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Ýlgili Makaleler
An Internet Database of Ultraviolet Continuum Light Curves for Seyfert Galaxies
Using the Multimission Archive at STScI (MAST), we have extractedspectra and determined continuum light curves for 175 Seyfert galaxiesthat have been observed with the International Ultraviolet Explorer andthe Faint Object Spectrograph on the Hubble Space Telescope. To obtainthe light curves as a function of Julian Date, we used fixed bins in theobject's rest frame and measured small regions (between 30 and 60Å) of each spectrum's continuum flux in the range 1150 to 3200Å. We provide access to the UV light curves and other basicinformation about the observations in tabular and graphical form via theInternet at http://www.chara.gsu.edu/PEGA/IUE.
| The magnetic nature of disk accretion onto black holes
Although disk accretion onto compact objects-white dwarfs, neutron starsand black holes-is central to much of high-energy astrophysics, themechanisms that enable this process have remained observationallydifficult to determine. Accretion disks must transfer angular momentumin order for matter to travel radially inward onto the compact object.Internal viscosity from magnetic processes and disk winds can both inprinciple transfer angular momentum, but hitherto we lacked evidencethat either occurs. Here we report that an X-ray-absorbing winddiscovered in an observation of the stellar-mass black hole binary GROJ1655 - 40 (ref. 6) must be powered by a magnetic process that can alsodrive accretion through the disk. Detailed spectral analysis andmodelling of the wind shows that it can only be powered by pressuregenerated by magnetic viscosity internal to the disk ormagnetocentrifugal forces. This result demonstrates that disk accretiononto black holes is a fundamentally magnetic process.
| On the origin of the iron Kα line cores in active galactic nuclei
X-ray observations made with Chandra and XMM-Newton have shown thatthere are relatively narrow cores to the iron Kα emission lines inactive galactic nuclei (AGN). Plausible origins for this core emissioninclude the outer regions of an accretion disc, a parsec-scale moleculartorus, and the optical broad-line region (BLR). Using data from theliterature it is shown that no correlation exists between the FeKα core width and the BLR (specifically Hβ) line width. Thisshows that in general the iron Kα core emission does not arisefrom the BLR. There is a similar lack of correlation between the widthof the Fe Kα core and black hole mass. The average Kα widthis about a factor of 2 lower than the Hβ width. It therefore seemslikely that, in many cases, the narrow core arises in the torus. Thereis a very wide range of observed Fe Kα core widths, however, andthis argues for multiple origins. The simplest explanation for theobserved line profiles in AGN is that they are due to a mixing of verynarrow emission from the inner edge of the torus, and broadened emissionfrom the accretion disc, in varying proportions from object to object.
| The near-infrared spectrum of Mrk 1239: direct evidence of the dusty torus?
We report 0.8-4.5 μm SpeX spectroscopy of the narrow-line Seyfert 1galaxy Mrk 1239. The spectrum is outstanding because the nuclearcontinuum emission in the near-infrared (NIR) is dominated by a strongbump of emission peaking at 2.2 μm, with a strength not reportedbefore in an active galactic nucleus. A comparison of the Mrk 1239spectrum to that of Ark 564 allowed us to conclude that the continuum isstrongly reddened by E(B-V) = 0.54. The excess of emission, confirmed byaperture photometry and additional NIR spectroscopy, follows a simpleblackbody curve at T~ 1200 K. This suggests that we may be observingdirect evidence of dust heated to near to the sublimation temperature,likely produced by the putative torus of the unification model. Althoughother alternatives are also plausible, the lack of star formation, thestrong polarization and low extinction derived for the emission linessupport the scenario where the hot dust is located between the narrowline region and the broad line region.
| Investigating a fluctuating-accretion model for the spectral-timing properties of accreting black hole systems
The fluctuating-accretion model of Lyubarskii and its extension byKotov, Churazov & Gilfanov seek to explain the spectral-timingproperties of the X-ray variability of accreting black holes in terms ofinward-propagating mass accretion fluctuations produced at a broad rangeof radii. The fluctuations modulate the X-ray emitting region as theymove inwards and can produce temporal-frequency-dependent lags betweenenergy bands, and energy-dependent power spectral densities (PSDs) as aresult of the different emissivity profiles, which may be expected atdifferent X-ray energies. Here, we use a simple numerical implementationto investigate in detail the X-ray spectral-timing properties of themodel and their relation to several physically interesting parameters,namely the emissivity profile in different energy bands, the geometricalthickness and viscosity parameter of the accretion flow, the strength ofdamping on the fluctuations and the temporal coherence (measured by the`quality factor', Q) of the fluctuations introduced at each radius. Wefind that a geometrically thick flow with large viscosity parameter isfavoured, and we confirm that the predicted lags are quite robust tochanges in the emissivity profile and physical parameters of theaccretion flow, which may help to explain the similarity of the lagspectra in the low/hard and high/soft states of Cyg X-1. We alsodemonstrate the model regime where the light curves in different energybands are highly spectrally coherent. We compare model predictionsdirectly to X-ray data from the narrow line Seyfert 1 galaxy NGC 4051and the black hole X-ray binary (BHXRB) Cyg X-1 in its high/soft state,and we show that this general scheme can reproduce simultaneously thetime lags and energy-dependence of the PSD.
