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Evolutionary unification in composite active galactic nuclei
In this paper, we present an evolutionary unification scenario,involving supermassive black holes (SMBHs) and starbursts (SBs) withoutflow (OF), that seems capable of explaining most of the observationalproperties (of at least part) of active galactic nuclei (AGN).The scenario includes a nuclear/circumnuclear SB closely associated withthe AGN where the narrow-line region (NLR), broad-line region (BLR) andbroad absorption line (BAL) region are produced in part by the OFprocess with shells and in compact supernova remnants (cSNRs).The OF process in BAL quasi-stellar objects (QSOs) with extreme infrared(IR) and FeII emission is studied. In addition, the FeII problemregarding the BLR of AGN is analysed. The correlations between the BAL,IR emission, FeII intensity and the intrinsic properties of the AGN arenot clearly understood. We suggest here that the behaviour of the BAL,IR and FeII emission in AGN can be understood within an evolutionary andcomposite model for AGN.In our model, strong BAL systems and FeII emission are present (andintense) in young IR objects. Parameters like the BALs, IR emission,FeII/Hβ intensity ratio, FeII equivalent width (EW), broad-linewidth, [OIII]λ5007-Å intensity and width, NLR size, X-rayspectral slope in radio quiet (RQ) AGN plus lobe separation, and lobe tocore intensity ratio in radio loud (RL) AGN are proposed to befundamentally time-dependent variables inside time-scales of the orderof 108 yr. Orientation/obscuration effects take the role of asecond parameter providing the segregation between Seyfert 1/Seyfert 2galaxies (Sy1/Sy2) and broad-/narrow-line radio galaxies (BLRG/NLRG).
| Investigating the nature of narrow-line Seyfert 1 galaxies with high-energy spectral complexity
With the commissioning of XMM-Newton came the discovery of 2.5-10keVspectral complexity in some narrow-line Seyfert 1 galaxies (NLS1). Thishigh-energy complexity can be manifested as sharp, spectral drops orgradual curvature in the spectrum. Models which are normally consideredare ionized reflection and partial covering. In this work, we define twosamples of NLS1: a complex sample whose members exhibit high-energycomplexity (C sample), and a general sample of NLS1 whose 2.5-10keVspectra do not strongly deviate from a simple power law (S sample). Wethan compare multiwavelength parameters of these two samples todetermine if there are any distinguishing characteristics in the complexNLS1. Considering historical light curves of each object, we find thatthe C sample is representative of NLS1 in a low X-ray flux state,whereas the members of the S sample appear to be in a typical fluxstate. Moreover, from measurements of αox withcontemporaneous ultraviolet (UV)/X-ray data, we find that the C sampleof NLS1 appear X-ray weaker at the time of the observation. For two NLS1in the C sample multi-epoch measurements of αox areavailable and suggest that αox approaches more normalvalues as the complexity between 2.5 and 10keV diminishes. This impliesthat a source could transit from one sample to the other as its X-rayflux varies. Secondly, there are indications that the C sample sources,on average, exhibit stronger optical FeII emission, with the three mostextreme (FeII/Hβ > 1.8) FeII emitters all displaying complexityin the 2.5-10keV band. It is an intriguing possibility that we may beable to identify X-ray complex NLS1 based on the extreme strength of themore easily observable optical FeII emission. However, it is not clearif the possible connection between FeII strength and spectral complexityis due to the FeII producing mechanism or because strong FeII emittersmay exhibit the greatest variability and consequently more likely to becaught in an extreme (low) flux state. Based on the current analysis, wecannot straightforwardly dismiss absorption or reflection as the causeof the X-ray complexity; by considering the multiple UV/X-rayobservations of 1H 0707-495 (a C sample member), we discuss a possiblemethod of distinguishing the two models provided further UV/X-rayobservations.
| 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.
