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How Do We See a Relativistic Accretion Disk during a Thermal Instability?
We calculated bolometric images of relativistic slim disks during aradiation-pressure-driven thermal instability. When the mass-accretionrate exceeds the critical one, an inner region of the standard accretiondisk bursts to change to a slim disk state having a large scale height.That is, the inner region of the disk becomes high temperature, and thethickness of the disk increases due to an increase of the radiationpressure. As a result, we found that the observed image of the diskduring the burst strongly depends on the inclination angle. That is,radiation from the innermost disk would be occulted by the disk outerrim for high inclination angles (i ≳ 70°). We also calculatedthe spectral energy distribution during a thermal instability. The Wienpeak of the spectrum of high inclination angles becomes softer than thatof low inclination angles due to the geometrical thickness. From thesefacts, even if a burst occurs in a black-hole candidate, we may notobserve the burst when the inclination angle is large. We may suggestthat numerous luminous black-hole candidates are still hidden in ourGalaxy.
| Revisiting the infrared spectra of active galactic nuclei with a new torus emission model
We describe improved modelling of the emission by dust in atoroidal-like structure heated by a central illuminating source withinactive galactic nuclei (AGNs). We have chosen a simple but realistictorus geometry, a flared disc, and a dust grain distribution functionincluding a full range of grain sizes. The optical depth within thetorus is computed in detail taking into account the differentsublimation temperatures of the silicate and graphite grains, whichsolves previously reported inconsistencies in the silicate emissionfeature in type 1 AGNs. We exploit this model to study the spectralenergy distributions (SEDs) of 58 extragalactic (both type 1 and type 2)sources using archival optical and infrared data. We find that both AGNand starburst contributions are often required to reproduce the observedSEDs, although in a few cases they are very well fitted by a pure AGNcomponent. The AGN contribution to the far-infrared luminosity is foundto be higher in type 1 sources, with all the type 2 requiring asubstantial contribution from a circumnuclear starburst. Our resultsappear in agreement with the AGN unified scheme, because thedistributions of key parameters of the torus models turn out to becompatible for type 1 and type 2 AGNs. Further support to theunification concept comes from comparison with medium-resolutioninfrared spectra of type 1 AGNs by the Spitzer observatory, showingevidence for a moderate silicate emission around 10 μm, which ourcode reproduces. From our analysis we infer accretion flows in the innernucleus of local AGNs characterized by high equatorial optical depths(AV~= 100), moderate sizes (Rmax < 100 pc) andvery high covering factors (f~= 80 per cent) on average.
| 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.
| 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.
| Local and Large-Scale Environment of Seyfert Galaxies
We present a three-dimensional study of the local (<=100h-1 kpc) and the large-scale (<=1 h-1 Mpc)environment of the two main types of Seyfert AGN galaxies. For thispurpose we use 48 Seyfert 1 galaxies (with redshifts in the range0.007<=z<=0.036) and 56 Seyfert 2 galaxies (with0.004<=z<=0.020), located at high galactic latitudes, as well astwo control samples of nonactive galaxies having the same morphological,redshift, and diameter size distributions as the corresponding Seyfertsamples. Using the Center for Astrophysics (CfA2) and Southern SkyRedshift Survey (SSRS) galaxy catalogs (mB~15.5) and our ownspectroscopic observations (mB~18.5), we find that within aprojected distance of 100 h-1 kpc and a radial velocityseparation of δv<~600 km s-1 around each of ourAGNs, the fraction of Seyfert 2 galaxies with a close neighbor issignificantly higher than that of their control (especially within 75h-1 kpc) and Seyfert 1 galaxy samples, confirming a previoustwo-dimensional analysis of Dultzin-Hacyan et al. We also find that thelarge-scale environment around the two types of Seyfert galaxies doesnot vary with respect to their control sample galaxies. However, theSeyfert 2 and control galaxy samples do differ significantly whencompared to the corresponding Seyfert 1 samples. Since the maindifference between these samples is their morphological typedistribution, we argue that the large-scale environmental differencecannot be attributed to differences in nuclear activity but rather totheir different type of host galaxies.