| High-metallicity, photoionized gas in intergalactic large-scale filaments
We present high-resolution ultraviolet spectra of absorption-linesystems towards the low-z quasi-stellar object (QSO) HS 0624+6907(zQSO= 0.3700). Coupled with ground-based imaging andspectroscopic galaxy redshifts, we find evidence that many of theseabsorbers do not arise in galaxy haloes but rather are trulyintergalactic gas clouds distributed within large-scale structures, andmoreover, the gas is cool (T < 105 K) and has relativelyhigh metallicity (Z > 0.9Zsolar). Hubble Space Telescope(HST) Space Telescope Imaging Spectrograph (STIS) data reveal a dramaticcluster of 13 HI Lyman α (Lyα) lines within a 1000 kms-1 interval at zabs= 0.0635. We find 10 galaxiesat this redshift with impact parameters ranging from ρ=135h-170 kpc to 1.37h-170Mpc. The velocities and velocity spread of the Lyα lines in thiscomplex are unlikely to arise in the individual haloes of the nearbygalaxies; instead, we attribute the absorption to intragroup medium gas,possibly from a large-scale filament viewed along its long axis.Contrary to theoretical expectations, this gas is not the shock-heatedwarm-hot intergalactic medium (WHIM); the width of the Lyα linesall indicate a gas temperature T<< 105 K, and metallines detected in the Lyα complex also favour photoionized, coolgas. No OVI absorption lines are evident, which is consistent withphotoionization models. Remarkably, the metallicity is near-solar,[M/H]=-0.05 +/- 0.4 (2σ uncertainty), yet the nearest galaxy whichmight pollute the intergalactic medium is at least135h-170 kpc away. Tidal stripping from nearbygalaxies appears to be the most likely origin of this highly enriched,cool gas. More than six Abell galaxy clusters are found within 4° ofthe sight line suggesting that the QSO line of sight passes near a nodein the cosmic web. At z~ 0.077, we find absorption systems as well asgalaxies at the redshift of the nearby clusters Abell 564 and Abell 559.We conclude that the sight line pierces a filament of gas and galaxiesfeeding into these clusters. The absorber at zabs= 0.07573associated with Abell 564/559 also has a high metallicity with [C/H]> -0.6, but again the closest galaxy is relatively far from the sightline (ρ= 293h-170 kpc). The Doppler parametersand HI column densities of the Lyα lines observed along the entiresight line are consistent with those measured towards other low-z QSOs,including a number of broad (b > 40kms-1) Lyα lines.
| XMM-Newton observations of the Seyfert 1 AGN H0557-385
We present XMM-Newton observations of the Seyfert 1 active galacticnucleus (AGN) H0557-385. We have conducted a study into the warmabsorber present in this source, and using high-resolution ReflectionGrating Spectrometer (RGS) data we find that the absorption can becharacterized by two phases: a phase with log ionization parameter ξof 0.50 (where ξ is in units of ergcms-1) and a column of0.2 × 1021cm-2, and a phase with log ξ of1.62 and a column of 1.3 × 1022cm-2. An ironKα line is detected. Neutral absorption is also present in thesource, and we discuss possible origins for this. On the assumption thatthe ionized absorbers originate as an outflow from the inner edge of thetorus, we use a new method for finding the volume filling factor. Bothphases of H0557-385 have small volume filling factors (<=1 per cent).We also derive the volume filling factors for a sample of 23 AGN usingthis assumption and for the absorbers with logξ > 0.7, we findreasonable agreement with the filling factors obtained through thealternative method of equating the momentum flow of the absorbers to themomentum loss of the radiation field. By comparing the filling factorsobtained by the two methods, we infer that some absorbers with logξ< 0.7 occur at significantly larger distances from the nucleus thanthe inner edge of the torus.
