An exploratory study of the hard X-ray variability properties of PG quasars with RXTE

We have monitored with the RXTE PCA the variability pattern of the 2-20 keV flux in four PG quasars (QSOs) from the Laor et al. (1994) sample. Six observations of each target at regular intervals of 1 day were performed. The sample comprises objects with extreme values of Balmer line width (and hence soft X-ray steepness) and spans about one order of magnitude in luminosity. The most robust result is that the variability amplitude decreases as energy increases. Several options for a possible ultimate driver of the soft and hard X-ray variability, such as the influx rate of Comptonizing relativistic particles, instabilities in the accretion flow or the number of X-ray active sites, are consistent with our results.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho

The Lyman Continuum Polarization Rise in the QSO PG 1222+228

Some QSOs show an abrupt, strong rise in polarization near rest wavelength 750 A. If this arises in the atmosphere of an accretion disk around a supermassive black hole, it may have diagnostic value. In PG 1222+228, the polarization rise occurs at the wavelength of a sharp drop in flux. We examine and reject interpretations of this feature involving a high velocity outflow. The observations agree with a model involving several intervening Lyman limit systems, two of which happen to coincide with the Lyman continuum polarization rise. After correction for the Lyman limit absorption, the continuum shortward of 912 A is consistent with a typical power-law slope, alpha = -1.8. This violates the apparent pattern for the Lyman limit polarization rises to occur only in ``candidate Lyman edge QSOs''. The corrected, polarized flux rises strongly at the wavelength of the polarization rise, resembling the case of PG 1630+377. The rise in polarized flux places especially stringent requirements on models.Comment: 19 pages, including 5 EPS figures. Uses aaspp4.sty. Accepted for publication in Publications of the Astronomical Society of the Pacific, 2000 Ma

Non-LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei. III. Integrated Spectra for Hydrogen-Helium Disks

We have constructed a grid of non-LTE disk models for a wide range of black hole mass and mass accretion rate, for several values of viscosity parameter alpha, and for two extreme values of the black hole spin: the maximum-rotation Kerr black hole, and the Schwarzschild (non-rotating) black hole. Our procedure calculates self-consistently the vertical structure of all disk annuli together with the radiation field, without any approximations imposed on the optical thickness of the disk, and without any ad hoc approximations to the behavior of the radiation intensity. The total spectrum of a disk is computed by summing the spectra of the individual annuli, taking into account the general relativistic transfer function. The grid covers nine values of the black hole mass between M = 1/8 and 32 billion solar masses with a two-fold increase of mass for each subsequent value; and eleven values of the mass accretion rate, each a power of 2 times 1 solar mass/year. The highest value of the accretion rate corresponds to 0.3 Eddington. We show the vertical structure of individual annuli within the set of accretion disk models, along with their local emergent flux, and discuss the internal physical self-consistency of the models. We then present the full disk-integrated spectra, and discuss a number of observationally interesting properties of the models, such as optical/ultraviolet colors, the behavior of the hydrogen Lyman limit region, polarization, and number of ionizing photons. Our calculations are far from definitive in terms of the input physics, but generally we find that our models exhibit rather red optical/UV colors. Flux discontinuities in the region of the hydrogen Lyman limit are only present in cool, low luminosity models, while hotter models exhibit blueshifted changes in spectral slope.Comment: 20 pages, 31 figures, ApJ in press, spectral models are available for downloading at http://www.physics.ucsb.edu/~blaes/habk

On the Baldwin Effect in Active Galactic Nuclei: I. The Continuum-Spectrum - Mass Relationship

We suggest that the Baldwin Effect is a result of the spectral dependence of the line-driving ionizing continuum on the black hole mass. We derive a relationship between the mass of the central black hole and the broad emission line luminosity in active galactic nuclei (AGN). Assuming the UV spectrum of AGN is emitted from an optically thick medium we find an expression for the characteristic energy of the ``UV bump'' in terms of the observable luminosity and emission-line width. We show empirically and analytically that the bump energy is anti-correlated with the black-hole mass and with the continuum luminosity. Our model reproduces the observed inverse correlation between equivalent width and continuum luminosity, yielding an explanation of the Baldwin effect from first principles. The model gives a good fit to the Baldwin Effect of the CIV line for a mean quasar EUV spectrum (Zheng et al. 1997) and for several model spectra. The model also predicts a correlation between the strength of the Baldwin Effect (the slope of the equivalent width as a function of luminosity) and the ionization potential, consistent with recent data.Comment: 19 pages Latex, 2 figures. Accepted for publication in the Astrophysical Journa