| 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).
| Discovery of Water Maser Emission in Eight AGNs with 70 m Antennas of NASA's Deep Space Network
We report the discovery of water maser emission in eight active galacticnuclei (AGNs) with the 70 m NASA Deep Space Network (DSN) antennas atTidbinbilla, Australia, and Robledo, Spain. The positions of the newlydiscovered masers, measured with the VLA, are consistent with theoptical positions of the host nuclei to within 1 σ (0.3" radio and1.3" optical) and most likely mark the locations of the embedded centralengines. The spectra of two sources, NGC 3393 and NGC 5495, display thecharacteristic spectral signature of emission from an edge-on accretiondisk, with orbital velocities of ~600 and ~400 km s-1,respectively. In a survey with DSN facilities of 630 AGNs selected fromthe NASA Extragalactic Database, we have discovered a total of 15 watermaser sources. The resulting incidence rate of maser emission amongnearby (vsys<7000 km s-1) Seyfert 1.8-2.0 andLINER systems is ~10% for a typical rms noise level of ~14 mJy over 1.3km s-1 spectral channels. As a result of this work, thenumber of nearby AGNs (vsys<7000 km s-1)observed with <20 mJy rms noise has increased from 130 to 449.
| Star Formation in Distant Red Galaxies: Spitzer Observations in the Hubble Deep Field-South
We present Spitzer 24 μm imaging of 1.5~40 μJy and conclude that the bulk of the DRGpopulation is dusty active galaxies. A mid-infrared (MIR) color analysiswith IRAC data suggests that the MIR fluxes are not dominated by buriedAGNs, and we interpret the high detection rate as evidence for a highaverage star formation rate of =130+/-30 Msolaryr-1. From this, we infer that DRGs are importantcontributors to the cosmic star formation rate density at z~2, at alevel of ~0.02 Msolar yr-1 Mpc-3 to ourcompleteness limit of KAB=22.9 mag.
| A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei
Seyfert galaxies commonly host compact jets spanning 10-100 pc scales,but larger structures are resolved out in long-baseline aperturesynthesis surveys. Previous, targeted studies showed thatkiloparsec-scale radio structures (KSRs) may be a common feature ofSeyfert and LINER galaxies, and the origin of KSRs may be starbursts oractive galactic nuclei (AGNs). We report a new Very Large Array surveyof a complete sample of Seyfert and LINER galaxies. Out of all of thesurveyed radio-quiet sources, we find that 44% (19 out of 43) showextended radio structures at least 1 kpc in total extent that do notmatch the morphology of the disk or its associated star-forming regions.The detection rate is a lower limit owing to the combined effects ofprojection and resolution. The infrared colors of the KSR host galaxiesare unremarkable compared to other Seyfert galaxies, and the large-scaleoutflows orient randomly with respect to the host galaxy axes. The KSRSeyfert galaxies instead stand out by deviating significantly from thefar-infrared-radio correlation for star-forming galaxies, with tendencytoward radio excess, and they are more likely to have a relativelyluminous, compact radio source in the nucleus; these results argue thatKSRs are powered by the AGNs rather than starbursts. The high detectionrate indicates that Seyfert galaxies generate radio outflows over asignificant fraction of their lifetime, which is much longer than thedynamical timescale of an AGN-powered jet but is comparable instead tothe buoyancy timescale. The likely explanation is that the KSRsoriginate from jet plasma that has been decelerated by interaction withthe nuclear interstellar medium (ISM). Based on a simple ram pressureargument, the kinetic power of the jet on kiloparsec scales is about 3orders of magnitude weaker than the power of the jet on 10-100 pcscales. This result is consistent with the interaction model, in whichcase virtually all of the jet power must be lost to the ISM within theinner kiloparsec.
| 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.