| Multiwavelength XMM-Newton observations of the Laor et al. sample of PG quasars
We present XMM-Newton/EPIC spectra for the Laor et al. sample of PalomarGreen (PG) quasars. We find that a power law provides a reasonable fitto the 2-5 keV region of the spectra. Excess soft X-ray emission below 2keV is present for all objects, with the exception of those known tocontain a warm absorber. However, a single power law is a poor fit tothe 0.3-10.0 keV spectrum and instead we find that a simple model,consisting of a broken power law (plus an iron line), provides areasonable fit in most cases. The equivalent width of the emission lineis constrained in just 12 objects but with low (<2σ)significance in most cases. For the sources whose spectra are wellfitted by the broken-power-law model, we find that various optical andX-ray line and continuum parameters are well correlated; in particular,the power-law photon index is well correlated with the FWHM of theHβ line and the photon indices of the low- and high-energycomponents of the broken power law are well correlated with each other.These results suggest that the 0.3-10 keV X-ray emission shares a common(presumably non-thermal) origin, as opposed to suggestions that the softexcess is directly produced by thermal disc emission or via anadditional spectral component. We present XMM-Newton Optical Monitor(OM) data, which we combine with the X-ray spectra so as to producebroad-band spectral energy distributions (SEDs), free from uncertaintiesdue to long-term variability in non-simultaneous data. Fitting theseoptical-UV spectra with a Comptonized disc model indicates that the softX-ray excess is independent of the accretion disc, confirming ourinterpretation of the tight correlation between the hard and soft X-rayspectra.
| An explanation for the soft X-ray excess in active galactic nuclei
We present a large sample of type 1 active galactic nuclei (AGN) spectrataken with XMM-Newton, and fit them with both the conventional model (apower law and blackbody) and the relativistically blurred photoionizeddisc reflection model of Ross & Fabian. We find that the discreflection model is a better fit. The disc reflection model successfullyreproduces the continuum shape, including the soft excess, of all thesources. The model also reproduces many features that wouldconventionally be interpreted as absorption edges. We are able to usethe model to infer the properties of the sources, specifically that themajority of black holes in the sample are strongly rotating, and thatthere is a deficit in sources with an inclination >70°. Weconclude that the disc reflection model is an important tool in thestudy of AGN X-ray spectra.
| 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.
| Discovery of Very High Energy Gamma Rays from 1ES 1218+30.4
The MAGIC collaboration has studied the high-frequency-peaked BL Lacobject 1ES 1218+30.4, at a redshift z=0.182, using the MAGIC imaging airCerenkov telescope located on the Canary Island of La Palma. A gamma-raysignal was observed with 6.4 σ significance. The differentialenergy spectrum for an energy threshold of 120 GeV can be fitted by asimple power law, yieldingFE(E)=(8.1+/-2.1)×10-7[E/(250GeV)]-3.0+/-0.4 TeV-1 m-2s-1. During the 6 days of observation in 2005 January, notime variability on timescales of days was found within the statisticalerrors. The observed integral flux above 350 GeV is nearly a factor of 2below the upper limit reported by the Whipple collaboration in 2003.
| 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).
| A Sample of IRAS Infrared-selected Seyfert 1.5 Galaxies: Infrared Color α(60, 25)-dominated Eigenvector 1
The well-documented E1 relationships are first extended to infraredcolor α(60, 25) and flux ratio [O III]/Hβn bycomparing emission-line properties to continuum properties in infraredwavelengths. Both direct correlations and a principal component analysisare used in a sample of 50 IRAS IR-selected Seyfert 1.5 galaxies. Inaddition, to confirm the correlations of E1 in Boroson & Green, oureigenvector 1 turns out to be dominated by the mid-infrared colorα(60, 25) and most strongly affected by RFe, [OIII]/Hβn, and EW(Hβb). Our analysisindicates that the objects with large E1 tend to coexist with relativelyyoung nuclear stellar populations, which implies that E1 is related tothe nuclear star formation history. The IR-dominated eigenvector 1 cantherefore be inferred to be interpreted as the ``age'' of an AGN. Inconfirmation of the work of Xu and coworkers, it is clear that theextreme Seyfert galaxies with both large RFe and large [OIII]/Hβn are rare in our universe.
| 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.