| A deep XMM-Newton observation of the ultraluminous X-ray source Holmberg II X-1: the case against a 1000-Msolar black hole
We present results from a 112-ks long look by XMM-Newton at theultraluminous X-ray source (ULX) Holmberg II X-1 (Ho II X-1), longthought to be the one of best candidates for the missing class ofintermediate-mass black holes (IMBHs). Our data comprises the firsthigh-quality XMM-Newton/RGS (reflection grating spectrometer) spectrumof an ULX, and an XMM-Newton/EPIC (European Photo Imaging Camera)spectrum with unprecedented signal-to-noise ratio. A detailed timinganalysis shows that any variability on time-scales of minutes to hoursis very weak (less than a few per cent fractional rms), though largeramplitude variations on much shorter time-scales could be hidden byphoton counting statistics. This result suggests that if Ho II X-1harbours an IMBH, then we are observing this source in a highly unusualand atypical state when compared with the known variability behaviour ofother accreting systems of large mass. Moreover, unlike galactic X-raybinaries, our spectral analysis indicates the possible presence of anoptically thick low-temperature corona. Taken together our timing andspectral analysis suggests that the compact companion is most likely ahigh-luminosity analogue of black hole binary systems similar to GRS1915+105, the galactic microquasar, harbouring a compact object of massno greater than 100 Msolar.
| The iron emission lines Kα and Kβ in active galactic nuclei A further piece of evidence for the Cerenkov line-like radiation
It has been recently observed by XMM-Newton that besides the emissionline of iron Kα at 6.4 keV there also exists in the Seyfert 1galaxy NGC 3783 the very intense emission line of iron Kβ at 7.0keV. Moreover, the ratio of equivalent widths (EWs) of these two linescan hardly be interpreted with the traditional mechanism of“photoelectric absorption fluorescence emission”. Byusing the mechanism of Cerenkov line-like radiation proposed by YOUJun-hian et al., the ratio of EWKα/EWKβmay be reasonably interpreted by means of a detailed model theoreticalcomputation. As shown by the results, the iron K lines in activegalactic nuclei (AGNs) are formed not only by the fluorescencemechanism, but also by the Cerenkov line-like radiation. Both coexist inAGNs, with the latter playing the dominant role.
| A FUSE Survey of High-Latitude Galactic Molecular Hydrogen
Measurements of molecular hydrogen (H2) column densities arepresented for the first six rotational levels (J=0-5) for 73extragalactic targets observed with the Far Ultraviolet SpectroscopicExplorer (FUSE). All of these have a final signal-to-noise ratio largerthan 10 and are located at Galactic latitude |b|>20deg.The individual observations were calibrated with the FUSE calibrationpipeline CalFUSE version 2.1 or higher and then carefully aligned invelocity. The final velocity shifts for all the FUSE segments arelisted. H2 column densities or limits are determined for thesix lowest rotational (J) levels for each H I component in the line ofsight, using a curve-of-growth approach at low column densities(<16.5) and Voigt-profile fitting at higher column densities.Detections include 65 measurements of low-velocity H2 in theGalactic disk and lower halo. Eight sight lines yield nondetections forGalactic H2. The measured column densities range fromlogN(H2)=14 to 20. Strong correlations are found betweenlogN(H2) and T01, the excitation temperature ofthe H2, as well as between logN(H2) and the levelpopulation ratios (log[N(J')/N(J)]). The average fraction ofnuclei in molecular hydrogen [f(H2)] in each sight line iscalculated; however, because there are many H I clouds in each sightline, the physics of the transition from H I to H2 cannot bestudied. Detections also include H2 in 16intermediate-velocity clouds in the Galactic halo (out of 35 IVCs).Molecular hydrogen is seen in one high-velocity cloud (the Leading Armof the Magellanic Stream), although 19 high-velocity clouds areintersected; this strongly suggests that dust is rare or absent in theseobjects. Finally, there are five detections of H2 in externalgalaxies.
| On the Fraction of X-Ray-obscured Quasars in the Local Universe
Recent wide-area hard X-ray and soft gamma-ray surveys have shown thatthe fraction of X-ray-obscured active galactic nuclei (AGNs) in thelocal universe significantly decreases with intrinsic luminosity. Inthis Letter we point out that two corrections have to be made to thesamples: (1) radio-loud AGNs have to be excluded, since their X-rayemission might be dominated by the jet component, and (2) Compton-thicksources have to be excluded too, since their hard X-ray and softgamma-ray emission are also strongly attenuated by Compton scattering.The soft gamma-ray-selected AGN samples obtained by Swift and INTEGRALprovide the best opportunity to study the fraction of obscured AGNs inthe local universe in the least biased way. We choose these samples tocheck if the corrections could alter the above result on the fraction ofobscured AGNs. We find that before the corrections both samples showsignificant anticorrelation between LX and NH,indicating an obvious decrease in the fraction of obscured AGNs withluminosity. However, after the corrections, we find only marginalevidence of anticorrelation (at the 98% confidence level) in the Swiftsample and no evidence at all in the INTEGRAL sample, which consists ofa comparable number of objects. We conclude that current samples onlyshow a marginal decrease in the fraction of obscured AGNs in the localuniverse and that much larger samples are required in order to reach amore robust conclusion.
