Belinda J. Wilkes

Atlas of quasar energy distributions

We present an atlas of the spectral energy distributions (SEDs) of normal, nonblazar, quasars over the whole available range (radio to 10 keV X-rays) of the electromagnetic spectrum. The primary (UVSX) sample includes 47 quasars for which the spectral energy distributions include X-ray spectral indices and UV data. Of these, 29 are radio quiet, and 18 are radio loud. The SEDs are presented both in figures and in tabular form, with additional tabular material published on CD-ROM. Previously unpublished observational data for a second set of quasars excluded from the primary sample are also tabulated. The effects of host galaxy starlight contamination and foreground extinction on the UVSX sample are considered and the sample is used to investigate the range of SED properties. Of course, the properties we derive are influenced strongly by the selection effects induced by quasar discovery techniques. We derive the mean energy distribution (MED) for radio-loud and radio-quiet objects and present the bolometric corrections derived from it. We note, however, that the dispersion about this mean is large (approximately one decade for both the infrared and ultraviolet components when the MED is normalized at the near-infrared inflection). At least part of the dispersion in the ultraviolet may be due to time variability, but this is unlikely to be important in the infrared. The existence of such a large dispersion indicates that the MED reflects only some of the properties of quasars and so should be used only with caution.

The Luminosity Function of X-Ray-selected Active Galactic Nuclei: Evolution of Supermassive Black Holes at High Redshift

We present a measure of the hard (2Y8 keV) X-ray luminosity function (XLF) of AGNs up to z � 5. At high redshifts, the wide area coverage of the Chandra Multiwavength Project is crucial to detect rare and luminous (LX > 10 44 ergs s � 1 ) AGNs. The inclusion of samples from deeper published surveys, such as the Chandra Deep Fields, allows us to span the lower LX range of the XLF. Our sample is selected from both the hard (z 6:3 ; 10 � 16 ergs cm � 2 s � 1 ) and soft (z > 3, f0:5Y2:0 keV > 1:0 ; 10 � 16 ergs cm � 2 s � 1 ) energy band detections. Within our optical magnitude limits (r 0 ; i 0 50%) regarding X-ray source identification (i.e., redshift). Wefind that the luminosity function is similar to that found in previous X-ray surveys up to z � 3 with an evolution dependent on both luminosity and redshift. At z > 3, there is a significant decline in the numbers of AGNs with an evolution rate similar to that found by studies of optically selected QSOs. Based on our XLF, we assess the resolved fraction of the cosmic X-ray background, the cumulative mass density of SMBHs, and the comparison of the mean accretion rate onto SMBHs and the star formation history of galaxies as a function of redshift. A coevolution scenario up to z � 2 is plausible, although at higher redshifts the accretion rate onto SMBHs drops more rapidly. Finally, we highlight the need for better statistics of high-redshift AGNs at zk3, which is achievable with the upcoming Chandra surveys. Subject headingg galaxies: active — quasars: general — surveys — X-rays: galaxies

An Empirical Ultraviolet Template for Iron Emission in Quasars as Derived from I Zwicky 1

We present an empirical template spectrum suitable for fitting and subtracting/studying the Fe II and Fe III emission lines in the rest frame ultraviolet spectra of quasars and active galactic nuclei, the first empirical ultraviolet iron template to cover the full range of 1250-3090 A. Iron emission is often a severe contaminant in optical-ultraviolet spectra of active galactic nuclei and quasars. Its presence complicates and limits the accuracy of measurements of both strong and weak emission lines and the continuum emission, affecting studies of line and continuum interrelations, the ionization structure, and elemental abundances in active galaxies and quasars. Despite the wealth of work on modeling the quasar Fe II emission and the need to account for this emission in observed quasar spectra, there is no ultraviolet template electronically available to aid this process. The iron template we present is based on Hubble Space Telescope spectra of the narrow line Seyfert 1 galaxy, I Zwicky 1 (I Zw 1, z = 0.061). The intrinsic narrow lines ( 900 km s-1) of this source and its rich iron spectrum make the template particularly suitable for use with most active galactic nuclei and quasar spectra. The iron emission spectrum, the line identifications, and the measurements of absorption and emission lines are presented and compared with the work of Laor et al. Comments on each individual line feature and the line fitting are available in Appendix A. The methods used to develop and apply the template are also described. We illustrate the application of the derived Fe II and Fe III templates by fitting and subtracting iron emission from the spectra of four high-redshift quasars and of the nearby quasar, 3C 273, confirming their general applicability to active galaxies despite the somewhat unusual properties of I Zw 1. We briefly discuss the small discrepancies between the observed iron emission of these quasars and the ultraviolet template, and compare the template with previously published ones. We discuss the advantages and limitations of the UV Fe II and Fe III templates and of the template fitting method. We conclude that the templates work sufficiently well to be a valuable and important tool for eliminating and studying the iron emission in active galaxies, at least until accurate theoretical iron emission models are developed. The Si IV+O IV] λ1400 feature in I Zw 1 is clearly strong relative to C IV λ1549, and C IV and C III] λ1909 are both relatively weak. This may partially be due to the higher densities and lower ionization parameter prevailing in narrow line Seyfert 1 galaxies and to the big blue bump shifting toward lower energies in more luminous Seyferts, such as I Zw 1. In I Zw 1 the narrow line width reveals that C III] is heavily blended with Si III] λ1892, Al III λλ1854,1863, and Fe III transitions. This suggests that the C III] line strength and width may be overestimated in many quasar line studies where the lines are broader and deblending is not possible. This affects density estimates of the broad line region. Photoionization modeling, including all these line features, and subsequent fitting to the spectra are required to estimate the true C III] strength. We also argue, based on earlier work, that (strong) iron emission may be connected with high densities and associated with outflows.