| Polarimetry of near-infrared flares from Sagittarius A*
Context: .We report new polarization measurements of the variablenear-infrared emission of the SgrA* counterpart associated with themassive 3-4×106 Mȯ Black Hole at theGalactic Center. Aims: .We investigate the physical processesresponsible for the variable emission from SgrA*. Methods: .Theobservations have been carried out using the NACO adaptive optics (AO)instrument at the European Southern Observatory's Very Large Telescope. Results: .We find that the variable NIR emission of SgrA* ishighly polarized and consists of a contribution of a non- or weaklypolarized main flare with highly polarized sub-flares. The flareactivity shows a possible quasi-periodicity of 20±3 minconsistent with previous observations. Conclusions: .The highlyvariable and polarized emission supports that the NIR emission isnon-thermal. The observations can be interpreted in a jet or temporarydisk model. In the disk model the quasi-periodic flux density variationscan be explained by spots on relativistic orbits around the central MBH.Alternative explanations for the high central mass concentrationinvolving boson or fermion balls are increasingly unlikely.
| 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.
| Variable iron-line emission near the black hole of Markarian 766
Aims.We investigate the link between ionised Fe X-ray line emission andcontinuum emission in a bright nearby AGN, Mrk 766. Methods: .Anew long (433 ks) XMM-Newton observation is analysed, together witharchival data from 2000 and 2001. The contribution from ionised lineemission is measured and its time variations on short (5-20 ks)timescales are correlated with the continuum emission. Results:.The ionised line flux is found to be highly variable and to be stronglycorrelated with the continuum flux, demonstrating an origin for theionised line emission that is co-located with the continuum emission.Most likely the emission is ionised reflection from the accretion discwithin a few A.U. of the central black hole, and its detection marks thefirst time that such an origin has been identified other than by fittingto spectral line profiles. Future observations may be able to measure atime lag and hence achieve reverberation mapping of AGN at X-rayenergies.
| The role of absorption and reflection in the soft X-ray excess of Active Galactic Nuclei. I. Preliminary results
The 2-10 keV continuum of AGN is well represented by a single power law,generally attributed to a hot Comptonizing medium, such as a coronaabove the accretion disk. At lower energies the continuum displays anexcess with respect to the extrapolation of this power law, called the"soft X-ray excess". Until now it was attributed either to reflection ofthe hard X-ray source by the accretion disk or to the presence of anadditional Comptonizing medium. An alternative solution is that a singlepower law correctly represents both the soft and the hard X-rayemission, and the soft X-ray excess is an artefact due to the absorptionof the primary power law by a relativistic wind. We examine theadvantages and drawbacks of the reflection versus absorption models. Weargue that in the absorption hypothesis, the absorbing medium should bein total pressure equilibrium to constrain the spectral distributionwhich otherwise would be too strongly variable in time and from oneobject to the other, as compared to observations. We conclude that someX-ray spectra, in particular those with strong soft X-ray excesses, canbe modelled by absorption in the 0.3-10 keV range. However, due to thelack of a complete grid of models and good data extending above 10 keV,we are not able to conclude that all objects can be accommodated by suchmodels. These absorption models imply either strong relativisticoutflowing winds with mass rates of the order of the Eddington value (oreven larger), or quasi-spherical inhomogeneous accretion flows. Onlyweak excesses can be modelled by reflection, unless the primarycontinuum is not directly seen. A reflection model absorbed by a modestrelativistic wind could be the best solution to the problem.
| Evidence for orbital motion of material close to the central black hole of Mrk 766
Time-resolved X-ray spectroscopy has been obtained for the narrow lineSeyfert galaxy Mrk 766 from XMM-Newton observations. We present analysisin the energy-time plane of EPIC pn data in the 4-8 keV band with energyresolution R ≃ 50. A component of Fe Kα emission detected inthe maps shows a variation of photon energy with time that appears bothto be statistically significant and to be consistent with sinusoidalvariation. We investigate the interpretation that there exists acomponent of line emission from matter in a Keplerian orbit around asupermassive black hole. The orbit has a period ~165 ks and aline-of-sight velocity ~13 500 km s-1. This yields a lowerlimit for the central mass of M > 4.9 ×105~Mȯ within a radius of 3.6 ×1013 cm (2.4 AU). The orbit parameters are consistent withhigher black hole masses, but the lack of any substantial gravitationalredshift of the orbit implies an upper limit to the black hole mass of4.5 × 107 Mȯ.