| Fe K Emission and Absorption in the XMM-EPIC Spectrum of the Seyfert Galaxy IC 4329a
We present a detailed analysis of the XMM-Newton long-look of theSeyfert galaxy IC 4329a. The Fe K bandpass is dominated by two resolvedpeaks at 6.4 and 7.0 keV, consistent with neutral or near-neutral FeKα and Kβ emission. There is a prominent redward asymmetry inthe 6.4 keV line, which could indicate emission from a Compton shoulder.Alternatively, models using dual relativistic disk lines are found todescribe the emission profile well. A low-inclination, moderatelyrelativistic dual disk-line model is possible if the contribution fromnarrow components, due to distant material, is small or absent. Ahigh-inclination, moderately relativistic profile for each peak ispossible if there are roughly equal contributions from both the broadand narrow components. Combining the XMM-Newton data with RXTEmonitoring data, we explore the time-resolved spectral behavior ontimescales from hours to 2 years. We find no strong evidence forvariability of the Fe K line flux on any timescale, likely due to theminimal level of continuum variability. We detect, at high significance,a narrow absorption line at 7.68 keV. This feature is most likely due toFe XXVI Kα absorption blueshifted to ~0.1c relative to thesystemic velocity, suggesting a high-velocity, highly ionized outflowcomponent. As is often the case with similar outflows seen inhigh-luminosity quasars, the power associated with the outflowrepresents a substantial portion of the total energy budget of the AGN.The outflow could arise from a radiatively driven disk wind, or it maybe in the form of a discrete, transient blob of ejected material.
| High-Resolution Absorption Spectroscopy of Multiphase, High-Metallicity Gas Associated with the Luminous Quasar HE 0226-4110
We present FUSE and HST STIS observations of the absorption-line systemnear the emission redshift of the radio-quiet, X-ray-bright quasar HE0226-4110 (z=0.495, V=15.2). The spectra cover the rest-frame wavelengthrange 610-1150 Å, and we detect a wide range of ionizationspecies, including four adjacent stages of oxygen: O III-VI, whichreveal a striking change in gas kinematics with ionization. Examinatonof the O VI λλ1031, 1037 doublet profiles reveals noevidence for partial coverage or unresolved saturated structure. O IIIis only detected in a narrow feature that is also traced by the H I andC III lines, suggesting that they arise in the same gas. Absorption atthe same velocity is also present in other species (N IV, O IV-VI, SIV, and possibly Ne VIII ), but the kinematics differ from the O III,implying production in separate gaseous phases. The combination of H I,O III , and C III information yields an estimate of both thephotoionization parameter and the metallicity of the O III-bearing gas:[O/H]=+0.12+0.16-0.03,logU=-2.29+0.02-0.23. We discuss two possiblelocations for the gas in this associated absorption-line system: thenarrow emission line region of the quasar, and the halo of the quasarhost galaxy. An additional narrow (and thus photoionized) component thatis only detected in O VI appears 58 km s-1 redward of the OIII-bearing gas with -0.35<~logU<~0.02. Additional structure isdetected in the associated absorber in the form of two broad componentsthat only appear in moderate- to high-ionization species.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. Also based onobservations made with the NASA-CNES-CSA Far Ultraviolet SpectroscopicExplorer, which is operated for NASA by Johns Hopkins University underNASA contract NAS 5-32985.
| On the X-Ray Baldwin Effect for Narrow Fe Kα Emission Lines
Most active galactic nuclei (AGNs) exhibit a narrow Fe Kα line at~6.4 keV in the X-ray spectra, due to the fluorescent emission from coldmaterial far from the inner accretion disk. Using XMM-Newtonobservations, Page et al. found that the equivalent width (EW) of thenarrow Fe Kα line decreases with increasing luminosity(EW~L-0.17+/-0.08), suggesting a decrease in the coveringfactor of the material emitting the line (presumably the torus). Bycombining the archival Chandra HETG observations of 34 type 1 AGNs withXMM observations in the literature, we build a much larger sample with101 AGNs. We find a similar X-ray Baldwin effect in the sample(EW~L-0.2015+/-0.0426) however, we note that theanticorrelation is dominated by the radio-loud AGNs in the sample, whoseX-ray spectra might be contaminated by the relativistic jet. Excludingthe radio-loud AGNs, we find a much weaker anticorrelation(EW~L-0.1019+/-0.0524). We present Monte Carlo simulationsshowing that such a weak anticorrelation can be attributed to therelative short timescale variations of the X-ray continuum.