Modeling the X-ray - UV Correlations in NGC 7469

We model the correlated X-ray - UV observations of NGC 7469, for which well sampled data in both these bands have been obtained recently in a multiwavelength monitoring campaign. To this end we derive the transfer function in wavelength \ls and time lag \t, for reprocessing hard (X-ray) photons from a point source to softer ones (UV-optical) by an infinite plane (representing a cool, thin accretion disk) located at a given distance below the X-ray source, under the assumption that the X-ray flux is absorbed and emitted locally by the disk as a black body of temperature appropriate to the incident flux. Using the observed X-ray light curve as input we have computed the expected continuum UV emission as a function of time at several wavelengths (\l \l 1315 \AA, \l \l 6962 \AA, \l \l 15000 \AA, \l \l 30000 \AA) assuming that the X-ray source is located one \sc radius above the disk plane, with the mass of the black hole $M$ and the latitude angle $\theta$ of the observer relative to the disk plane as free parameters. We have searched the parameter space of black hole masses and observer azimuthal angles but we were unable to reproduce UV light curves which would resemble, even remotely, those observed. We also explored whether particular combinations of the values of these parameters could lead to light curves whose statistical properties (i.e. the autocorrelation and cross correlation functions) would match those corresponding to the observed UV light curve at \l \l 1315 \AA. Even though we considered black hole masses as large as $10^9$ M$_{\odot}$ no such match was possible. Our results indicate that some of the fundamental assumptions of this model will have to be modified to obtain even approximate agreement between the observed and model X-ray - UV light curves.Comment: 16 pages, 13 figures, ApJ in pres

PKS 1004+13: A High-Inclination, Highly-Absorbed Radio-Loud QSO -- The First Radio-Loud BAL QSO at Low Redshift?

The existence of BAL outflows in only radio-quiet QSOs was thought to be an important clue to mass ejection and the radio-loud - radio-quiet dichotomy. Recently a few radio-loud BAL QSOs have been discovered at high redshift. We present evidence that PKS 1004+13 is a radio-loud BAL QSO. It would be the first known at low-redshift (z = 0.24), and one of the most radio luminous. For PKS 1004+13, there appear to be broad absorption troughs of O VI, N V, Si IV, and C IV, indicating high-ionization outflows up to about 10,000 km/s. There are also two strong, broad (~500 km/s), high-ionization, associated absorption systems that show partial covering of the continuum source. The strong UV absorption we have detected suggests that the extreme soft-X-ray weakness of PKS 1004+13 is primarily the result of absorption. The large radio-lobe dominance indicates BAL and associated gas at high inclinations to the central engine axis, perhaps in a line-of-sight that passes through an accretion disk wind.Comment: To appear in Ap.J. Letters, 1999 (June or July); 4 pages, 5 figure

Polarimetric Imaging of the Massive Black Hole at the Galactic Center

The radio source Sgr A* in the Galactic center emits a polarized spectrum at millimeter and sub-millimeter wavelengths that is strongly suggestive of relativistic disk accretion onto a massive black hole. We use the well-constrained mass of Sgr A* and a magnetohydrodynamic model of the accretion flow to match both the total flux and polarization from this object. Our results demonstrate explicitly that the shift in the position angle of the polarization vector, seen at wavelengths near the peak of the mm to sub-mm emission from this source, is a signal of relativistic accretion flow in a strong gravitational field. We provide maps of the polarized emission to illustrate how the images of polarized intensity from the vicinity of the black hole would appear in upcoming observations with very long baseline radio interferometers (VLBI). Our results suggest that near-term VLBI observations will be able to directly image the polarized Keplerian portion of the flow near the horizon of the black hole.Comment: 12 pages, 2 figures, Accepted for publciation in ApJ Letter