| Molecular hydrogen and [FeII] in active galactic nuclei - II. Results for Seyfert 2 galaxies
Near-infrared spectroscopy is used to study the kinematics andexcitation mechanisms of H2 and [FeII] lines in a sampledominated by Seyfert 2 galaxies. The spectra simultaneously cover theJHK bands, allowing us to compare line fluxes emitted in the interval0.8-2.4 μm and avoiding aperture and seeing effects. TheH2 lines are systematically narrower than the narrow-lineregion lines, suggesting that, very likely, the H2 does notoriginate from the same parcel of gas that forms the narrow-line region.Emission-line ratios between H2 lines favour thermalexcitation mechanisms for the molecular gas in active galactic nuclei.It was found that non-thermal excitation contributes, at most, 30 percent of the observed H2. Thermal excitation is also confirmedby the rather similar vibrational and rotational temperatures in theobjects (~2000 K). The mass of hot H2 ranges from102 to 103Msolar, with nearly half ofobjects showing values of <500 Msolar. It shows that thefraction of molecular mass present in the nuclear region and emitting inthe near-infrared is a very small fraction of the warm molecular masspresent in the centre. A diagnostic diagram composed of the line ratiosH2/Brγ and [FeII]/Paβ proves to be a useful toolin the near-infrared for separating emission-line objects by theirdegree of nuclear activity. We found that active galactic nuclei arecharacterized by H2 2.121 μm/Brγ and [FeII] 1.257μm/Paβ flux ratios between 0.6 and 2. Starburst/HII galaxiesdisplay line ratios <0.6 while low-ionization nuclear emission-lineregions are characterized by values larger than 2 in either ratio.
| Scaling variability from stellar to supermassive black holes
We investigate the correspondence between the variability seen in thestellar and supermassive black holes. Galactic black hole (GBH) powerdensity spectra (PDS) are generally complex, and dependent on spectralstate. In the low/hard state the high-frequency rollover in the PDSmoves in a way which is not simply related to luminosity. Hence thisfeature can only be used as an approximate indicator rather than as anaccurate tracer of black hole mass in active galactic nuclei (AGNs). TheX-ray spectrum in the high/soft state is dominated by the disc in theGBH, which is rather stable. We show that the PDS of the Comptonizedtail in GBHs can be much more variable, and that it is this which shouldbe compared to AGNs due to their much lower disc temperature. Thisbandpass effect removes a problem in interpreting the (often highlyvariable) narrow-line Seyfert 1 (NLS1) galaxies as the counterparts ofthe high mass accretion rate GBHs. Additionally, we speculate that someNLS1s (e.g. Akn 564) are counterparts of the very high state. TheComptonized tail in this state is also highly variable, but with PDSwhich can be roughly described as band-limited noise. This shape issimilar to that seen in the low/hard state, so merely seeing suchband-limited noise in the power spectrum of an AGN does not necessarilyimply low luminosity. We also stress that Cygnus X-1, often used forcomparison with AGNs, is not a typical black hole system due to itspersistent nature. In particular, the shape of its power spectrum in thehigh/soft state is markedly different from that of other (transient) GBHsystems in this state. The fact that the NLS1s NGC 4051 and MCG -6-30-15do appear to show a power spectrum similar to that of the high/softstate of Cyg X-1 may lend observational support to theoreticalspeculation that the hydrogen ionization disc instability does notoperate in AGNs.
| XMM-Newton spectroscopy of high-redshift quasars
XMM-Newton observations of 29 high-redshift (z > 2) quasars,including seven radio-quiet, 16 radio-loud and six broad absorption line(BAL) objects, are presented; due to the high redshifts, the rest-frameenergy bands extend up to ~30-70 keV. Over 2-10 keV, the quasars can bewell fitted in each case by a simple power law, with no strong evidencefor iron emission lines. The lack of iron lines is in agreement bothwith dilution by the radio jet emission (for the radio-loud quasars) andthe X-ray Baldwin effect. No Compton reflection humps at higher energies(i.e. above 10 keV in the rest frame) are detected either. Over thebroad-band (0.3-10 keV), approximately half (nine out of 16) of theradio-loud quasars are intrinsically absorbed, with the values ofNH generally being 1 × 1022 to 2 ×1022cm-2 in the rest frames of the objects. Noneof the seven radio-quiet objects shows excess absorption, while four ofthe six BAL quasars are absorbed. The radio-loud quasars have flattercontinuum slopes than their radio-quiet counterparts(ΓRL~ 1.55; ΓRQ~ 1.98 over 2-10 keV),while, after modelling the absorption, the underlying photon index forthe six BAL quasars is formally consistent with the non-BAL radio-quietobjects.