| Seyfert Galaxies and the Hard X-Ray Background: Artificial Chandra Observations of z=0.3 Active Galaxies
Deep X-ray surveys have resolved much of the X-ray background radiationbelow 2 keV into discrete sources, but the background above 8 keVremains largely unresolved. The obscured (type 2) active galactic nuclei(AGNs) that are expected to dominate the hard X-ray background have notyet been detected in sufficient numbers to account for the observedbackground flux. However, deep X-ray surveys have revealed large numbersof faint quiescent and starburst galaxies at moderate redshifts. Inhopes of recovering the missing AGN population, it has been suggestedthat the defining optical spectral features of low-luminosity Seyfertnuclei at large distances may be overwhelmed by their host galaxies,causing them to appear optically quiescent in deep surveys. We test thispossibility by artificially redshifting a sample of 23 nearby,well-studied active galaxies to z=0.3, testing them for X-ray AGNsignatures, and comparing them to the objects detected in deep X-raysurveys. We find that these redshifted galaxies have propertiesconsistent with the deep-field normal and optically bright, X-ray-faintgalaxy (OBXF) populations, supporting the hypothesis that the numbers ofAGNs in deep X-ray surveys are being underestimated and suggesting thatOBXFs should not be ruled out as candidate AGN hosts that couldcontribute to the hard X-ray background source population.
| The K-band properties of Seyfert 2 galaxies
Aims. It is well known that the [O iii]λ5007 emission line andhard X-ray (2-10 keV) luminosities are good indicators of AGN activitiesand that the near and mid-infrared emission of AGN originates fromre-radiation of dusty clouds heated by the UV/optical radiation from theaccretion disk. In this paper we present a study of the near-infraredK-band (2.2 μm) properties for a sample of 65 Seyfert 2 galaxies. Methods: .By using the AGN/Bulge/Disk decomposition technique, weanalyzed the 2MASS K_S-band images for Seyfert 2 galaxies in order toderive the K_S-band magnitudes for the central engine, bulge, and diskcomponents. Results: .We find that the K_S-band magnitudes of thecentral AGN component in Seyfert 2 galaxies are tightly correlated withthe [O iii]λ5007 and the hard X-ray luminosities, which suggeststhat the AGN K-band emission is also an excellent indicator of thenuclear activities at least for Seyfert 2 galaxies. We also confirm thegood relation between the central black hole masses and bulge's K-bandmagnitudes for Seyfert 2s.
| Extragalactic H_2O masers and X-ray absorbing column densities
Having conducted a search for the λ 1.3 cm (22 GHz) water vaporline towards galaxies with nuclear activity, large nuclear columndensities or high infrared luminosities, we present H2O spectra for NGC2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and400 Lȯ. The H2O maser in UGC 5101 is by far the mostluminous yet found in an ultraluminous infrared galaxy. NGC 3393 revealsthe classic spectrum of a "disk maser", represented by three distinctgroups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below1000 km s-1. Based on the literature and archive data, X-rayabsorbing column densities are compiled for the 64 galaxies withreported maser sources beyond the Magellanic Clouds. For NGC 2782 andNGC 5728, we present Chandra archive data that indicate the presence ofan active galactic nucleus in both galaxies. Modeling the hard nuclearX-ray emission, NGC 2782 is best fit by a high energy reflectionspectrum with NH 1024 cm-2. ForNGC 5728, partial absorption with a power law spectrum indicatesNH 8 × 1023 cm-2. Thecorrelation between absorbing column and H2O emission is analyzed. Thereis a striking difference between kilo- and megamasers with megamasersbeing associated with higher column densities. All kilomasers (L_H_2O< 10 Lȯ) except NGC 2273 and NGC 5194 areCompton-thin, i.e. their absorbing columns are <1024cm-2. Among the H{2}O megamasers, 50% arise fromCompton-thick and 85% from heavily obscured (>1023cm-2) active galactic nuclei. These values are not larger butconsistent with those from samples of Seyfert 2 galaxies not selected onthe basis of maser emission. The similarity in column densities can beexplained by small deviations in position between maser spots andnuclear X-ray source and a high degree of clumpiness in thecircumnuclear interstellar medium.
