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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.
| 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.
| 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.
| Toward a clean sample of ultra-luminous X-ray sources
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
| 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.
| Galaxy Cluster Archaeology
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| 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.
| 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.
| 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.
| 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.
| The XMM-Newton View of the Relativistic Spectral Features in AX J0447-0627
The XMM-Newton observation of the optically type 1 active galacticnucleus (AGN) AX J0447-0627 (z=0.214) unambiguously reveals a complex,bright, and prominent set of lines in the 4-8 keV rest-frame energyrange. Although from a phenomenological point of view, the observedproperties can be described by a simple power-law model plus five narrowGaussian lines (at rest-frame energies of ~4.49, ~5.55, ~6.39, ~7.02,and ~7.85 keV), we find that a model comprising a power law(Γ~2.2), a reflected relativistic continuum, a narrow Fe IKα line from neutral material, as well as a broad Fe Kαrelativistic line from a ionized accretion disk, represents a goodphysical description of the data. The ``double-horned'' profile of therelativistic line implies an inclination of the accretion disk of~45° and an origin in a narrow region of the disk, fromRin~19GM/c2 to Rout~30GM/c2.The narrow Fe I Kα line from neutral material is probably producedfar from the central black hole, most likely in the putative moleculartorus. Although some of these properties have already been found inother type 1 AGNs and discussed in the literature, at odds with theobjects reported so far we measure high equivalent widths of theobserved lines: ~1.4 keV for the double-horned relativistic line and~0.4 keV for the narrow line.
| Chandra High-Resolution X-Ray Spectroscopy of the Fe K Line in the Seyfert 1 Galaxy NGC 3783
We report on the results of detailed X-ray spectroscopy of the Fe Kregion in the Seyfert 1 galaxy NGC 3783 using the Chandra High EnergyGrating Transmission Spectrometer (HETGS). There were five observationsover an interval of ~125 days in 2001, each with an exposure time of~170 ks. The combined data constitute the highest signal-to-noise Fe Kspectrum having the best velocity resolution in the Fe K band to date(FWHM~1860 km s-1). The combined data show a resolved FeKα line core (FWHM=1700+410-390 kms-1) with a center energy of 6.397+/-0.003 keV, consistentwith an origin in neutral or lowly ionized Fe, located between the BLRand NLR, as found by Kaspi et al. in 2002. We also find that excess fluxaround the base of the Fe Kα line core can be modeled with eithera Compton-scattering ``shoulder'' or an emission line (with about thesame flux as the line core) from a relativistic accretion disk, havingan inclination angle of 11° or less. This disk-line model is as goodas a Compton-shoulder model for the base of the Fe Kα line core.In the latter model, we measured the column density to be7.5+2.7-0.6×1023 cm-2,which corresponds to a Thomson optical depth of ~0.60, so theline-emitting matter is not quite Compton-thick. An intrinsic width of1500+460-340 km s-1 FWHM is stillrequired in this model. Moreover, more complicated scenarios involvingboth a Compton shoulder and a disk line cannot be ruled out. We confirman absorption feature due to He-like Fe(FWHM=6405+5020-2670 km s-1) found inprevious studies.
| Type 2 Counterparts of Narrow-Line Seyfert 1 Galaxies
Unified models based on viewing angle successfully explain the observeddifferences between type 1 and type 2 Seyfert galaxies. The existence ofa range in accretion rates (m˙~0.001-1) relative to the Eddingtonrate (from broad-line Seyfert 1's to narrow-line Seyfert 1's, or NLS1's)and the unification of Seyfert galaxies imply that there must be type 2counterparts of NLS1's, that is, Seyfert 2's with high accretion ratesor small black hole masses. One such Seyfert 2, NGC 5506, has alreadybeen unmasked based on near-infrared spectroscopy. Here we confirm theabove result and present evidence for two additional type 2 counterpartsof NLS1's based on XMM-Newton observations. The three active galacticnuclei NGC 7314, 7582, and 5506, with type 1.9/2 optical spectra, showextremely rapid variability, by factors of greater than 2.4, ~1.3, and~1.7 over 200, 350, and 300 s, respectively, and steep 2-12 keV spectra(Γ>~2) in their intrinsic X-ray emission, characteristic ofNLS1's. These observations establish the ``obscured NLS1 galaxies'' as asubclass of Seyfert 2 galaxies.