| Interstellar polarization and the position-angle orientations of type 1 Seyfert galaxies
We comment on recent spectropolarimetric studies that compare theobserved polarization position angles (PAs) of type 1 Seyfert galaxiesnear Hα with the observed orientations of their radio-source axeson the sky. For a Seyfert galaxy in which scattering occurs mainly in anequatorial scattering region, the polarization PA is expected to beparallel to the radio axis, while for a case in which light scatterspredominantly in the polar regions, the Hα polarization PA shouldbe perpendicular to the radio axis. In practice, these correlations aredifficult to establish because a Galactic interstellar polarizationcontribution can introduce a significant uncertainty into thepolarization PA determination, even when the magnitude of interstellarpolarization is small. We show how such uncertainties may affect theanalysis of PA alignments, and present spectropolarimetric observationsof a probe star along the line of sight to the type 1 Seyfert galaxy Mrk871 that allow us to assess the intrinsic Hα polarization and PAof Mrk 871. These results suggest that spectropolarimetric observationsof such probe stars should form an integral part of future polarizationstudies of Seyfert galaxies.
| 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.
| Where are the X-ray quasi-periodic oscillations in active galaxies?
In this paper, we address the question of whether existing X-rayobservations of Seyfert galaxies are sufficiently sensitive to detectquasi-periodic oscillations (QPOs) similar to those observed in theX-ray variations of Galactic black holes (GBHs). We use data fromXMM-Newton and simulated data based on the best Rossi X-ray TimingExplorer (RXTE) long-term monitoring light curves to show that if X-rayQPOs are present in Seyfert X-ray light curves - with similar shapes andstrengths to those observed in GBHs, but at lower frequenciescommensurate with their larger black hole masses - they would beexceedingly difficult to detect. Our results offer a simple explanationfor the present lack of QPO detections in Seyferts. We discuss theimprovements in telescope size and monitoring patterns needed to makeQPO detections feasible. The most efficient type of future observatoryfor searching for X-ray QPOs in active Galactic nuclei (AGN) is an X-rayAll-Sky Monitor (ASM). A sufficiently sensitive ASM would be ideallysuited to detect low-frequency QPOs in nearby AGN. The detection of AGNQPOs would strengthen the AGN-GBH connection, and could serve aspowerful diagnostics of the black hole mass and the structure of theX-ray emitting region in AGN.
| Constraints on hot metals in the vicinity of the Galaxy
We have searched for evidence of soft X-ray absorption by hot metals inthe vicinity of the Galaxy in the spectra of a small sample of 15 type Iactive galactic nuclei (AGN) observed with the high-resolution X-raygratings on board Chandra. This is an extension of our previous surveyof hot OVII and OVIII absorbing gas in the vicinity of the Galaxy. Thestrongest absorption signatures within a few hundred km s-1of their rest-frame energies are most likely to be due to warm absorbingoutflows from the nearest AGN, which are back-lighting the local hotgas. We emphasize that absorption signatures in the spectra of somedistant AGN that are kinematically consistent with the recessionalvelocity of the AGN are most likely to be due to hot local gas. Alongthe sightline towards PG 1211+143, PDS 456 and MCG-6-30-15 there is avery large absorbing Fe column density which is kinematically consistentwith absorption by hot, local Fe. The sightlines to these three AGN passthrough the limb of the Northern Polar Spur (NPS), a local bubble formedfrom several supernovae which, if rich in Fe, may account for a largelocal Fe column.We obtain limits on the column density of local, highly ionized N, Ne,Mg, Si along all of the sightlines in our sample. We correlate thecolumn density limits with those of highly ionized O along the samesightlines. Assuming the hot local gas is in collisionally ionizedequilibrium, we obtain limits on the temperature and relative abundancesof the metals in the hot local gas. Our limits on the ionic columndensities in the local hot gas seem to be consistent with those observedin the hot halo gas of edge-on normal spiral galaxies.
| 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.