| Gas metallicity in the narrow-line regions of high-redshift active galactic nuclei
We analyze optical (UV rest-frame) spectra of X-ray selected narrow-lineQSOs at redshift 1.5 z 3.7 found in the Chandra Deep FieldSouth and of narrow-line radio galaxies at redshift 1.2 z 3.8 to investigate the gas metallicity of the narrow-line regions andtheir evolution in this redshift range. Such spectra are also comparedwith UV spectra of local Seyfert 2 galaxies. The observational data areinconsistent with the predictions of shock models, suggesting that thenarrow-line regions are mainly photoionized. The photoionization modelswith dust grains predict line flux ratios which are also in disagreementwith most of the observed values, suggesting that the high-ionizationpart of the narrow-line regions (which is sampled by the availablespectra) is dust-free. The photoionization dust-free models provide twopossible scenarios which are consistent with the observed data:low-density gas clouds (nH 103cm-3) with a sub-solar metallicity (0.2 Z_gas/Z_ȯ 1.0), or high-density gas clouds (nH 105 cm-3) with a wide range of gas metallicity(0.2 Z_gas/Z_ȯ 5.0). Regardless of the specificinterpretation, the observational data do not show any evidence for asignificant evolution of the gas metallicity in the narrow-line regionswithin the redshift range 1.2 z 3.8. Instead, we find atrend for more luminous active galactic nuclei to have more metal-richgas clouds (luminosity-metallicity relation), which is in agreement withthe same finding in the studies of the broad-line regions. The lack ofevolution for the gas metallicity of the narrow-line regions impliesthat the major epoch of star formation in the host galaxies of theseactive galactic nuclei is at z 4.
| 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.
| Adaptive Optics Observations of Active Galactic Nuclei: Zooming into the Central Parsec Region
First results from the AGN-Heidelberg program aimed at resolving thecentral pc region of the closest Active Galactic Nuclei in the IR arepresented. The core region of prototype active nuclei are clearlyunveiled at IR waves and at distances from the nucleus - few pc- wherecircumnuclear starforming regions appear not to be present. Within thatperspective, classical active nuclei as Circinus and NGC 1097, revealwith unprecedented detail clear channels of material being driven to thecore whereas others as Centaurus A and NGC 1566, show a `clean' coreenvironment. At the very center, a central compact region of about 2 pcscale is resolved in Circinus but not in the other cases challengingthus the universal presence of the putative obscuring torus.
| X-ray variability of NGC 3227 and 5506 and the nature of active galactic nucleus `states'
We use X-ray monitoring data obtained over a broad range of time-scalesto measure the broad-band power spectral density functions (PSDs) of twoSeyfert galaxies: the broad-line Seyfert 1 NGC 3227 and the Seyfert 2NGC 5506, which has recently been identified as an obscured narrow-lineSeyfert 1 (NLS 1). Using a Monte Carlo fitting technique, we demonstratethat both PSDs are reminiscent of the PSD of black hole X-ray binaries(BHXRBs) in the high/soft state, and specifically rule out alow/hard-state PSD shape in NGC 3227. This result demonstrates that, atleast where variability is concerned, broad-line Seyferts with hardX-ray spectra (photon index Γ~ 1.6) are not simply the analoguesof the low/hard state in BHXRBs, and the dichotomy of NLS 1 andbroad-line Seyferts cannot be simply interpreted in terms of the twostates. We show that the PSD normalization in NGC 3227 is stronglyenergy-dependent, with larger variability amplitudes at lower energies,unlike NGC 5506 which shows little energy dependence of variability. Wedemonstrate that this difference is caused by spectral pivoting of thecontinuum in NGC 3227 at high energies, which is probably also relatedto the large amplitude of variability seen in the 2-10 keV band in thisactive galactic nucleus (AGN). Using the new PSD data and new results inthe literature, we replot the PSD break time-scale versus mass plot forall AGNs with PSD breaks measured so far, and demonstrate that higheraccretion rate AGNs appear to have relatively shorter break time-scalesfor their black hole mass than lower accretion rate AGNs.