| Discovery of an X-Ray-luminous Galaxy Cluster at z=1.4
We report the discovery of a massive, X-ray-luminous cluster of galaxiesat z=1.393, the most distant X-ray-selected cluster found to date. XMMUJ2235.3-2557 was serendipitously detected as an extended X-ray source inan archival XMM-Newton observation of NGC 7314. VLT FORS2 R- and z-bandsnapshot imaging reveals an overdensity of red galaxies in both angularand color spaces. The galaxy enhancement is coincident in the sky withthe X-ray emission; the cluster red sequence at R-z~=2.1 identifies itas a high-redshift candidate. Subsequent FORS2 multiobject spectroscopyunambiguously confirms the presence of a massive cluster based on 12concordant redshifts in the interval 1.381 cluster found withXMM-Newton, the relative ease and efficiency of discovery demonstratesthat it should be possible to build large samples of z>1 clustersthrough the joint use of X-ray and large ground-based telescopes.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA member statesand NASA.Based on observations obtained at the European Southern Observatoryusing the Very Large Telescope, on Cerro Paranal (ESO programs72.A-0706, 73.A-0737, 74.A-0023, and 274.A-5024).
| Narrow Iron Kα Lines in Active Galactic Nuclei: Evolving Populations?
We assemble a sample consisting of 66 active galactic nuclei (AGNs) fromthe literature and from the XMM-Newton archive in order to investigatethe origin of the 6.4 keV narrow iron Kα line (NIKAL). The X-rayBaldwin effect of the NIKAL is confirmed in this sample. We find thatthe equivalent width (EW) of the NIKAL is more strongly inverselycorrelated with the Eddington ratio (E) than with the 2-10 keVX-ray luminosity. Our sample favors the dusty torus origin, with theX-ray Baldwin effect being caused by the changing opening angle of thedusty torus. The EW-E relation can be derived from a toy model ofthe dusty torus. If the unification scheme is valid in all AGNs, we canderive the Baldwin effect from the ratio of type II AGNs to the totalpopulation given by Chandra and Hubble Space Telescope deep surveys.Thus, the evolution of populations could be reflected in the NIKAL'sBaldwin effect.
| The relativistic shift of narrow spectral features from black-hole accretion discs
Transient spectral features have been discovered in the X-ray spectra ofActive Galactic Nuclei, mostly in the 5-7 keV energy range. Severalinterpretations were proposed for the origin of these features. Weexamined a model of Doppler boosted blue horns of the iron lineoriginating from a spot in a black hole accretion disc, taking intoaccount different approximations of general relativistic light rays andthe resulting shift of energy of photons. We provide a practical formulafor the blue horn energy of an intrinsically narrow line and assess itsaccuracy by comparing the approximation against an exact value,predicted under the assumption of a planar accretion disc. The mostaccurate approximation provides excellent agreement with the spotorbital radius down to the marginally stable orbit of a non-rotatingblack hole.
| A catalogue of ultraluminous X-ray sources in external galaxies
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.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/429/1125
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| The star formation history of Seyfert 2 nuclei
We present a study of the stellar populations in the central ~200 pc ofa large and homogeneous sample comprising 79 nearby galaxies, most ofwhich are Seyfert 2s. The star formation history of these nuclei isreconstructed by means of state-of-the-art population synthesismodelling of their spectra in the 3500-5200 Åinterval. Aquasar-like featureless continuum (FC) is added to the models to accountfor possible scattered light from a hidden active galactic nucleus(AGN).We find the following. (1) The star formation history of Seyfert 2nuclei is remarkably heterogeneous: young starbursts, intermediate-ageand old stellar populations all appear in significant and widely varyingproportions. (2) A significant fraction of the nuclei show a strong FCcomponent, but this FC is not always an indication of a hidden AGN: itcan also betray the presence of a young, dusty starburst. (3) We detectweak broad Hβ emission in several Seyfert 2s after cleaning theobserved spectrum by subtracting the synthesis model. These are mostlikely the weak scattered lines from the hidden broad-line regionenvisaged in the unified model, given that in most of these casesindependent spectropolarimetry data find a hidden Seyfert 1. (4) The FCstrengths obtained by the spectral decomposition are substantiallylarger for the Seyfert 2s which present evidence of broad lines,implying that the scattered non-stellar continuum is also detected. (5)There is no correlation between the star formation in the nucleus andeither the central or overall morphology of the parent galaxies.
| The 6.7-keV Kα complex of He-like iron in transient plasmas*
Time-dependent numerical simulations of the Kα complex of FeXXVare carried out as a function of temperature-density-radiation fieldvariations in high-temperature astrophysical and laboratory plasmas. Inaddition to several well-known features, the transient and steady-statespectra reveal the effects due to (a) time-dependent thermal andnon-thermal radiation fields, (b) photo- and collisional excitation andionization, and (c) high densities, on the `quartet' of principal w, x,y, z lines, and dielectronic satellites. The highly detailed models showprecisely how, assuming a temporal-temperature correlation, the X-rayintensity varies between 6.6 and 6.7 keV and undergoes a `spectralinversion' in the w and z line intensities, characterizing anionization- or a recombination-dominated plasma. The dielectronicsatellite intensities are the most temperature-dependent features, butinsensitive to density variations, and significantly contribute to theKα complex for T < 6.7 keV leading to asymmetric profiles. The6.7-keV Kα complex should be a potential diagnostic of X-rayflares in active galactic nuclei, afterglows in gamma-ray bursts, andother non-equilibrium sources with the high-resolution measurementspossible from the upcoming mission Astro-E2. It is also shown that highelectron densities attenuate the line intensities in simulationsrelevant to laboratory plasmas, such as in inertial confinement fusion,laser, or magnetic Z-pinch devices.