| Non-linear X-ray variability in X-ray binaries and active galaxies
We show that the rms-flux relation recently discovered in the X-raylight curves of active galactic nuclei (AGN) and X-ray binaries (XRBs)implies that the light curves have a formally non-linear, exponentialform, provided the rms-flux relation applies to variations on alltime-scales (as it appears to). This phenomenological model implies thatstationary data will have a lognormal flux distribution. We confirm thisresult using an observation of Cyg X-1, and further demonstrate that ourmodel predicts the existence of the powerful millisecond flares observedin Cyg X-1 in the low/hard state, and explains the general shape andamplitude of the bicoherence spectrum in that source. Our model predictsthat the most variable light curves will show the most extremenon-linearity. This result can naturally explain the apparent non-linearvariability observed in some highly variable narrow line Seyfert 1(NLS1) galaxies, as well as the low states observed on long time-scalesin the NLS1 NGC 4051, as being nothing more than extreme manifestationsof the same variability process that is observed in XRBs and lessvariable AGN. That variability process must be multiplicative (withvariations coupled together on all time-scales) and cannot be additive(such as shot-noise), or related to self-organized criticality, orresult from completely independent variations in many separate emittingregions. Successful models for variability must reproduce the observedrms-flux relation and non-linear behaviour, which are more fundamentalcharacteristics of the variability process than the power spectrum orspectral-timing properties. Models where X-ray variability is driven byaccretion rate variations produced at different radii remain the mostpromising.
| 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.
| The continuum and narrow-line region of the narrow-line Seyfert 1 galaxy Mrk 766
We present the first spectroscopic observations in the interval 0.8-4.0μm, complemented with existing Hubble Space Telescope ultraviolet(UV) and optical spectroscopy, of the narrow-line Seyfert 1 galaxy Mrk766. The near-infrared spectrum is characterized by numerous permittedlines of HI, HeI, HeII and FeII, and forbidden lines of [SII], [SIII]and [FeII] among others. Highly ionized species such as [SiIX], [SiX],[SIX] and [MgVII] are also observed. The continuum emission has acomplex shape, with contributions from the central engine, circumnuclearstellar population and dust. This last component is demonstrated by thepresence of an excess of emission peaking at 2.25 μm, well fitted bya blackbody function with Tbb= 1200 K. That temperature isclose to the evaporation temperature of graphite grains. As such, itprovides strong evidence of hot dust, probably very close to thenucleus. Consistent modelling of the line spectrum and the broad-bandcontinuum by composite models, which account for the photoionizing fluxof the central engine and shocks, shows that shock velocities rangebetween 100 and 500 km s-1, the pre-shock densities between100 and 1000 cm-3 and the radiation fluxes from the activecentre between 109 and 5 × 1012 photoncm-2 s-1 eV-1 at 1 Ryd with spectralindices αUV=-1.5 and αX=-0.4. Adoptingsilicate grains, dust-to-gas ratios are between 10-6 and 4× 10-4 by mass. The emitting clouds are at an averagedistance of 160 pc from the centre, with high-velocity clouds closer andlow-velocity clouds further from the centre. The N/H relative abundancededuced from the fit of the [NII] 6548+/[OIII] 5007+ line ratio could betwice the solar value. On the other hand, Fe is depleted from thegaseous phase by a factor >2, most probably trapped into grains.Ratios of calculated to observed line ratios to Hβ indicate anaverage contribution of the broad-line region to the observed Hβ ofapproximately 40 per cent.
| Stellar velocity dispersion in narrow-line Seyfert 1 galaxies
Several authors have recently explored, for narrow-line Seyfert 1galaxies (NLS1s), the relationship between black hole mass(MBH) and stellar velocity dispersion (σ*).Their results are more or less in agreement and seem to indicate thatNLS1s fill the region below the fit obtained by Tremaine et al., showinga range of σ* similar to that of Seyfert 1 galaxies,and a lower MBH. Until now, the [OIII] width has been used inplace of the stellar velocity dispersion, but some indications havebegun to arise against the effectiveness of the gaseous kinematics inrepresenting the bulge potential, at least in NLS1s. Bian & Zhaohave stressed the urgency of producing true σ*measurements. Here, we present new stellar velocity dispersions obtainedthrough direct measurements of the CaII absorption triplet (~8550Å) in the nuclei of eight NLS1 galaxies. The resultingσ* values and a comparison withσ[OIII] confirm our suspicion that [OIII] typicallyoverestimates the stellar velocity dispersion. We demonstrate that NLS1sfollow the MBH-σ* relation as Seyfert 1,quasars and non-active galaxies.
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