| MCG-6-30-15: long time-scale X-ray variability, black hole mass and active galactic nuclei high states
We present a detailed study of the long time-scale X-ray variability ofthe Seyfert 1 Galaxy MCG-6-30-15, based on eight years of frequentmonitoring observations with the Rossi X-ray Timing Explorer. Whencombined with the published short-time-scale XMM-Newton observations, wederive the power-spectral density (PSD) covering six decades offrequency from ~10-8 to ~10-2 Hz. As with NGC4051, another narrow-line Seyfert 1 galaxy (NLS1), we find that the PSDof MCG-6-30-15 is a close analogue of the PSD of a galactic black holeX-ray binary system (GBH) in a `high' rather than a `low' state. As withNGC 4051 and the GBH Cygnus X-1 in its high state, a smoothly bendingmodel is a better fit to the PSD of MCG-6-30-15, giving a derived breakfrequency of 7.6+10-3× 10-5 Hz.Assuming linear scaling of the break frequency with black hole mass, weestimate the black hole mass in MCG-6-30-15 to be~2.9+1.8-1.6×106Msolar.Although, in the X-ray band, it is one of the best observed Seyfertgalaxies, there has as yet been no accurate determination of the mass ofthe black hole in MCG-6-30-15. Here we present a mass determinationusing the velocity dispersion (MBH-σ*)technique and compare it with estimates based on the width of theHα line. Depending on the calibration relationship assumed for theMBH-σ* relationship, we derive a mass ofbetween 3.6 and 6 × 106Msolar, consistentwith the mass derived from the PSD.Using the newly derived mass and break time-scale, and revisedreverberation masses for other active galactic nuclei (AGN) fromPeterson et al., we update the black hole mass-break-time-scale diagram.The observations are still generally consistent with narrow-line Seyfert1 galaxies having shorter break time-scales, for a given mass, thanbroad-line AGN, probably reflecting a higher accretion rate. However,the revised, generally higher, masses (but unchanged break time-scales)are also consistent with perhaps all of the X-ray bright AGN studied sofar being high-state objects. This result may simply be a selectioneffect, based on their selection from high-flux X-ray all-skycatalogues, and their consequent typically high X-ray/radio ratios,which indicate high-state systems.
| The relationship between X-ray variability amplitude and black hole mass in active galactic nuclei
We have investigated the relationship between the X-ray variabilityamplitude and black hole mass for a sample of 46 radio-quiet activegalactic nuclei observed by ASCA. 33 of the objects in our sampleexhibited significant variability over a time-scale of ~40 ks. Wedetermined the normalized excess variance in the 2-10 keV light curvesof these objects and found a significant anticorrelation between excessvariance and black hole mass. Unlike most previous studies, we havequantified the variability using nearly the same time-scale for allobjects. Moreover, we provide a prescription for estimating theuncertainties in variance which accounts both for measurementuncertainties and for the stochastic nature of the variability. We alsopresent an analytical method to predict the excess variance from a modelpower spectrum accounting for binning, sampling and windowing effects.Using this, we modelled the variance-mass relation assuming all objectshave a universal twice-broken power spectrum, with the position of thebreaks being dependent on mass. This accounts for the general form ofthe variance-mass relationship but is formally a poor fit and there isconsiderable scatter. We investigated this scatter as a function of theX-ray photon index, luminosity and Eddington ratio. After accounting forthe primary dependence of excess variance on mass, we find nosignificant correlation with either luminosity or X-ray spectral slope.We do find an anticorrelation between excess variance and the Eddingtonratio, although this relation might be an artefact owing to theuncertainties in the mass measurements. It remains to be establishedthat enhanced X-ray variability is a property of objects with steepX-ray slopes or large Eddington ratios. Narrow-line Seyfert 1 galaxies,in particular, are consistent with being more variable than theirbroad-line counterparts solely because they tend to have smaller masses.
