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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).
| 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.
| An atlas of calcium triplet spectra of active galaxies
We present a spectroscopic atlas of active galactic nuclei covering theregion around the λλ8498, 8542, 8662 calcium triplet(CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26Seyfert 1s, three starburst and six normal galaxies. The spectra pertainto the inner ~300 pc in radius, and thus sample the central kinematicsand stellar populations of active galaxies. The data are used to measurestellar velocity dispersions (σ*) with bothcross-correlation and direct fitting methods. These measurements arefound to be in good agreement with each other and with those in previousstudies for objects in common. The CaT equivalent width is alsomeasured. We find average values and sample dispersions ofWCaT of 4.6 +/- 2.0, 7.0 +/- 1.0 and 7.7 +/- 1.0 Å forSeyfert 1s, Seyfert 2s and normal galaxies, respectively. We furtherpresent an atlas of [SIII]λ9069 emission-line profiles for asubset of 40 galaxies. These data are analysed in a companion paperwhich addresses the connection between stellar and narrow-line regionkinematics, the behaviour of the CaT equivalent width as a function ofσ*, activity type and stellar population properties.
| 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.
| 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.
| 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.
| Molecular gas properties of 12-μm Seyfert galaxies - I. The southern sample
We have used the 15-m Swedish European Southern Observatory (ESO)Submillimetre Telescope (SEST) to observe the J= 1 -> 0 and J= 2-> 1 transition lines of CO in 30 Southern hemisphere Seyfertgalaxies from the extended 12-μm sample of Rush, Malkan &Spinoglio. We detected CO J= 1 -> 0 in 16 out of the 30 Seyfertgalaxies and CO J= 2 -> 1 in 17 out of the 30 Seyfert galaxies. Fromthe observed spectra, various CO gas properties have been derivedincluding the luminosity of the CO gas and, using a standard conversionfactor, the H2 mass. The average H2 gas mass forSeyfert 1 galaxies was 3 × 109 Msolar for COJ= 1 -> 0 and 1 × 109 Msolar for CO J= 2-> 1, while in comparison the H2 gas mass for Seyfert 2type galaxies was 11 × 109 Msolar for CO J=1 -> 0 and 3 × 109 MsolarCO J= 2 -> 1.From this small sample of Seyfert galaxies, we tentatively support theconclusion that type 2 Seyfert galaxies contain more molecular gas thantheir type 1 counterparts.
| Long-term infrared photometry of Seyferts
Long-term (up to 10 000 d) monitoring has been undertaken for 41Seyferts in the near-infrared (1.25-3.45 μm). All but two showedvariability, with amplitudes at K in the range <0.1 to >1.1 mag.The time-scale for detectable change is from about one week to a fewyears.Where contemporary observations of variability in X-rays, ultraviolet(UV) or visible light exist, it is found that the near-infrared variesin a similar way, though in some cases the shorter-wavelength infrared(IR) bands are diluted by underlying galaxy radiation.A simple cross-correlation study indicates that there is evidence fordelays of up to several hundred d between the variations seen at theshortest wavelengths (U or J) and the longest (L) in many galaxies. Inparticular, the data for Fairall 9 now extend to twice the intervalcovered in earlier publications and the delay between its UV and IRoutputs is seen to persist.An analysis of the fluxes shows that, for any given galaxy, the coloursof the variable component of its nucleus are usually independent of thelevel of activity. The state of activity of the galaxy can beparameterized.Taken over the whole sample, the colours of the variable components fallwithin moderately narrow ranges. In particular, the H-K colour isappropriate to a blackbody of temperature 1600 K. The H-K excess for aheavily reddened nucleus can be determined and used to findEB-V, which can be compared to the values found from thevisible region broad line ratios.Using flux-flux diagrams, the flux within the aperture from theunderlying galaxies can often be determined without the need for modelsurface brightness profiles. In many galaxies it is apparent that theremust be an additional constant contribution from warm dust.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Chandra Multiwavelength Project. I. First X-Ray Source Catalog
The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14deg2) survey of serendipitous Chandra X-ray sources, aimingto establish fair statistical samples covering a wide range ofcharacteristics (such as absorbed active galactic nuclei, high-zclusters of galaxies) at flux levels(fX~10-15to10-14 ergs s-1cm-2) intermediate between the Chandra deep surveys andprevious missions. We present the first ChaMP catalog, which consists of991 near on-axis, bright X-ray sources obtained from the initial sampleof 62 observations. The data have been uniformly reduced and analyzedwith techniques specifically developed for the ChaMP and then validatedby visual examination. To assess source reliability and positionaluncertainty, we perform a series of simulations and also use Chandradata to complement the simulation study. The false source detection rateis found to be as good as or better than expected for a given limitingthreshold. On the other hand, the chance of missing a real source israther complex, depending on the source counts, off-axis distance (orPSF), and background rate. The positional error (95% confidence level)is usually less than 1" for a bright source, regardless of its off-axisdistance, while it can be as large as 4" for a weak source (~20 counts)at a large off-axis distance (Doff-axis>8'). Wehave also developed new methods to find spatially extended or temporaryvariable sources, and those sources are listed in the catalog.