| A Galactic Origin for the Local Ionized X-Ray Absorbers
Recent Chandra and XMM-Newton observations of distant quasars have shownstrong local (z~0) X-ray absorption lines from highly ionized gas,primarily He-like oxygen. The nature of these X-ray absorbers, i.e.,whether they are part of the hot gas associated with the Milky Way orpart of the intragroup medium in the Local Group, remains a puzzle dueto the uncertainties in the distance. We present in this paper a surveyof 20 AGNs with Chandra and XMM-Newton archival data. About 40% of thetargets show local O VII He α absorption with column densitiesaround 1016 cm-2; in particular, O VII absorptionis present in all the high-quality spectra. We estimate that the skycovering fraction of this O VII-absorbing gas is at least 63%, at 90%confidence, and likely to be unity given enough high-quality spectra. Onthe basis of (1) the expected number of absorbers along sight linestoward distant AGNs, (2) joint analysis with X-ray emissionmeasurements, and (3) mass estimation, we argue that the observed X-rayabsorbers are part of the hot gas associated with our Galaxy. Futureobservations will significantly improve our understanding of thecovering fraction and provide robust tests of this result.
| The Radius-Luminosity Relationship for Active Galactic Nuclei: The Effect of Host-Galaxy Starlight on Luminosity Measurements
We have obtained high-resolution images of the central regions of 14reverberation-mapped active galactic nuclei (AGNs) using the HubbleSpace Telescope Advanced Camera for Surveys High Resolution Camera toaccount for host-galaxy starlight contamination of measured AGNluminosities. We measure the host-galaxy starlight contribution to thecontinuum luminosity at 5100 Å through the typical ground-basedslit position and geometry used in the reverberation-mapping campaigns.We find that removing the starlight contribution results in asignificant correction to the luminosity of each AGN both for lowerluminosity sources, as expected, but also for the higher luminositysources such as the PG quasars. After accounting for the host galaxystarlight, we revisit the well-known broad-line region radius-luminosityrelationship for nearby AGNs. We find the power-law slope of therelationship for the Hβ line to be 0.518+/-0.039, shallower thanwhat was previously reported and consistent with the slope of 0.5expected from the naive theoretical assumption that all AGNs have, onaverage, the same ionizing spectrum and the same ionization parameterand gas density in the Hβ line-emitting region.
| The MBH-σ* Relation in Local Active Galaxies
We examine whether active galaxies obey the same relation between blackhole mass and stellar velocity dispersion as inactive systems, using thelargest published sample of velocity dispersions for active nuclei todate. The combination of 56 original measurements with objects from theliterature not only increases the sample from the 15 consideredpreviously to 88 objects but allows us to cover an unprecedented rangein both stellar velocity dispersion (30-268 km s-1) and blackhole mass (105-108.6 Msolar). In theMBH-σ* relation of active galaxies, we finda lower zero point than the best-fit relation of Tremaine et al. forinactive galaxies, and an upper limit on the intrinsic scatter of 0.4dex. There is also evidence of a flatter slope at low black hole masses.We discuss potential contributors to the observed offsets, includingvariations in the geometry of the broad-line region, evolution in theMBH-σ* relation, and differential growthbetween black holes and galaxy bulges.
| Inner-Shell Absorption Lines of Fe VI-Fe XVI: A Many-Body Perturbation Theory Approach
We provide improved atomic calculation of wavelengths, oscillatorstrengths, and autoionization rates relevant to the 2-->3 inner-shelltransitions of Fe VI-XVI, the so-called Fe M-shell unresolved transitionarray (UTA). A second-order many-body perturbation theory is employed toobtain accurate transition wavelengths, which are systematically largerthan previous theoretical results by 15-45 mÅ. For a fewtransitions of Fe XVI and Fe XV for which laboratory measurements exist,our new wavelengths are accurate to within a few mÅ. Using thesenew calculations, the apparent discrepancy in the velocities between theFe M-shell UTA and other highly ionized absorption lines in the outflowof NGC 3783 disappears. The oscillator strengths in our new calculationagree well with the previous theoretical data, while the newautoionization rates are significantly larger, especially for lowercharge states. We attribute this discrepancy to the missingautoionization channels in the previous calculation. The increasedautoionization rates may slightly affect the column density analysis ofthe Fe M-shell UTA for sources with high column density and very lowturbulent broadening. The complete set of atomic data is provided as anelectronic table.