| Principal components in active galactic nuclei variability data and the estimation of the flux contributions from different components
It has been found that the near-infrared flux variations of Seyfertgalaxies satisfy relations of the formFi~αij+βijFj,where Fi, Fj are the fluxes in filters i and j;and αi,j, βi,j are constants. Theserelations have been used to estimate the constant contributions of thenon-variable underlying galaxies. The paper attempts a formal treatmentof the estimation procedure, allowing for the possible presence of athird component, namely non-variable hot dust. In an analysis of asample of 38 Seyfert galaxies, inclusion of the hot dust componentimproves the model fit in approximately half the cases. All derived dusttemperatures are below 300 K, in the range 540-860 K or above 1300 K. Anoteworthy feature is the estimation of confidence intervals for thecomponent contributions: this is achieved by bootstrapping. It is alsopointed out that the model implies that such data could be fruitfullyanalysed in terms of principal components.
| FeXXV and FeXXVI lines from low-velocity, photoionized gas in the X-ray spectra of active galactic nuclei
We have calculated the equivalent widths of the absorption linesproduced by FeXXV and FeXXVI in a Compton-thin, low-velocityphotoionized material illuminated by the nuclear continuum in activegalactic nuclei. The results, plotted against the ionization parameterand the column density of the gas, are a complement to those presentedby Bianchi & Matt for the emission lines from the same ionicspecies. As an extension to the work by Bianchi & Matt, we alsopresent a qualitative discussion on the different contributions to theHe-like iron emission line complex in the regimes where recombination orresonant scattering dominates, providing a useful diagnostic tool tomeasure the column density of the gas. Future high-resolution missions(e.g. Astro-E2) will allow us to fully take advantage of these plasmadiagnostics. In the meantime, we compare our results with an up-to-datelist of Compton-thick and unobscured (at least at the iron line energy)Seyfert galaxies with emission and/or absorption lines from H- andHe-like iron observed with Chandra and XMM-Newton.
| An 8.4-GHz Long Baseline Array observation of the unusual Seyfert galaxy NGC 7213
We have observed the type 1.5 Seyfert galaxy NGC 7213 with theAustralian Long Baseline Array (LBA) at 8.4 GHz to discover whether thisobject has the high brightness temperature compact core suggested bylow-frequency variability. Confirmation would support the hypothesisthat radio-intermediate Seyfert galaxies have Doppler-boosted radiojets. Our observation confirms the existence of this core but with aflux density of almost a factor of 6 less than observed 12 yr earlier.Though few studies exist on the long-term radio variability of Seyferts,a decline of this magnitude does appear to be rare.
| The X-ray spectrum of NGC 7213 and the Seyfert-LINER connection
We present an XMM-Newton observation of the Seyfert-LINER(low-ionization nuclear emission-line region) galaxy NGC 7213. The RGSsoft X-ray spectrum is well fitted with a power law plus soft X-raycollisionally ionized thermal plasma (kT=0.18+0.03-0.01 keV). We confirm the presence ofFeI, FeXXV and FeXXVI Kα emission in the EPIC spectrum and settighter constraints on their equivalent widths of82+10-13, 24+9-11 and24+10-13 eV, respectively. We compare the observedproperties together with the inferred mass accretion rate of NGC 7213with those of other Seyfert and LINER galaxies. We find that NGC 7213has intermediate X-ray spectral properties lying between those of theweak active galactic nucleus found in the LINER M81 andhigher-luminosity Seyfert galaxies. There appears to be a continuoussequence of X-ray properties from the Galactic Centre through LINERgalaxies to Seyferts, probably determined by the amount of materialavailable for accretion in the central regions.
