Dan Maoz

Reverberation Measurements for 17 Quasars and the Size-Mass-Luminosity Relations in Active Galactic Nuclei

Correlated variations in the line and continuum emission from active galactic nuclei (AGNs) can be used to determine the size and geometry of the broad emission-line regions (BLRs). We have spectro- photometrically monitored a well-de—ned sample of 28 Palomar-Green quasars in order to obtain mea- surements of their BLRs and to investigate the relationships between quasar luminosity, central black hole mass, and BLR size in AGNs. Spectrophotometry was obtained every 1¨4 months for 7.5 yr, yield- ing 20¨70 observing epochs per object. Both the continuum and emission-line —uxes of all of the quasars were observed to change during the duration of the observing program. Seventeen of the 28 objects were observed with adequate sampling independent observing epochs) to search for correlated variations (Z20 between the Balmer emission lines and the continuum —ux. For each of these 17 objects, a signi—cant correlation was observed, with the Balmer-line variations lagging those of the continuum by D100 days (rest frame). Our work increases the available luminosity range for studying the size-mass-luminosity relations in AGNs by 2 orders of magnitude and doubles the number of objects suitable for such studies. Combining our results with comparable published data available for Seyfert 1 galaxies, we —nd the BLR size scales with the rest-frame 5100 luminosity as L0.70B0.03. This determination of the scaling of the Ae size of the BLR as a function of luminosity is signi—cantly diUerent from those previously published and suggests that the eUective ionization parameter in AGNs may be a decreasing function of luminosity. We are also able to constrain, subject to our assumption that gravity dominates the motions of the BLR gas, the scaling relationship between the mass of the central black holes and the luminosity in AGNs. We —nd that the central mass scales with 5100 luminosity as M P L0.5B0.1. This is inconsistent with all Ae AGNs having optical luminosity that is a constant fraction of the Eddington luminosity. Subject headings: galaxies: activequasars: emission linesquasars: general

THE RELATIONSHIP BETWEEN LUMINOSITY AND BROAD-LINE REGION SIZE IN ACTIVE GALACTIC NUCLEI

We reinvestigate the relationship between the characteristic broad-line region size (RBLR) and the Balmer emission-line, X-ray, UV, and optical continuum luminosities. Our study makes use of the best available determinations of RBLR for a large number of active galactic nuclei (AGNs) from Peterson et al. Using their determinations of RBLR for a large sample of AGNs and two different regression methods, we investigate the robustness of our correlation results as a function of data subsample and regression technique. Although small systematic differences were found depending on the method of analysis, our results are generally consistent. Assuming a power-law relation RBLR ∝ Lα, we find that the mean best-fitting α is about 0.67 ± 0.05 for the optical continuum and the broad Hβ luminosity, about 0.56 ± 0.05 for the UV continuum luminosity, and about 0.70 ± 0.14 for the X-ray luminosity. We also find an intrinsic scatter of ~40% in these relations. The disagreement of our results with the theoretical expected slope of 0.5 indicates that the simple assumption of all AGNs having on average the same ionization parameter, BLR density, column density, and ionizing spectral energy distribution is not valid and there is likely some evolution of a few of these characteristics along the luminosity scale.

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On Uncertainties in Cross‐Correlation Lags and the Reality of Wavelength‐dependent Continuum Lags in Active Galactic Nuclei

We describe a model-independent method of assessing the uncertainties in cross-correlation lags determined from the light curves of active galactic nuclei (AGNs) and use this method to investigate the reality of lags between UV and optical continuum variations in well-studied AGNs. Our results confirm the existence of such lags in NGC 7469. We find that the continuum variations at 1825, 4845, and 6962 A follow those at 1315 A by , and days, respectively, based on the centroids of the cross-correlation functions; the error intervals quoted correspond to 68% confidence levels, and each of these lags is greater than zero at no less than 97% confidence. We do not find statistically significant interband continuum lags in NGC 5548, NGC 3783, or Fairall 9. Wavelength-dependent continuum lags may be marginally detected in the case of NGC 4151. However, on the basis of theoretical considerations, wavelength-dependent continuum lags in sources other than NGC 7469 are not expected to have been detectable in previous experiments. We also confirm the existence of a statistically significant lag between X-ray and UV continuum variations in the blazar PKS 2155–304.