| Statistical properties of local active galactic nuclei inferred from the RXTE 3-20 keV all-sky survey
We compiled a sample of 95 AGNs serendipitously detected in the 3-20 keVband at Galactic latitude |b|>10o during the RXTE slewsurvey (XSS, Revnivtsev et al. 2004), and utilize it to study thestatistical properties of the local population of AGNs, including theX-ray luminosity function and absorption distribution. We find thatamong low X-ray luminosity (L3-20< 1043.5 ergs-1) AGNs, the ratio of absorbed (characterized by intrinsicabsorption in the range 1022cm-21041 erg s-1 estimated here issmaller than the earlier estimated total X-ray volume emissivity in thelocal Universe, suggesting that a comparable X-ray flux may be producedtogether by lower luminosity AGNs, non-active galaxies and clusters ofgalaxies. Finally, we present a sample of 35 AGN candidates, composed ofunidentified XSS sources.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/469
| RXTE all-sky slew survey. Catalog of X-ray sources at |b|>10o
We report results of a serendipitous hard X-ray (3-20 keV), nearlyall-sky (|b|>10o) survey based on RXTE/PCA observationsperformed during satellite reorientations in 1996-2002. The survey is80% (90%) complete to a 4σ limiting flux of ≈ 1.8 (2.5) ×10-11 erg s-1 cm-2 in the 3-20 keVband. The achieved sensitivity in the 3-8 keV and 8-20 keV subbands issimilar to and an order of magnitude higher than that of the previouslyrecord HEAO-1 A1 and HEAO-1 A4 all-sky surveys, respectively. A combined7× 103 sq deg area of the sky is sampled to flux levelsbelow 10-11 erg s-1 cm-2 (3-20 keV). Intotal 294 sources are detected and localized to better than 1 deg. 236(80%) of these can be confidently associated with a known astrophysicalobject; another 22 likely result from the superposition of 2 or 3closely located known sources. 35 detected sources remain unidentified,although for 12 of these we report a likely soft X-ray counterpart fromthe ROSAT all-sky survey bright source catalog. Of the reliablyidentified sources, 63 have local origin (Milky Way, LMC or SMC), 64 areclusters of galaxies and 100 are active galactic nuclei (AGN). The factthat the unidentified X-ray sources have hard spectra suggests that themajority of them are AGN, including highly obscured ones(NH>1023 cm-2). For the first timewe present a log N-log S diagram for extragalactic sources above4× 10-12 erg s-1 cm-2 at 8-20keV.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/927
| The relation between AGN hard X-ray emission and mid-infrared continuum from ISO spectra: Scatter and unification aspects
We use mid-infrared spectral decomposition to separate the 6 μmmid-infrared AGN continuum from the host emission in the ISO lowresolution spectra of 71 active galaxies and compare the results toobserved and intrinsic 2-10 keV hard X-ray fluxes from the literature.We find a correlation between mid-infrared luminosity and absorptioncorrected hard X-ray luminosity, but the scatter is about an order ofmagnitude, significantly larger than previously found with smallerstatistics. Main contributors to this scatter are likely variations inthe geometry of absorbing dust, and AGN variability in combination withnon-simultaneous observations. There is no significant differencebetween type 1 and type 2 objects in the average ratio of mid-infraredand hard X-ray emission, a result which is not consistent with the mostsimple version of a unified scheme in which an optically andgeometrically thick torus dominates the mid-infrared AGN continuum. Mostprobably, significant non-torus contributions to the AGN mid-IRcontinuum are masking the expected difference between the two types ofAGN.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/465Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries: France, Germany, TheNetherlands, and the UK) with the participation of ISAS and NASA.