| Determining Central Black Hole Masses in Distant Active Galaxies and Quasars. II. Improved Optical and UV Scaling Relationships
We present four improved empirical relationships useful for estimatingthe central black hole mass in nearby AGNs and distant luminous quasarsalike using either optical or UV single-epoch spectroscopy. These massscaling relationships between line widths and luminosity are based onrecently improved empirical relationships between the broad-line regionsize and luminosities in various energy bands and are calibrated to theimproved mass measurements of nearby AGNs based on emission-linereverberation mapping. The mass scaling relationship based on theHβ line luminosity allows mass estimates for low-redshift sourceswith strong contamination of the optical continuum luminosity by stellaror nonthermal emission, while that based on the C IV λ1549 linedispersion allows mass estimates in cases where only the line dispersion(as opposed to the FWHM) can be reliably determined. We estimate thatthe absolute uncertainties in masses given by these mass scalingrelationships are typically around a factor of 4. We include in anappendix mass estimates for all of the Bright Quasar Survey (PG) quasarsfor which direct reverberation-based mass measurements are notavailable.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the Data Archive at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Spectral Variability of Quasars in the Sloan Digital Sky Survey. II. The C IV Line
We examine the variability of the high-ionization C IV λ1549 linein a sample of 105 quasars observed at multiple epochs by the SloanDigital Sky Survey. We find a strong correlation between the change inthe C IV line flux and the change in the line width, but no correlationsbetween the change in flux and changes in line center and skewness. Therelation between line flux change and line width change is consistentwith a model in which a broad line base varies with greater amplitudethan the line core. The objects studied here are more luminous and athigher redshift than those normally studied for variability, ranging inredshift from 1.65 to 4.00 and in absolute r-band magnitude from roughly-24 to -28. Using moment analysis line-fitting techniques, we measureline fluxes, centers, widths, and skewnesses for the C IV line at twoepochs for each object. The well-known Baldwin effect is seen for theseobjects, with a slope of β=-0.22. The sample has a median intrinsicBaldwin effect slope of βint=-0.85 the C IV lines inthese high-luminosity quasars appear to be less responsive to continuumvariations than those in lower luminosity AGNs. In addition, we find noevidence for variability of the well-known blueshift of the C IV linewith respect to the low-ionization Mg II λ2798 line in thehighest flux objects, indicating that this blueshift might be useful asa measure of orientation.Presented as part of a dissertation to the Department of Astronomy andAstrophysics of the University of Chicago, in partial fulfillment of therequirements for the Ph.D. degree.
| Reverberation Measurements of the Inner Radius of the Dust Torus in Nearby Seyfert 1 Galaxies
The most intense monitoring observations yet made in the optical andnear-infrared wave bands were carried out for Seyfert 1 galaxies NGC5548, NGC 4051, NGC 3227, and NGC 7469 by the MAGNUM telescope, andclear time-delayed responses of the K-band flux variations to the V-bandflux variations were found for all of these galaxies. Their H-K colortemperatures of 1500-1800 K, estimated from their observed fluxvariation gradients, support a view that the bulk of the K flux shouldoriginate in the thermal radiation of hot dust surrounding the centralengine and that the lag time should correspond to light-travel distancebetween them. Cross-correlation analysis measures their lag times to be47-53 (NGC 5548), 11-18 (NGC 4051), about 20 (NGC 3227), and 65-87 (NGC7469) days. The lag times are tightly correlated with the opticalluminosities, as expected from dust reverberation(Δt~L0.5), while weakly with the central virial masses,which suggests that the inner radii of the dust tori around activenuclei have one-to-one correspondences with their central luminosities.In the lag time versus central luminosity diagram, the K-band lag timesplace an upper boundary on the similar lag times of broad emission linesin the literature, which not only supports the unified scheme of AGNsbut also implies a physical transition from the BLR out to the dusttorus that encircles the BLR. Correlated short-term V-band and X-rayflux variations in NGC 5548 are also found with a delay of 1 or 2 days,indicating the thermal reprocessing of X-ray emission by the centralaccretion flow.
| The First INTEGRAL AGN Catalog
We present the first INTEGRAL AGN catalog, based on observationsperformed from launch of the mission in 2002 October until 2004 January.The catalog includes 42 AGNs, of which 10 are Seyfert 1, 17 are Seyfert2, and 9 are intermediate Seyfert 1.5. The fraction of blazars is rathersmall, with five detected objects, and only one galaxy cluster and nostarburst galaxies have been detected so far. A complete subset consistsof 32 AGNs with a significance limit of 7 σ in the INTEGRAL ISGRI20-40 keV data. Although the sample is not flux limited, thedistribution of sources shows a ratio of obscured to unobscured AGNs of1.5-2.0, consistent with luminosity-dependent unified models for AGNs.Only four Compton-thick AGNs are found in the sample. Based on theINTEGRAL data presented here, the Seyfert 2 spectra are slightly harder(Γ=1.95+/-0.01) than Seyfert 1.5 (Γ=2.10+/-0.02) and Seyfert1 (Γ=2.11+/-0.05).