Accurate galactic N(H) values towards quasars and AGN

Integrated Galactic 21-cm column densities toward about 174 quasars and AGN are presented. The data are corrected for stray radiation, using the technique of Lockman et al. (1986). Because of the 21-arcmin beam size of the 140-ft telescope used in the study, the uncertainty in N(H) due to angular variations in the H I of the Galaxy at high latitudes is minimized. The column densities are accurate to about 1 X 10 to the 19th atoms/sq cm, or 5 percent, whichever is larger. Errors above N(H) of about 4 X 10 to the 20th atoms/sq cm are dominated by opacity uncertainties. 24 references.

Chandra and Spitzer Unveil Heavily Obscured Quasars in the Chandra/SWIRE Survey*

Using the large multi-wavelength data set in the chandra/SWIRE Survey (0.6 square degrees in the Lockman Hole), we show evidence for the existence of highly obscured (Compton-thick) AGN, estimate a lower limit to their surface density and characterize their multi-wavelength properties. Two independent selection methods based on the X-ray and infrared spectral properties are presented. The two selected samples contain 1) 5 X-ray sources with hard X-ray spectra and column densities > 10^24 cm-2, and 2) 120 infrared sources with red and AGN-dominated infrared spectral energy distributions (SEDs). We estimate a surface density of at least 25 Compton-thick AGN per square degree detected in the infrared in the chandra/SWIRE field of which ~40% show distinct AGN signatures in their optical/near-infrared SEDs, the remainings being dominated by the host-galaxy emission. Only ~33% of all Compton-thick AGN are detected in the X-rays at our depth (F(0.3-8 keV)>10^-15 erg/cm2/s. We report the discovery of two sources in our sample of Compton-thick AGN, SWIRE_J104409.95+585224.8 (z=2.54) and SWIRE_J104406.30+583954.1 (z=2.43), which are the most luminous Compton-thick AGN at high-z currently known. The properties of these two sources are discussed in detail with an analysis of their spectra, SEDs, luminosities and black-hole masses.

X-RAY AND OPTICAL VARIABILITY IN NGC 4051 AND THE NATURE OF NARROW-LINE SEYFERT 1 GALAXIES

We report on the results of a three-year program of coordinated X-ray and optical monitoring of the narrow-line Seyfert 1 galaxy NGC 4051. The rapid continuum variations observed in the X-ray spectra are not detected in the optical, although the time-averaged X-ray and optical continuum fluxes are well correlated. Variations in the flux of the broad Hβ line are found to lag behind the optical continuum variations by 6 days (with an uncertainty of 2-3 days), and combining this with the line width yields a virial mass estimate of ~1.1 × 106 M☉, at the very low end of the distribution of active galactic nucleus masses measured by line reverberation. Strong variability of He II λ4686 is also detected, and the response time measured is similar to that of Hβ but with a much larger uncertainty. The He II λ4686 line is almost 5 times broader than Hβ, and it is strongly blueward asymmetric, as are the high-ionization UV lines recorded in archival spectra of NGC 4051. The data are consistent with the Balmer lines arising in a low-to-moderate-inclination disklike configuration and the high-ionization lines arising in an outflowing wind, of which we observe preferentially the near side. Previous observations of the narrow-line region morphology of this source suggest that the system is inclined by ~50°, and if this is applicable to the broad Hβ-emitting region, a central mass of ~1.4 × 106 M☉ can be inferred. During the third year of monitoring, both the X-ray continuum and the He II λ4686 line went into extremely low states, although the optical continuum and the Hβ broad line were both still present and variable. We suggest that the inner part of the accretion disk may have gone into an advection-dominated state, yielding little radiation from the hotter inner disk.