| XMM-Newton observations of an absorbed z= 0.67 QSO: no dusty torus?
We present XMM-Newton observations of AX J0341.4-4453, a mildly reddenedAV < 7 quasi-stellar object (QSO) at a redshift of z=0.672. The XMM-Newton spectrum shows a large obscuring columnNH~ 1023 cm-2 corresponding toAV~ 70, in agreement with previous results based on the lowerlimit of the ASCA hardness ratio. The X-ray spectrum is represented by a`scattering' model with Γ~ 2.0 and where the scattered power-lawnormalization is a few per cent of the hard component. No FeK line isdetected with a 90 per cent upper limit on its equivalent width of ~360eV. The large discrepancy between the column density observed in X-raysand that inferred from the Balmer decrement can be explained by dustsublimation near the nucleus. Then, the X-ray and the opticalobscuration come from two different regions: the X-ray close to theaccretion disc, the optical at much larger > 0.25 pc scales.
| BeppoSAX Average Spectra of Seyfert Galaxies
We have studied the average 3-200 keV spectra of Seyfert galaxies oftype 1 and 2, using data obtained with BeppoSAX. The average Seyfert 1spectrum is well fitted by a power-law continuum with photon spectralindex Γ~1.9, a Compton reflection component R~0.6-1 (depending onthe inclination angle between the line of sight and the reflectingmaterial), and a high-energy cutoff at around 200 keV; there is also aniron line at 6.4 keV characterized by an equivalent width of 120 eV.Seyfert 2 galaxies, on the other hand, show stronger neutral absorption[NH=(3-4)×1022 atoms cm-2], asexpected, but are also characterized by an X-ray power law that issubstantially harder (Γ~1.75) and with a cutoff at lower energies(Ec~130 keV); the iron line parameters are insteadsubstantially similar to those measured in type 1 objects. There areonly two possible solutions to this problem: to assume more reflectionin Seyfert 2 galaxies than observed in Seyfert 1 galaxies or morecomplex absorption than estimated in the first instance. The firstpossibility is ruled out by the Seyfert 2 to Seyfert 1 ratio, while thesecond provides an average Seyfert 2 intrinsic spectrum very similar tothat of the Seyfert 1. The extra absorber is likely an artifact due tosumming spectra with different amounts of absorption, although we cannotexclude its presence in at least some individual sources. Our resultargues strongly for a very similar central engine in both types ofgalaxies, as expected under the unified theory.
| The Seyfert Population in the Local Universe
The magnitude-limited catalog of the Southern Sky Redshift Survey(SSRS2) is used to characterize the properties of galaxies hostingactive galactic nuclei (AGNs). Using emission-line ratios, we identify atotal of 162 (3%) Seyfert galaxies out of the parent sample with 5399galaxies. The sample contains 121 Seyfert 2 galaxies and 41 Seyfert 1galaxies. The SSRS2 Seyfert galaxies are predominantly in spirals oftypes Sb and earlier or in galaxies with perturbed appearance as theresult of strong interactions or mergers. Seyfert galaxies in thissample are twice as common in barred hosts as the non-Seyfert galaxies.By assigning galaxies to groups using a percolation algorithm, we findthat the Seyfert galaxies in the SSRS2 are more likely to be found inbinary systems when compared with galaxies in the SSRS2 parent sample.However, there is no statistically significant difference between theSeyfert and SSRS2 parent sample when systems with more than two galaxiesare considered. The analysis of the present sample suggests that thereis a stronger correlation between the presence of the AGN phenomenonwith internal properties of galaxies (morphology, presence of bar,luminosity) than with environmental effects (local galaxy density, groupvelocity dispersion, nearest neighbor distance).Partly based on observations at European Southern Observatory (ESO),under the ESO-ON agreement to operate the 1.52 m telescope.