| The exceptional X-ray variability of the dwarf Seyfert nucleus NGC 4395
An analysis of the X-ray variability of the low-luminosity Seyfertnucleus NGC 4395, based on a long XMM-Newton observation, is presented.The power spectrum shows a clear break from a flat spectrum (α~ 1)to a steeper spectrum (α~ 2) at a frequency fbr=0.5-3.0 × 10-3 Hz, comparable to the highestcharacteristic frequency found previously in a Seyfert galaxy. Thisextends the measured MBH-fbr values to lowerMBH than previous studies of Seyfert galaxies, and isconsistent with an inverse scaling of variability frequency with blackhole mass. The variations observed are among the most violent seen in anactive galactic nuclei to date, with the fractional rms amplitude(Fvar) exceeding 100 per cent in the softest band. Theamplitude of the variations seems intrinsically higher in NGC 4395 thanmost other Seyfert galaxies, even after accounting for the differencesin characteristic frequencies. The origin of this difference is notclear, but it is unlikely to be a high accretion rate(L/LEdd<~ 20 per cent for NGC 4395). The variationsclearly follow the linear rms-flux relation, further supporting the ideathat this is a ubiquitous characteristics of accreting black holes. Thevariations are highly coherent between different energy bands with anyfrequency-dependent time delay limited to <~1 per cent.
| Galactic Winds
Galactic winds are the primary mechanism by which energy and metals arerecycled in galaxies and are deposited into the intergalactic medium.New observations are revealing the ubiquity of this process,particularly at high redshift. We describe the physics behind thesewinds, discuss the observational evidence for them in nearbystar-forming and active galaxies and in the high-redshift universe, andconsider the implications of energetic winds for the formation andevolution of galaxies and the intergalactic medium. To inspire futureresearch, we conclude with a set of observational and theoreticalchallenges.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| The Relationship of Hard X-Ray and Optical Line Emission in Low-Redshift Active Galactic Nuclei
In this paper we assess the relationship of the population of activegalactic nuclei (AGNs) selected by hard X-rays to the traditionalpopulation of AGNs with strong optical emission lines. First, we studythe emission-line properties of a new hard-X-ray-selected sample of 47local AGNs (classified optically as Type 1 and 2 AGNs). We find that thehard X-ray (3-20 keV) and [O III] λ5007 optical emission-lineluminosities are well-correlated over a range of about 4 orders ofmagnitude in luminosity (mean luminosity ratio 2.15 dex with a standarddeviation of σ=0.51 dex). Second, we study the hard X-rayproperties of a sample of 55 local AGNs selected from the literature onthe basis of the flux in the [O III] line. The correlation between thehard X-ray (2-10 keV) and [O III] luminosity for the Type 1 AGNs isconsistent with what is seen in the hard-X-ray-selected sample. However,the Type 2 AGNs have a much larger range in the luminosity ratio, andmany are very weak in hard X-rays (as expected for heavily absorbedAGNs). We then compare the hard X-ray (3-20 keV) and [O III] luminosityfunctions of AGNs in the local universe. These have similar faint-endslopes, with a luminosity ratio of 1.60 dex (0.55 dex smaller than themean value for individual hard-X-ray-selected AGNs). We conclude that atlow redshift, selection by narrow optical emission lines will recovermost AGNs selected by hard X-rays (with the exception of BL Lacobjects). However, selection by hard X-rays misses a significantfraction of the local AGN population with strong emission lines.
| The Swift/BAT High-Latitude Survey: First Results
We present preliminary results from the first 3 months of the SwiftBurst Alert Telescope (BAT) high Galactic latitude survey in the 14-195keV band. The survey reaches a flux of ~10-11 ergscm-2 s-1 and has ~2.7 arcmin (90% confidence)positional uncertainties for the faintest sources. This represents themost sensitive survey to date in this energy band. These data confirmthe conjectures that a high-energy-selected active galactic nucleus(AGN) sample would have very different properties from those selected inother bands and that it represents a ``true'' sample of the AGNpopulation. We have identified 86% of the 66 high-latitude sources.Twelve are Galactic-type sources, and 44 can be identified withpreviously known AGNs. All but five of the AGNs have archival X-rayspectra, enabling us to estimate the line-of-sight column densities andother spectral properties. Both of the z>0.11 objects are blazars.The median redshift of the others (excluding radio-loud objects) is0.012. We find that the column density distribution of these AGNs isbimodal, with 64% of the nonblazar sources having column densitiesNH>=1022 cm-2. None of the sourceswith logLX>43.5 (cgs units) show high column densities,and very few of the lower LX sources have low columndensities. Based on these data, we expect the final BAT catalog to have>200 AGNs and reach fluxes of less than ~10-11 ergscm-2 s-1 over the entire sky.
