of various clouds to reprocess the underlying continuum, and can arise in a chaotic environment with no preferred pressure, gas density, or ionization parameter.Comment: 9 pages including 1 ps figure. LaTeX format using aaspp4.st

Dielectronic recombination data for dynamic finite-density plasmas II. The oxygen isoelectronic sequence

Dielectronic recombination (DR) and radiative recombination (RR) data for oxygen-like ions forming fluorine-like ions have been calculated as part of the assembly of a level-resolved DR and RR database necessary for modelling of dynamic finite-density plasmas (Badnell et al. 2003). Total DR and RR rate coefficients for F+ to Zn22+ are presented and the results discussed. By comparison between perturbative and R-matrix results, we find that RR/DR interference effects are negligible even for the lowest-charged F+ member. We also find that the 2→2 low-temperature DR (no change in the principal quantum number of the core electrons) does not scale smoothly with nuclear charge Z due to resonances straddling the ionization limit, thereby making explicit calculations for each ion necessary. These RR and DR data are suitable for modelling of solar and cosmic plasmas under conditions of collisional ionization equilibrium, photoionization equilibrium, and non-equilibrium ionization