| The Host Galaxies of Narrow-Line Seyfert 1 Galaxies: Evidence for Bar-Driven Fueling
We present a study of the host galaxy morphologies of narrow- andbroad-line Seyfert 1 galaxies (NLS1's and BLS1's) based on broadbandoptical images from the Hubble Space Telescope archives. We find thatlarge-scale stellar bars, starting at ~1 kpc from the nucleus, are muchmore common in NLS1's than BLS1's. Furthermore, the fraction of NLS1spirals that have bars increases with decreasing full width athalf-maximum of the broad component of Hβ. These results suggest alink between the large-scale bars, which can support high fueling ratesto the inner kiloparsecs, and the high mass accretion rates associatedwith the supermassive black holes in NLS1's.
| High-energy sources before INTEGRAL. INTEGRAL reference catalog
We describe the INTEGRAL reference catalog which classifies previouslyknown bright X-ray and gamma-ray sources before the launch of INTEGRAL.These sources are, or have been at least once, brighter than ~ 1 mCrababove 3 keV, and are expected to be detected by INTEGRAL. This catalogis being used in the INTEGRAL Quick Look Analysis to discover newsources or significantly variable sources. We compiled several publishedX-ray and gamma-ray catalogs, and surveyed recent publications for newsources. Consequently, there are 1122 sources in our INTEGRAL referencecatalog. In addition to the source positions, we show an approximatespectral model and expected flux for each source, based on which wederive expected INTEGRAL counting rates. Assuming the default instrumentperformances and at least ~ 105 s exposure time for anypart of the sky, we expect that INTEGRAL will detect at least ~ 700sources below 10 keV and ~ 400 sources above 20 keV over the missionlife.The Catalog is available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/411/L59
| The history of the iron Kalpha line profile in the Piccinotti AGN ESO 198-G24
This paper presents ASCA (July 1997), XMM-Newton (December 2000) andBeppoSAX (January 2001) observations of the Piccinotti Seyfert 1 galaxyESO 198-G24. The BeppoSAX 0.1-200 keV spectrum exhibits reprocessingfeatures, probably produced by an X-ray illuminated, relativisticaccretion disk subtending a solid angle la 2 pi . During the XMM-Newtonobservation the fluorescent iron Kalpha line profile(centroid energy Ec =~ 6.4 keV) was broad and twice asbright as in the BeppoSAX observation. An additional emission feature(Ec =~ 5.7 keV), detected at the 96.3% confidence level, maybe part of a relativistic, double-peaked profile. By contrast, in theearlier ASCA observation the line profile is dominated by a remarkablynarrow ``core'' (intrinsic width, sigma < 50 eV). If this componentis produced by reflection off the inner surface of a molecular torus,its large Equivalent Width ( =~ 300 eV) most likely represents the``echo'' of a previously brighter flux state, in agreement with thedynamical range covered by the historical X-ray light curve in ESO198-G24.