Observational Clues to the Progenitors of Type Ia Supernovae

Type Ia supernovae (SNe Ia) are important distance indicators, element factories, cosmic-ray accelerators, kinetic-energy sources in galaxy evolution, and end points of stellar binary evolution. It has long been clear that a SN Ia must be the runaway thermonuclear explosion of a degenerate carbon-oxygen stellar core, most likely a white dwarf (WD). However, the specific progenitor systems of SNe Ia, and the processes that lead to their ignition, have not been identified. Two broad classes of progenitor binary systems have long been considered: single-degenerate (SD), in which a WD gains mass from a nondegenerate star; and double-degenerate (DD), involving the merger of two WDs. New theoretical work has enriched these possibilities with some interesting updates and variants. We review the significant recent observational progress in addressing the progenitor problem. We consider clues that have emerged from the observed properties of the various proposed progenitor populations, from studies of SN Ia sites—...

MULTIWAVELENGTH MONITORING OF THE DWARF SEYFERT 1 GALAXY NGC 4395. I. A REVERBERATION-BASED MEASUREMENT OF THE BLACK HOLE MASS

A reverberation-mapping program on NGC 4395, the least luminous known Seyfert 1 galaxy, undertaken with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope yields a measurement of the mass of the central black hole MBH = (3.6 ± 1.1) × 105 M☉. The observations consist of two visits of five orbits each, in 2004 April and July. During each of these visits, the UV continuum varied by at least 10% (rms), and only C IV λ1549 showed corresponding variations large enough to reliably determine the emission-line lag, which was measured to be of order 1 hr for both visits. The size of the C IV-emitting region is about a factor of 3 smaller than expected if the slope of the broad-line region radius-luminosity relationship is identical to that for the Hβ emission line. NGC 4395 is underluminous even for its small black hole mass; the Eddington ratio of ~1.2 × 10-3 is lower than that of any other active galactic nucleus for which a black hole mass measurement has been made by emission-line reverberation.

Long-term optical variability properties of the Palomar—Green quasars

We present results from a monitoring programme of 42 quasars from the Palomar–Green sample. The objects were observed for seven years at the Wise Observatory, as part of a long-term effort to monitor AGN of various types. This is the most extensive programme of its kind carried out to date on a well-defined, optically selected quasar sample. The typical sampling interval is ∼ 40 d. One-third of the quasars were observed at ∼ 60 epochs, and the rest at ∼ 30 epochs in two bands (B and R) with a photometric accuracy of ∼ 0.01 mag. We present light curves for all of the sources, and discuss the sample variability properties. All of the quasars in the sample varied during the campaign with intrinsic rms amplitudes of 5 <σB<34 per cent and 4 <σR<26 per cent. The rms amplitude and colour for the entire sample are σB=14 per cent, σR=12 per cent and σB-R=5 per cent. On time-scales of 100 to 1000 d the power spectra of the sources have a power-law shape, Pν≺ν−γ, with γ≈ 2.0 and a spread ≲ 0.6. At least half of the quasars, particularly those that are most variable, become bluer when they brighten, and the rest do not show this behaviour. We quantify this phenomenon, which has been observed previously mainly in Seyfert galaxies. The quasars which are most variable tend also to exhibit asymmetry in their variations, in the sense that the brightening phases last longer than the fading phases. We have searched for correlations between the measured variability properties and other parameters of the quasars, such as luminosity, redshift, radio loudness and X-ray slope. We find several new correlations, and reproduce some of the correlations reported by previous studies. Among them are an anticorrelation of variability amplitude with luminosity, a trend of the autocorrelation time-scale with luminosity, and an increase in variability amplitude with Hβ equivalent width. However, all of these trends have a large scatter despite the low observational uncertainties.