| The Host Galaxies of Narrow-Line Seyfert 1 Galaxies: Nuclear Dust Morphology and Starburst Rings
We present a study of the nuclear morphology of a sample of narrow- andbroad-line Seyfert 1 galaxies (NLS1s and BLS1s, respectively) based onbroadband images in the Hubble Space Telescope archives. In our previousstudy we found that large-scale stellar bars at >1 kpc from thenucleus are more common in NLS1s than BLS1s. In this paper we find thatNLS1s preferentially have grand-design dust spirals within ~1 kpc oftheir centers. We also find that NLS1s have a higher fraction of nuclearstar-forming rings than BLS1s. We find that many of the morphologicaldifferences are due to the presence or absence of a large-scale stellarbar within the spiral host galaxy. In general, barred Seyfert 1 galaxiestend to have grand-design dust spirals at their centers, confirming theresults of other researchers. The high fraction of grand-design nucleardust spirals and stellar nuclear rings observed in NLS1s' host galaxiessuggests a means for efficient fueling of their nuclei to support theirhigh Eddington ratios.
| The small dispersion of the mid IR - hard X-ray correlation in active galactic nuclei
Context: .We investigate mid-infrared and X-ray properties of the dustytorus in unification scenarios for active galactic nuclei. Aims:.We use the relation between mid IR and hard X-ray luminosities toconstrain AGN unification scenarios. Methods: .With VISIR at theVLT, we have obtained the currently highest angular resolution (0.35arcsec FWHM) narrow-band mid infrared images of the nuclei of 8 nearbySeyfert galaxies. Combining these observations with X-ray data from theliterature we study the correlation between their mid IR and hard X-rayluminosities. Results: .We find that the rest frame 12.3 μm(LMIR) and 2-10 keV (LX) luminosities arecorrelated at a highly significant level. The best fit power-law to ourdata is log{LMIR} ∝ (1.60 ± 0.22 )log{LX}, showing a much smaller dispersion than earlierstudies. Conclusions: .The similarity in thelog{LMIR}/log{LX} ratio between Sy1s and Sy2s evenusing high angular resolution MIR data implies that the similarity isintrinsic to AGN and not caused by contamination from extra-nuclearemission. This supports clumpy torus models. The exponent of thecorrelation constrains the inner geometry of the torus.
| The structure and X-ray radiation spectra of illuminated accretion disks in AGN. III. Modeling fractional variability
Context: .Random magnetic flares above the accretion disks of ActiveGalactic Nuclei can account for the production of the primary radiationand for the rapid X-ray variability that have been frequently observedin these objects. The primary component is partly reprocessed in thedisk atmosphere, forming a hot spot underneath the flare source andgiving rise to distinct spectral features. Aims: .Extending thework of Czerny et al. (2004, A&A, 420, 1), we model the fractionalvariability amplitude due to distributions of hot spots co-orbiting onthe accretion disk around a supermassive black hole. We compare ourresults to the observed fractional variability spectrum of the Seyfertgalaxy MCG-6-30-15. Methods: .According to defined radialdistributions, our code samples random positions for the hot spotsacross the disk. The local spot emission is computed as reprocessedradiation coming from a compact primary source above the disk. Thestructure of the hot spot and the anisotropy of the re-emission aretaken into account. We compute the fractional variability spectraexpected from such spot ensembles and investigate dependencies on theparameters describing the radial spot distribution. We consider thefractional variability F{ var} with respect to the spectralmean and the so-called point-to-point definition F{ pp}. Ourmethod includes relativistic corrections due to the curved space-time inthe vicinity of a rotating supermassive black hole at the disk center;the black hole's angular momentum is a free parameter and is subject tothe fitting procedure. Results: .We confirm that therms-variability spectra involve intrinsic randomness at a significantlevel when the number of flares appearing during the total observationtime is too small. Furthermore, the fractional variability expressed byF{ var} is not always compatible with F{ pp}. Inthe special case of MCG-6-30-15, we can reproduce the short-timescalevariability and model the suppressed variability in the energy range ofthe Kα line without any need to postulate reprocessing fartheraway from the center. The presence of the dip in the variabilityspectrum requires an increasing rate of energy production by the flarestoward the center of the disk. The depth of the feature is wellrepresented only if we assume a fast rotation of the central black holeand allow for considerable suppression of the primary flare emission.The modeled line remains consistent with the measured equivalent widthof the iron Kα line complex. The model can reproduce thefrequently observed suppression of the variability in the spectral rangearound 6.5 keV, thereby setting constraints on the black hole spin andon the disk inclination.
| Spectral line variability amplitudes in active galactic nuclei
We present the results of a long-term variability campaign of verybroad-line AGNs with line widths broader than FWHM > 5000 kms-1. The main goal of our investigation was to study whetherthe widths of the optical broad emission lines are correlated with theoptical intensity variations on timescales of years. Our AGN sampleconsisted of 10 objects. We detected a significant correlation betweenoptical continuum variability amplitudes and Hβ emission linewidths (FWHM) and, to a lesser degree, between Hβ line intensityvariations and Hβ equivalent widths. We add the spectroscopic dataof variable AGNs from the literature to supplement our sample. The AGNsfrom other optical variability campaigns with different line-widthshelped to improve the statistical significance of our very broad-lineAGN sample. After including the data on 35 additional galaxies, thecorrelation between optical continuum variability amplitudes and Hβemission line widths becomes even more significant and the probabilitythat this is a random correlation drops to 0.7 percent.