Quasar energy distributions. I - Soft X-ray spectra of quasars

As the initial stage of a study of quasar energy distributions (QEDs), Einstein IPC spectra of 24 quasars are presented. These are combined with previously reported IPC spectra to form a sample of 33 quasars with well-determined soft X-ray slopes. A correlation analysis shows that radio loudness, rather than redshift or luminosity, is fundamentally related to the X-ray slope. This correlation is not followed by higher energy spectra of active galaxies. Two components are required to explain both sets of results. The best-fit column densities are systematically smaller than the Galactic values. The same effect is not present in a sample of BL Lac objects, implying that the effect is intrinsic to the quasars and is caused by a low-energy turnup in the quasar spectra. 74 references.

A Chandra Survey of Broad Absorption Line Quasars

We have carried out a survey with the Chandra X-Ray Observatory of a sample of 10 bright broad absorption line (BAL) quasars (QSOs). Eight of 10 sources are detected. The six brightest sources have only high-ionization BALs (hiBALs), while the four faintest all show low-ionization BALs (loBALs). We perform a combined spectral fit for hiBAL QSOs (384 counts total; 0.5-6 keV) to determine the mean spectral parameters of this sample. We derive an underlying best-fit power-law slope Γ = 1.8 ± 0.35, which is consistent with the mean slope for radio-quiet QSOs from ASCA, but BAL QSOs require a (rest-frame) absorbing column of 6.5 × 1022 cm-2, with a partial covering fraction of ~80%. The optical-to-X-ray spectral slope (αox from 2500 A to 2 keV) varies from 1.7 to 2.4 across the full sample, consistent with previous results that BAL QSOs appear to be weak soft X-ray emitters. Removing the absorption component from our best-fit spectral model yields a range of αox from 1.55 to 2.28. All six hiBAL QSOs have deabsorbed X-ray emission consistent with non-BAL QSOs of similar luminosity. The spectral energy distributions of the hiBAL QSOs—both the underlying power-law slope and αox—provide the first conclusive evidence that BAL QSOs have appeared to be X-ray weak because of intrinsic absorption and that their underlying emission is consistent with non-BAL QSOs. By contrast, the removal of the best-fit absorption column detected in the hiBAL QSOs still leaves the four loBAL QSOs with values of αox > 2 that are unusually X-ray faint for their optical luminosities, which is consistent with other evidence that loBALs have higher column density, dustier absorbers. Important questions of whether BAL QSOs represent a special line of sight toward a QSO nucleus or rather an early evolutionary or high-accretion phase in a QSO lifetime remain to be resolved, and the unique properties of loBAL QSOs will be an integral part of that investigation.

Multiwavelength Observations of Short-Timescale Variability in NGC 4151. IV. Analysis of Multiwavelength Continuum Variability

For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA=6%) variations (uncorrelated with those at lower energies) were seen at soft gamma-ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (ales-2.5)

Steps toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei. XVI. A 13 Year Study of Spectral Variability in NGC 5548

We present the final installment of an intensive 13 year study of variations of the optical continuum and broad Hemission line in the Seyfert 1 galaxy NGC 5548. The database consists of 1530 optical continuum measurements and 1248 Hmeasurements. The Hvariations follow the continuum variations closely, with a typical time delay of about 20 days. However, a year-by-year analysis shows that the magnitude of emission-line time delay is correlated with the mean continuum flux. We argue that the data are consistent with the simple model prediction between the size of the broad-line region and the ionizing luminosity, r / L 1=2 ion . Moreover, the apparently linear nature of the correlation between the Hresponse time and the nonstellar optical continuum Fopt arises as a consequence of the changing shape of the continuum as it varies, specifically Fopt / F 0:56 UV . Subject headings: galaxies: active — galaxies: individual (NGC 5548) — galaxies: nuclei — galaxies: Seyfert