| Relativistic spectral features from X-ray-illuminated spots and the measure of the black hole mass in active galactic nuclei
Narrow spectral features in the 5-6 keV range were recently discoveredin the X-ray spectra of a few active galactic nuclei. We discuss thepossibility that these features are due to localized spots which occuron the surface of an accretion disc following its illumination byflares. We present detailed line profiles as a function of orbital phaseof the spot and its radial distance from a central black hole.Comparison of these computed profiles with observed features can help toestimate parameters of the system. In principle, this method can providea powerful tool to measure the mass of super-massive black holes inactive galactic nuclei. By comparing our simulations with the Chandraand XMM-Newton results, we show, however, that spectra from presentgeneration X-ray satellites are not of good enough quality to exploitthe method fully and determine the black hole mass with sufficientaccuracy. This task has to be deferred to future missions with highthroughput and high energy resolution, such as Constellation-X and Xeus.
| Spectropolarimetry of Compton-thin Seyfert 2 galaxies
We present new spectropolarimetry of a sample of nearby Compton-thinSeyfert 2 galaxies (i.e. those with NH < 1023cm-2). We show that the detection rate of scattered broadHα in this sample is considerably higher than in Seyfert 2galaxies as a whole. Our results also show that in this low obscurationset it is possible to find scattered broad Hα even when the globalproperties of the galaxy are largely dominated by the host galaxy andnot the active galactic nucleus. These results argue against theexistence of a population of `pure' Seyfert 2 galaxies.
| A long hard look at MCG-6-30-15 with XMM-Newton- II. Detailed EPIC analysis and modelling
The bright Seyfert 1 galaxy MCG-6-30-15 has provided some of the bestevidence to date for the existence of supermassive black holes in activegalactic nuclei. Observations with ASCA revealed an X-ray iron lineprofile shaped by strong Doppler and gravitational effects. In thispaper the shape of the iron line, its variability characteristics andthe robustness of this spectral interpretation are examined using thelong XMM-Newton observation taken in 2001. A variety of spectral models,both including and excluding the effects of strong gravity, are comparedto the data in a uniform fashion. The results strongly favour models inwhich the spectrum is shaped by emission from a relativistic accretiondisc. It is far more difficult to explain the 3-10 keV spectrum usingmodels dominated by absorption (by either warm or partially coveringcold matter), emission line blends, curved continua or additionalcontinuum components. These provide a substantially worse fit to thedata and fail to explain other observations (such as the simultaneousBeppoSAX spectrum). This reaffirms the veracity of the relativistic`disc line' interpretation. The short term variability in the shape ofthe energy spectrum is investigated and explained in terms of atwo-component emission model. Using a combination of spectralvariability analyses, the spectrum is successfully decomposed into avariable power-law component (PLC) and a reflection-dominated component(RDC). The former is highly variable while the latter is approximatelyconstant throughout the observation, leading to the well-known spectralvariability patterns. Consideration of the long term X-ray monitoring ofMCG-6-30-15 by RXTE demonstrates that the long XMM-Newton observationsampled the `typical' state of the source. These results and those ofother analyses of the large XMM-Newton data set are summarized and theirimplications for understanding this enigmatic source are discussed.
| The hard X-ray spectrum of the Seyfert galaxy IRAS 18325-5926: reflection from an ionized disc and variable iron K emission
We report our analysis of the X-ray spectra of the Seyfert galaxy IRAS18325-5926 (= Fairall 49) obtained from various X-ray observatoriesprior to XMM-Newton, including new results from two RXTE and oneBeppoSAX observations. A relatively steep continuum slope (Γ~=2.2) in the 2-15 keV band is confirmed. The continuum spectrum observedwith the BeppoSAX PDS shows a possible roll-over at energies above 30keV, indicating a Comptonizing corona cooler than in other Seyfertnuclei. The X-ray spectrum above 2 keV is best explained with a modelincluding reflection from a highly ionized disc with significantrelativistic blurring. The iron Kα emission feature is then mainlydue to FeXXV. The seven recent observations show that the iron Kemission flux appears to follow the continuum between the observationsseparated by a few months to years, although some exceptions suggestthat the linestrength may be determined in a more complex way.