| The correlation of narrow line emission and X-ray luminosity in active galactic nuclei
Aims.We combine emission line and X-ray luminosities for 45 sources fromthe Chandra Deep Field South (CDF-S), and seven HELLAS sources, toobtain a new sample of 52 X-ray selected type-II active galactic nuclei(AGNs). Eighteen of our sources are very luminous with a typical,absorption-corrected 2-10 keV luminosity of few × 1044erg s-1 (type-II QSOs). Methods. We compare theemission line properties of the new sources with emission line and X-rayluminosities of known low redshift, mostly lower luminosity AGNs byusing a composite spectrum. Results. We find thatL[OIII]/L2-10 andL[OII]/L2-10 decrease with L(2-10 keV) such thatL[OIII]/L2-10 ∝L2-10-0.42. The trend was already evident, yetneglected in past low redshift samples. This lead to erroneouscalibration of the line-to-X-ray luminosity in earlier AGN samples. Theanalysis of several type-I samples shows the same trend with a similarslope but a median L[OIII]/L2-10 which is largerby a factor of about two compared with optically selected type-IIsamples. We interpret this shift as due to additional reddening intype-II sources and comment in general on the very large extinction inmany type-II objects and the significantly smaller average reddening ofthe SDSS type-II AGNs. The decrease ofL[OIII]/L2-10 with L(2-10 keV) is large enough tosuggest that a significant fraction of high luminosity high redshifttype-II AGNs have very weak emission lines that may have escapeddetection in large samples. A related decrease of EW([O iii]λ5007) with optical continuum luminosity is demonstrated by ananalysis of 12 000 type-I SDSS AGNs. The new correlations found here areimportant for deriving accurate luminosity functions for AGNs and theirneglect may explain past discrepancies between emission line and X-raysamples.
| Modeling the warm absorber in active galactic nuclei
We present a wide grid of models for the structure and transmissionproperties of warm absorbers in active galactic nuclei (AGN). Contraryto commonly used constant density models, our absorbing cloud is assumedto be under constant total (gas plus radiation) pressure. Thisassumption implies the coexistence of material at different temperaturesand ionization states, which is a natural consequence of pressure andthermal equilibrium. Our photoionization code allows us to compute theprofiles of the density, the temperature, the gas pressure, theradiation pressure and the ionization state across the cloud, and tocalculate the radiative transfer of continuum and lines includingCompton scattering. Therefore, equivalent widths of both saturated andunsaturated lines are properly modeled. For each pair of the incidentspectrum slope and the ionization parameter at the cloud surface thereis a natural upper limit to the total column densities of the cloud dueto thermal instabilities. These maximum values are comparable to theobservational constraints on the column density of warm absorbers whichmay give support to constant total pressure models. In all models wenote considerable absorption around 6.4 keV which modifies the intrinsicrelativistically broadened iron line profile originating in an accretiondisk illuminated atmosphere. Our models can be applied to fitting thespectroscopic data from the XMM-Newton and Chandra satellites.
| A new model for the Warm Absorber in NGC 3783: a single medium in total pressure equilibrium
Context.Many active galactic nuclei exhibit X-ray features typical ofthe highly ionized gas called "Warm Absorber" (WA). Such a materialappears to be stratified, displaying zones of different density,temperature, and ionization. In this paper, we investigate thepossibility of modelling the WA gas in NGC 3783 as a single medium intotal pressure equilibrium. Aims.Our goal is to demonstrate thatthe WA can be well modelled assuming constant total pressure, incontrast to the current descriptions that are based on the presence ofmultiple regions, each in constant density. The assumption of totalpressure equilibrium yields a more physical description of the WA,resulting in the natural stratification of the ionized gas, andproviding an explanation for the presence of lines from differentionization states, as observed in WA spectra. Methods.We have usedthe photoionization code TITAN, developed by our team, to compute a gridof constant total pressure models with the purpose of fitting the WA inNGC 3783. We have compared our models to the 900 ks Chandra spectrum ofNGC 3783 and to previous studies where the WA was described by multiplezones of constant density. Results.In the case of NGC 3783, the WAfeatures can be well reproduced by a clumpy, ionized gas with cosmicabondances, ionization parameter ξ= 2500 erg cm s-1,column density {N_H} = 4 × 1022 cm-2, andconstant total pressure. Conclusions.We have shown that the WA inNGC 3783 can be modelled by a single medium in total pressureequilibrium; this is probably the case for other WAs currently describedby multi-zone, constant density models. In addition, our workdemonstrates that the TITAN code is well adapted to the study of the WAin active galactic nuclei, opening new prospects for the use of TITAN bya larger community.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
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