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

Radio Properties of Low Redshift Broad Line Active Galactic Nuclei

The question as to whether the distribution of radio-loudness in active galactic nuclei (AGN) is actually bimodal has been discussed extensively in the literature. Futhermore, there have been claims that radio-loudness depends on black hole mass and Eddington ratio. We investigate these claims using the low redshift broad line AGN sample of Greene & Ho (2007), which consists of 8434 objects at z < 0.35 from the Sloan Digital Sky Survey Fourth Data Release (SDSS DR4). We obtained radio fluxes from the Very Large Array Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey for the SDSS AGN. Out of the 8434 SDSS AGN, 821 have corresponding observed radio fluxes in the FIRST survey. We calculated the radio-loudness parameter (R) for all objects above the FIRST detection limit (1 mJy), and an upper limit to R for the undetected objects. Using these data, the question of radio bimodality is investigated for different subsets of the total sample. We find no clear demarcation between the radio-loud (RL, R > 10) and radio-quiet (RQ, R < 10) objects, but instead fill in a more radio-intermediate population in a continuous fashion for all subsamples. We find that 4.7% of the AGN in the flux-limited subsample are RL based on core radio emission alone. We calculate the radio-loud fraction (RLF) as both a function of black hole mass and Eddington ratio. The RLF decreases (from 13% to 2%) as Eddington ratio increases over 2.5 order of magnitude. The RLF is nearly constant (~5%) over 4 decades in black hole mass, except for an increase at masses greater than 10^8 solar masses. We find for the FIRST detected subsample that 367 of the RL AGN have black hole masses less than 10^8 solar masses, a large enough number to indicate that RL AGN are not a product of only the most massive black holes in the local universe.Comment: 28 pages, 14 figures, accepted to A

On Quasar Masses and Quasar Host Galaxies

The mass of massive black holes in quasar cores can be deduced using the typical velocities of Hb-emitting clouds in the Broad Line Region (BLR) and the size of this region. However, this estimate depends on various assumptions and is susceptible to large systematic errors. The Hb-deduced black hole mass in a sample of 14 bright quasars is found here to correlate with the quasar host galaxy luminosity, as determined with the Hubble Space Telescope (HST). This correlation is similar to the black hole mass vs. bulge luminosity correlation found by Magorrian et al. in a sample of 32 nearby normal galaxies. The similarity of the two correlations is remarkable since the two samples involve apparently different types of objects and since the black hole mass estimates in quasars and in nearby galaxies are based on very different methods. This similarity provides a ``calibration'' of the Hb-deduced black hole mass estimate, suggesting it is accurate to +-0.5 on log scale. The similarity of the two correlations also suggests that quasars reside in otherwise normal galaxies, and that the luminosity of quasar hosts can be estimated to +-0.5 mag based on the quasar continuum luminosity and the Hb line width. Future imaging observations of additional broad-line active galaxies with the HST are required in order to explore the extent, slope, and scatter of the black hole mass vs. host bulge luminosity correlation in active galaxies.Comment: Accepted for publication in ApJ Letters, 7 pages, aas2pp4.st

Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei: I. Comparing the Photoionization and Reverberation Techniques

The masses and emission-line region sizes of Active Galactic Nuclei (AGNs) can be measured by ``reverberation-mapping'' (measuring the lag of the emission-line luminosity after changes in the continuum). We use tis technique to calibrate similar size and mass estimates made by photoionization models of the AGN line-emitting regions. We compile a sample of 19 AGNs with reliable reverberation and spectroscopy data, twice the number available previously. The data provide strong evidence that the BLR size and the emission-line width measure directly the central mass. Two methods are used to estimate the distance of the broad emission-line region (BLR) from the ionizing source: the photoionization method (available for many AGNs but has large intrinsic uncertainties), and the reverberation method (gives very reliable distances, but available for only a few objects). The distance estimate is combined with the velocity dispersion, derived from the broad Hb line profile, to estimate the virial mass. Comparing the central masses calculated with the reverberation method to those calculated using a photoionization model, we find a highly significant, nearly linear correlation. This provides a calibration of the photoionization method on the objects with presently available reverberation data, which should enable mass estimates for all AGNs with measured Hb line width. Comparing the BLR sizes given by the two methods also enables us to estimate the ionizing EUV luminosity which is directly unobservable. We find it to be typically ten times the visible (monochromatic luminosity at 5100A). The inferred Eddington ratio of the individual objects in our sample are 0.001-0.03 (visible luminosity) and 0.01-0.3 (ionizing luminosity).Comment: 27 pages Latex, 8 figures. Accepted for publication in the Astrophysical Journa