| The intrinsic emission of Seyfert galaxies observed with BeppoSAX/PDS. I. Comparison of the average spectra of the three classes of Seyfert galaxies
We present a study of the hard X-ray spectrum (>15 keV) of differentclasses of Seyfert galaxies observed with BeppoSAX/PDS. Using hard X-raydata, we avoid absorption effects modifying the Seyfert emission andhave direct access to the central engine of these sources. The aim ofthis study is first to characterize the general properties of the hardX-ray spectrum of Seyfert 1, 1.5 and 2 galaxies and secondly to comparetheir intrinsic emission to test unified models according to which allthe classes have the same nucleus.\ We compute the average spectrum of14 Sy 1, 9 Sy 1.5 and 22 Sy 2 galaxies observed by the PDS (15-136 keV).The average spectrum of Sy 1 differs from that of Sy 2, the firstrequiring the presence of a high energy cutoff which is absent in thesecond. We also show that the reflection component is possibly moreimportant in the Sy 2 emission. The nature of Sy 1.5 galaxies isambiguous as they have a negligible reflection component (like Sy 1) anddo not require a cutoff (like Sy 2).
| X-Ray versus Optical Observations of Active Galactic Nuclei: Evidence for Large Grains?
Recently, Maiolino et al. constructed a sample of active galactic nucleifor which both the reddening E(B-V) and the column density NHto the nucleus could be determined. For most of the galaxies in theirsample, they found that E(B-V)/NH is substantially smallerthan for the diffuse interstellar medium of our Galaxy. They assertedthat either the dust-to-gas ratio is lower than in the Galaxy or thegrains are so large that they do not extinct or redden efficiently inthe optical. We show that there is no systematic increase in E(B-V) withNH for the Maiolino et al. galaxies, which suggests that theX-ray absorption and optical extinction occur in distinct media. In alater paper, Maiolino et al. suggested that the observed lines of sightfor the previous Maiolino et al. galaxies pass through the ``torus''that obscures the broad-line region and nuclear continuum in Seyfert 2galaxies and argued that the torus grains are larger than Galacticgrains. There is no reason to believe that the lines of sight for thesegalaxies pass through the torus, since the observed column densities arelower than those typically observed in Seyfert 2 galaxies. We suggestinstead that the X-ray absorption occurs in material located off thetorus and/or accretion disk, while the optical extinction occurs inmaterial located beyond the torus. The X-ray absorbing material couldeither be dust-free or contain large grains that do not extinctefficiently in the optical. There is no conclusive evidence that thegrains in active galactic nuclei are systematically larger than those inthe diffuse interstellar medium of our Galaxy. We discuss an alternativeway to probe the properties of dust in Seyfert tori but find thatobservations of Seyfert 2 nuclei with higher resolution than currentlyavailable will be needed in order to place stringent limits on the dust.
| Ubiquitous Variability of X-Ray-absorbing Column Densities in Seyfert 2 Galaxies
We present a study of the variations in the absorbing column density of25 X-ray-defined Seyfert 2 galaxies, as inferred from hard X-rayobservations, on timescales from months to several years. We show that asignificant variation of NH (from 20% to 80%) is observed inalmost all (22 of 25) of the sources with multiple X-ray observations,although X-ray absorption never vanishes. For a subsample of 11 sourcesobserved at least five times, the typical variation time, as defined bya structure function, is less than 1 yr for both heavily absorbed(NH~1023 cm-2) and moderately absorbed(NH~1022 cm-2) sources. Thesevariations rule out the simplest version of the unified models, based ona homogeneous obscuring torus, and suggest the presence of clumpycircumnuclear material on a scale well below a parsec. We propose amodification of the torus model in which an overabundance of slightlydusty broad emission-line region (BELR) clouds obscures the BELR. TheBELR needs, like the torus, to have an axisymmetric structure. Thismodel is closely related to that of Elvis for type 1 active galacticnuclei (AGNs). For lightly obscured AGNs (NH~1022cm-2), the structure function shows an increase at atimescale of ~5 yr, indicating a second absorber, most probably on a5-10 pc scale associated with the host galaxy.