Early type galaxies, dark halos, and gravitational lensing statistics

We present calculations of the expected statistics of gravitational lensing of quasars in the Hubble Space Telescope snapshot survey. We first model early-type (elliptical and S0) galaxies using their observed surface brightness profiles and dynamically inferred mass-to-light ratios. Our work improves upon previous calculations, which have generally approximated the galaxy potentials by isothermal spheres. For standard cosmologies, the predicted number of lensed quasars in the survey is 1.1-2.8, 98% of which have image separations less than 2″, compared to the four lenses observed, two of which have separations greater than 2″. These constant mass-to-light ratio models are rejected

Type-Ia Supernova Rates and the Progenitor Problem: A Review

The identity of the progenitor systems of type-Ia supernovae (SNe Ia) is a major unsolved problem in astrophysics. SN Ia rates are providing some striking clues. We review the basics of SN rate measurement, preach about some sins of SN rate measurement and analysis, and illustrate one of these sins with an analogy about Martian scientists. We review the recent progress in measuring SN Ia rates in various environments and redshifts, and their use to reconstruct the SN Ia delay-time distribution (DTD) — the SN rate versus time that would follow a hypothetical brief burst of star formation. A good number of DTD measurements, using a variety of methods, appear to be converging. At delays 1 < t < 10 Gyr, these measurements show a similar, ~t–1, power-law shape. The DTD peaks at the shortest delays probed. This result supports the idea of a double-degenerate progenitor origin for SNe Ia. Single-degenerate progenitors may still play a role in producing short-delay SNe Ia, or perhaps all SNe Ia, if the red-giant donor channel is more efficient than is found by most theoretical models. The DTD normalization enjoys fairly good agreement (though perhaps some tension), among the various measurements, with a Hubble time–integrated DTD value of about 2 ± 1 SNe Ia per 1000 M⊙ (stellar mass formed with a low–mass turnover initial mass function). The local WD binary population suggests that the WD merger rate can explain the Galactic SN Ia rate, but only if sub-Chandra mergers lead to SN Ia events. We point to some future directions that should lead to progress in the field, including measurement of the bivariate (delay and stretch) SN Ia response function.

The delay-time distribution of Type Ia supernovae from Sloan II

We derive the delay-time distribution (DTD) of Type Ia supernovae (SNe Ia) using a sample of 132 SNe Ia, discovered by the Sloan Digital Sky Survey II (SDSS2) among 66 000 galaxies with spectral-based star formation histories (SFHs). To recover the best-fitting DTD, the SFH of every individual galaxy is compared, using Poisson statistics, to the number of SNe that it hosted (zero or one), based on the method introduced in Maoz et al. This SN sample differs from the SDSS2 SN Ia sample analysed by Brandt et al., using a related, but different, DTD recovery method. Furthermore, we use a simulation-based SN detection-efficiency function, and we apply a number of important corrections to the galaxy SFHs and SN Ia visibility times. The DTD that we find has 4σ detections in all three of its time bins: prompt (τ 2.4 Gyr), indicating a continuous DTD, and it is among the most accurate and precise among recent DTD reconstructions. The best-fitting power-law form to the recovered DTD is t−1.07 ± 0.07, consistent with generic ∼t−1 predictions of SN Ia progenitor models based on the gravitational-wave-induced mergers of binary white dwarfs. The time-integrated number of SNe Ia per formed stellar mass is NSN/M = 0.00130 ± 0.00015  M⊙−1, or about 4 per cent of the stars formed with initial masses in the 3 - 8 M⊙ range. This is lower than, but largely consistent with, several recent DTD estimates based on SN rates in galaxy clusters and in local-volume galaxies, and is higher than, but consistent with NSN/M estimated by comparing volumetric SN Ia rates to cosmic SFH.