Multiwavelength XMM-Newton observations of the Laor et al. sample of PG quasars

We present XMM-Newton/EPIC spectra for the Laor et al. sample of Palomar Green quasars. We find that a power-law provides a reasonable fit to the 2-5 keV region of the spectra. Excess soft X-ray emission below 2 keV is present for all objects, with the exception of those known to contain a warm absorber. A single power-law is, however, a poor fit to the 0.3-10.0 keV spectrum and instead we find that a simple model, consisting of a broken power-law (plus an iron line), provides a reasonable fit in most cases. The equivalent width of the emission line is constrained in just twelve objects but with low (<2 sigma) significance in most cases. For the sources whose spectra are well-fit by the broken power-law model, we find that various optical and X-ray line and continuum parameters are well-correlated; in particular, the power-law photon index is well-correlated with the FWHM of the Hbeta line and the photon indices of the low and high energy components of the broken power-law are well-correlated with each other. These results suggest that the 0.3-10 keV X-ray emission shares a common (presumably non-thermal) origin, as opposed to suggestions that the soft excess is directly produced by thermal disc emission or via an additional spectral component. We present XMM-Newton/OM data which we combine with the X-ray spectra so as to produce broad-band spectral energy distributions, free from uncertainties due to long-term variability in non-simultaneous data. Fitting these optical-UV spectra with a Comptonized disc model indicates that the soft X-ray excess is independent of the accretion disc, confirming our interpretation of the tight correlation between the hard and soft X-ray spectra.Comment: Accepted for publication in MNRA

The Soft X-Ray Properties of a Complete Sample of Optically Selected Quasars II. Final Results

We present the final results of a ROSAT PSPC program to study the soft X-ray emission properties of a complete sample of low $z$ quasars. The main results are: 1. There is no evidence for significant soft excess emission or excess foreground absorption by cold gas in 22 of the 23 quasars. 2. The mean 0.2-2 keV continuum of quasars agrees remarkably well with an extrapolation of the mean 1050-350A continuum recently determined by Zheng et al. (1996), indicating that there is no steep soft component below 0.2 keV. 3. The occurrence of warm absorbers in quasars is rather rare, in sharp contrast to lower luminosity AGN. 4. The strongest correlation found is between the spectral slope, alpha_x, and the Hb FWHM. This remarkably strong correlation may result from a dependence of alpha_x on L/L_Edd, as seen in Galactic black hole candidates. 5. There appears to exist a distinct class of ``X-ray weak'' quasars. These may be quasars where the direct X-ray source is obscured, and only scattered X-rays are observed. 6. Thin accretion disk models cannot reproduce the observed optical to soft X-ray spectral shape. An as yet unknown physical mechanism maintains a strong correlation between the optical and soft X-ray emission. 7. The well known difference in alpha_x between radio-loud and radio-quiet quasars may be due only to their different Hb FWHM. 8. The agreement of the 21 cm and X-ray columns implies that He in the diffuse H II component of the Galactic ISM is ionized to He II or He III (shortened abstract).Comment: 19 pages of text only, uses aas2pp4.sty file, to appear in ApJ vol. 447, 3/1/97, complete postscript version of 34 pages including 5 tables and 8 figures available at http://physics.technion.ac.il/~laor/rosat/paper.p

The Vertical Structure and Ultraviolet Spectrum of Accretion Disks Heated by Internal Dissipation in Active Galactic Nuclei

We present an improved calculation of the vertical structure and ultraviolet spectrum of a dissipative accretion disk in an AGN. We calculate model spectra in which the viscous stress is proportional to the total pressure, the gas pressure only and the geometric mean of the radiation and gas pressures (cf. Laor & Netzer 1989: LN89). As a result of a more complete treatment of absorptive opacity, we find greater overall spectral curvature than did LN89, as well as larger amplitudes in both the Lyman and HeII photoionization edges. The local black body approximation is not a good description of the near UV spectrum. With relativistic corrections (appropriate to non-rotating black holes) included, we find that the near UV spectrum hardens with increasing m-dot / m_8 (m-dot is the accretion rate in Eddington units, m_8 the black hole mass in units of 10^8 M_Sun). The near UV spectrum is consistent with observations if m-dot/ m_8 \sim 10^{-3}, but disks this cold would have large, and unobserved, absorption features at the Lyman edge. The edge amplitude is reduced when m-dot/m_8 is larger, but then the near-UV slope is too hard to match observations. We conclude that models in which conventional disks orbit non-rotating black holes do not adequately explain UV continuum production in AGN.Comment: AAS LaTe

Magnetic Stress at the Marginally Stable Orbit: Altered Disk Structure, Radiation, and Black Hole Spin Evolution

Magnetic connections to the plunging region can exert stresses on the inner edge of an accretion disk around a black hole. We recompute the relativistic corrections to the thin-disk dynamics equations when these stresses take the form of a time-steady torque on the inner edge of the disk. The additional dissipation associated with these stresses is concentrated relatively close outside the marginally stable orbit, scaling as r to the -7/2 at large radius. As a result of these additional stresses: spin-up of the central black hole is retarded; the maximum spin-equilibrium accretion efficiency is 36%, and occurs at a/M=0.94; the disk spectrum is extended toward higher frequencies; line profiles (such as Fe K-alpha) are broadened if the line emissivity scales with local flux; limb-brightening, especially at the higher frequencies, is enhanced; and the returning radiation fraction is substantially increased, up to 58%. This last effect creates possible explanations for both synchronized continuum fluctuations in AGN, and polarization rises shortward of the Lyman edge in quasars. We show that no matter what additional stresses occur, when a/M < 0.36, the second law of black hole dynamics sets an absolute upper bound on the accretion efficiency.Comment: 11 pages, 15 figures, accepted for publication in the Astrophysical Journa