| The X-ray variability of the narrow-line type 1 Seyfert galaxy IRAS 13224-3809 from an XMM-Newton observation
We report on the XMM-Newton timing properties of the most X-rayvariable, radio-quiet, narrow-line type 1 Seyfert galaxy IRAS13224-3809. IRAS 13224-3809 continues to display the extremely variablebehaviour that was previously observed with ROSAT and ASCA; however, nogiant, rapid flaring events are observed. We detect variations by afactor as high as ~8 during the 64-ks observation and the variability ispersistent throughout the light curve. Dividing the light curve into9-min segments, we found almost all of the segments to be variable at>3σ. When the time-averaged cross-correlation function iscalculated for the 0.3-0.8 keV band with the 3-10 keV band, thecross-correlation profile is skewed, indicating a possible smearing ofthe signal to longer times (the soft band leading the hard). Acorrelation between count rate and hardness ratio is detected in fourenergy bands. In three cases, the correlation is consistent withspectral hardening at lower count rates, which can be explained in termsof a partial-covering model. The other band displays the reverse effect,showing spectral hardening at higher count rates. We can explain thistrend as a more variable power-law component compared with the softcomponent. We also detect a delay between the 0.3-1.5 keV count rate andthe 0.8-1.5 to 0.3-0.8 keV hardness ratio, implying flux-inducedspectral variability. Such delays and asymmetries in thecross-correlation functions could suggest reprocessing of soft and hardphotons. In general, much of the timing behaviour can be attributed toerratic eclipsing behaviour associated with the partial coveringphenomenon, in addition to intrinsic variability in the source. Thevariability behaviour of IRAS 13224-3809 suggests a complicatedcombination of effects which we have started to disentangle with thispresent analysis.
| 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).
| O VII and O VIII Absorption by Hot Gas in the Vicinity of the Galaxy
We searched for evidence of soft X-ray absorption by hot gas in thevicinity of the Galaxy in a small sample of 15 type I active galacticnuclei (AGNs) observed with the high resolution X-ray gratings on boardChandra. We find that around half of the sight lines in our sampleexhibit absorption due to local H- or He-like oxygen (or both) atconfidence levels ranging from >90% to >3 σ. Depending on thesight line, the absorption can be identified with hot gas in particularlocal structures, the Local Group (LG), or the putative local hotintergalactic medium (IGM). Several sight lines in our sample coincidewith sight lines in a study of O VI absorption by local gas, so anassumption of collisional ionization equilibrium (CIE) allows us toconstrain the temperature of the local hot gas. In the southern Galactichemisphere, we identify absorption along the sight line to Fairall 9with the Magellanic Stream, and this gas has105.75
| High-Resolution X-Ray Spectroscopy of the Fe K Complex in IC 4329A
We report the detection of complex Fe K line emission from a ChandraHigh Energy Transmission Grating Spectrometer (HETGS) observation of theSeyfert 1 galaxy IC 4329A. The line emission is double-peaked, one peakcentered at ~6.3 keV and the other at ~6.9 keV in the source rest frame.When modeled by Gaussians, the lower energy peak is resolved by thehigh-energy grating at greater than 99% confidence, while the higherenergy peak is resolved at only less than 90% confidence. Thebest-fitting widths are ~21,000 and ~4000 km s-1 FWHM for the~6.3 and ~6.9 keV peaks, respectively. If the peaks correspond to twodistinct emission lines, then the peak energies are redshifted withrespect to the expected line energies of Fe I Kα and Fe XXVILyα by at least 650 and 950 km s-1, respectively.Alternatively, the Fe K line profile may be due to a single line from arelativistic accretion disk. In that case, the inclination angle of thedisk is required to be 24+9-1 degrees, the outerradius is constrained to several tens of gravitational radii, and theradial line emissivity is flatter than r-0.7. Anotherpossibility is that both peaks are due to distinct lines but that eachone is relativistically broadened by a disk. In that case, the lowerenergy peak could correspond to emission from Fe in a low ionizationstate and the high-energy peak to Fe XXVI Lyα emission. Then theinclination angle is even less, restricted to a few degrees. However,the radial emissivity law is allowed to be steeper (~r-2.5),and the outer radius does not have to be fine-tuned. Yet anotherscenario is that the lower energy peak originates in a disk but thehigher energy peak originates in more distant matter. The diskinclination angle is then intermediate between the last two cases, butthe emissivity is again required to be flat. We cannot rule out Fe XXVHe-like absorption modifying the observed line profile. However, thedata and the inferred emission-line parameters are insensitive to thepresence of a Compton reflection continuum. Including Compton reflectiondoes, however, allow a steeper radial emissivity law for therelativistic line. Future missions, such as Astro-E2, will be able tobreak a lot of the degeneracy in the physically distinct models that canall account for the Chandra data. Since IC 4329A is one of the brightestSeyfert 1 galaxies, it should be a good astrophysical laboratory forstudying the ionization structure of accretion disks around supermassiveblack holes.
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