| Molecular Hydrogen Excitation around Active Galactic Nuclei
We report R~3000 Very Large Telescope ISAAC K-band spectroscopy of thenuclei (i.e., central 100-300 pc) of nine galaxies hosting an activegalactic nucleus. For five of these we also present spectra of thecircumnuclear region out to 1 kpc. We have measured a number ofmolecular hydrogen lines in the ν=1-0, 2-1, and 3-2 vibrationaltransitions, as well as the Brγ and He I recombination lines andthe Na I stellar absorption feature. Although only three of the galaxiesare classified as type 1 Seyfert galaxies in the literature, broadBrγ (FWHM>~1000 km s-1) is seen in seven of theobjects. The ν=1-0 emission appears thermalized at temperaturesT~1000 K. However, the ν=2-1 and ν=3-2 emission show evidence ofbeing radiatively excited by far-ultraviolet photons. Thephotodissociation region models that fit the data best are, as for theultraluminous infrared galaxies in Davies et al., those for which theH2 emission arises in dense clouds illuminated by intensefar-ultraviolet radiation. The Na I stellar absorption line is clearlyseen in six of the nuclear spectra of these AGNs, indicating thepresence of a significant population of late-type stars. It is possiblethat these stars are a result of the same episode of star formation thatgave rise to the stars heating the photodissociation regions. It seemsunlikely that the AGN is the dominant source of excitation for thenear-infrared H2 emission: in two of the nuclear spectraH2 was not detected at all, and in general we find noevidence of suppression of the 2-1 S(3) line, which may occur inX-ray-irradiated gas. Our data do not reveal any significant differencebetween the nuclear and circumnuclear line ratios, suggesting that thephysical conditions of the dominant excitation mechanism are similarboth near the AGN and in the larger scale environment around it, andthat star formation is an important process even in the central 100 pcaround AGNs.Based on observations at the European Southern Observatory VLT(69.B-0075).
| A Variability Study of the Seyfert 2 Galaxy NGC 6300 with XMM-Newton
We present the results of timing analysis of the XMM-Newton observationof the Seyfert 2 galaxy NGC 6300. The hard X-ray spectrum above 2 keVconsists of a Compton-thin absorbed power law, as is often seen inSeyfert 2 galaxies. We clearly detected rapid time variability on atimescale of about 1000 s from the light curve above 2 keV. The excessvariance of the time variability (σ2rms) iscalculated to be ~0.12, and the periodogram of the light curve is wellrepresented by a power-law function with a slope of 1.75. In contrast toprevious results from Seyfert 2 nuclei, these variabilitycharacteristics are consistent with those of Seyfert 1 galaxies. Thisconsistency suggests that NGC 6300 has a black hole mass and accretionproperties similar to Seyfert 1 galaxies. Using the relation betweentime variability and central black hole mass by Hayashida et al., theblack hole mass of NGC 6300 is estimated to be ~2.8×105Msolar. Taking the uncertainty of this method into account,the black hole mass is less than 107 Msolar.Taking the bolometric luminosity of 3.3×1043 ergss-1 into consideration, this yields an accretion rate of>0.03 of the Eddington value, comparable to estimates from Seyfert 1galaxies using this method. The time variability analysis suggests thatNGC 6300 actually has a Seyfert 1 nucleus obscured by thick matter andmore generally provides a new pillar of support for the unified model ofSeyfert galaxies based on obscuration.
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