| A Search for H2O Maser Emission in Southern Active Galactic Nuclei and Star-forming Galaxies: Discovery of a Maser in the Edge-on Galaxy IRAS F01063-8034
We report the cumulative results of five surveys for H2Omaser emission at 1.35 cm wavelength in 131 active galactic nuclei(AGNs) and star-forming galaxies, conducted at the Parkes Observatorybetween 1993 and 1998. We detected one new maser, in the edge-on galaxyIRAS F01063-8034, which exhibits a single ~0.1 Jy spectral feature at4282+/-6 km s-1 (heliocentric) with an unusually large54+/-16 km s-1 half-power full width. The centroid velocityof the emission increased to 4319.6+/-0.6 km s-1 (38+/-2 kms-1 width) over the 13 days between discovery andconfirmation of the detection. A similarly broad-line width and largechange in velocity has been noted for the maser in NGC 1052, wherein jetactivity excites the emission. Neither optical spectroscopy,radio-infrared correlations, nor infrared colors provide compellingevidence of unusual activity in the nucleus of IRAS F01063-8034. Sincethe galaxy appears to be outwardly normal at optical and infraredwavelengths, detection of an H2O maser therein is unique. Themaser emission is evidence that the galaxy harbors an AGN that isprobably obscured by the edge-on galactic disk. The detection highlightsthe possibility that undetected AGNs could be hidden in other relativelynearby galaxies. No other maser emission features have been identifiedat velocities between 3084 and 6181 km s-1.
| BeppoSAX/PDS identification of the true counterpart of the Piccinotti source H0917-074
High energy emission has been discovered serendipitously by theBeppoSAX/PDS telescope in the ~ 1.3deg field of view aroundthe Piccinotti source H0917-074. A re-pointing of BeppoSAX/NFI hasallowed the association of this emission with the Seyfert 2 galaxyMCG-1-24-12 which lies within the original HEAO1/A2 error box ofH0917-074. This is the first PDS serendipitous discovery of a Seyfert 2galaxy and the first detection of MCG-1-24-12 in the X-ray domain. Themeasured 2-10 keV flux of MCG-1-24-12 is ~ 1 x 10-11 ergcm-2 s-1 compatible with the Piccinotti HEAO-1/A2observation. This is a factor of ~ 6 greater than that observed fromEXO0917.3-0722, originally suggested as the counterpart of thePiccinotti source. The 2-10 keV spectrum of MCG-1-24-12 shows thepresence of Fe Kalpha emission together with an absorptionfeature at ~ 8.7 keV. At high energies, the Seyfert 2 still dominatesand the observed 20-100 keV flux is ~ 4 x 10-11 ergcm-2 s-1.
| The BeppoSAX view of bright Compton-thin Seyfert 2 galaxies
We present the analysis of 31 observations (17 of which are publishedhere for the first time) of 20 bright Compton thin Seyfert 2 s, in the0.1-200 keV band, performed with the BeppoSAX satellite. The sampleconsists of all Seyfert 2 s in the BeppoSAX public archive, with a 2-10keV flux higher than 5*E-12 erg cm-2s-1. The good statistics available and the broad energy bandpermit a detailed study of the main continuum components of thesesources, i.e. the primary power-law, the reflected component, the softemission and the high-energy cut-off. The main results of our analysisare: (1) the 3-200 keV intrinsic power-law has a mean photon index Gamma= 1.79 +/- 0.01, with a dispersion of sigma =0.23. (2) The high-energyexponential cut-off at E ~ 100-300 keV is not an ubiquitous property ofSeyfert galaxies: in ~ 30% of the objects the continuum power-law doesnot drop up to energies of 300 keV or more. (3) A reflected component ispresent in almost all the sources (17 out of 21). The small variationsof this component with respect to the intrinsic continuum, in objectswith multiple observations, suggests that the reflector is not theaccretion disk, but must be located much farther from the nucleus. (4)The range of ratios between the reflected and intrinsic componentssuggests that the circumnuclear medium is not homogeneous, and asignificant fraction of the solid angle is covered by a gas thicker thanthat along the line of sight. (5) The iron Kalpha line is present is allbut one the sources. The equivalent width is in the typical range ofSeyfert 1 s (EW=100-300 eV) in sources with low absorption(NH < 3x 1023 cm-2), and increasesin more absorbed objects, as expected according to unified models. (6)The energy resolution of BeppoSAX is in general too low to measure theiron line width. However, in 6 cases we measured a significant linebroadening.
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