Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
PG 0844+349 revisited - is there any outflow?
Aims.The detection of high velocity absorption lines from highly ionizedmaterial has been reported recently from the X-ray spectrum of thenarrow line Seyfert 1 galaxy PG 0844+349 and were interpreted as arelativistic outflow from the source. We studied this outflow because itwould have important implications for our understanding of narrow lineSeyfert 1 galaxies. Methods: .To address the reality of theseabsorption features, we analyzed a long XMM observation with the MOScameras on this object. Results: .We did not detect the absorptionlines claimed, and a re-analysis of the previous data set with the mostrecent detector calibrations did not confirm the earlier results. TheX-ray spectra can be well modeled with a power law plus a bremsstrahlungcomponent. With this description, the soft and the harder 2-10 keV X-rayflux changed by a similar fraction (≃25%) between the twoobservations with only small changes in the spectral form. An analysisof the hardness ratio variations within a single observation does notshow any strong correlation between the hardness ratio and the continuumluminosity and we do not detect substantial lags between the hard andsoft band fluxes.
| Bimodal spectral variability of Cygnus X-1 in an intermediate state
We report the results of an observation of Cygnus X-1performed on June 7-11, 2003 with INTEGRAL that we combine withsimultaneous radio observations with the Ryle telescope. Both spectraland variability properties of the source indicate that CygnusX-1 was in an Intermediate State. The INTEGRAL spectrum showsa high-energy cut-off or break around 100 keV. The shape of this cut-offdiffers from pure thermal Comptonisation, suggesting the presence of anon-thermal component at higher energies. The average broad bandspectrum is well represented by hybrid thermal/non-thermalComptonisation models. However, models with mono-energetic injection, ormodels with an additional soft component are favoured over standardpower-law acceleration models. During the 4 day long observation thebroad band (3-200 keV) luminosity varied by up to a factor of 2.6 andthe source showed an important spectral variability. A principalcomponent analysis demonstrates that most of this variability occursthrough 2 independent modes. The first mode consists in changes in theoverall luminosity on time scale of hours with almost constant spectra(responsible for 68% of the variance) that are strikingly uncorrelatedwith the variable radio flux. We interpret this variability mode asvariations of the dissipation rate in the corona, possibly associatedwith magnetic flares. The second variability mode consists in a pivotingof the spectrum around ~10 keV (27% of the variance). It acts on alonger time-scale: initially soft, the spectrum hardens in the firstpart of the observation and then softens again. This pivoting pattern isstrongly correlated with the radio (15 GHz) emission: radio fluxes arestronger when the INTEGRAL spectrum is harder We propose that thepivoting mode represents a "mini" state transition from a nearly HighSoft State to a nearly Low Hard State, and back. This mini-transitionwould be caused by changes in the soft cooling photons flux in the hotComptonising plasma associated with an increase of the temperature ofthe accretion disc. The jet power then appears to be anti-correlatedwith the disc luminosity and unrelated to the coronal power. This is insharp contrast with previous results obtained for the Low Hard State,suggesting a different mode of coupling between the jet, the cold disc,and the corona in Intermediate States. From this interpretation we alsoinfer that the bolometric luminosity jumps by a factor of about 2 duringthe transition hard to soft, suggesting a radiatively inefficientaccretion flow in the Low Hard State.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
| 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.
| 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.
| Chandra Observations of the Narrow-Line Seyfert 1 Galaxy RX J2217.9-5941
We report the results of two Chandra ACIS-S observations from 2003February and August of the highly X-ray variable narrow-line Seyfert 1galaxy RX J2217.9-5941. Observations spanning the time from the ROSATAll-Sky Survey (RASS) through an ASCA observation in 1998 indicateapparently monotonically decreasing flux by a factor of 30. The Chandraobservations reveal increased emission over that seen with ASCA,supporting a persistent variability rather than an X-ray outburst event.However, the cause of the strong X-ray variability remains unclear. OurChandra observations confirm the steep soft X-ray spectrum in the0.2-2.0 keV band found during the RASS observation(αX=2.7). The spectral shape of the source appears tobe variable, with the spectrum becoming softer when the source becomesfainter. Best-fitting models to the data include an absorbed brokenpower law, a blackbody plus a power law, and a power law with partialcovering absorption. The latter model suggests a variable partialcovering absorber in the line of sight, which can explain in part thevariability seen in RX J2217.9-5941. We suggest that there might be apopulation of narrow-line Seyfert 1 galaxies that are, at least attimes, highly absorbed.
| The gas content of peculiar galaxies: Strongly interacting systems
A study of the gas content in 1038 interacting galaxies, essentiallyselected from Arp, Arp & Madore, Vorontsov-Velyaminov catalogues andsome of the published literature, is presented here. The data on theinterstellar medium have been extracted from a number of sources in theliterature and compared with a sample of 1916 normal galaxies. The meanvalues for each of the different ISM tracers (FIR, 21 cm, CO lines,X-ray) have been estimated by means of survival analysis techniques, inorder to take into account the presence of upper limits. From the datait appears that interacting galaxies have a higher gas content thannormal ones. Galaxies classified as ellipticals have both a dust and gascontent one order of magnitude higher than normal. Spirals have in mostpart a normal dust and HI content but an higher molecular gas mass. TheX-ray luminosity also appears higher than that of normal galaxies ofsame morphological type, both including or excluding AGNs. We consideredthe alternative possibilities that the molecular gas excess may derivefrom the existence of tidal torques which produce gas infall from thesurrounding regions or from a different metallicity which affects the Xconversion factor between the observed CO line luminosity and the H_2calculated mass. According to our tests, it appears that interactinggalaxies possess a higher molecular mass than normal galaxies but with asimilar star formation efficiency.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/422/941
| Close and distant reprocessing media in Mkn 509 studied with BeppoSAX
We present the broad band analysis of two BeppoSAX observations of theSeyfert 1 Mkn 509. In 2000 the source was in a typical flux state,F2-10 keV=2.7× 10-11 erg cm-2s-1, while in 1998 it was found in a high flux state,F2-10 keV=5.7× 10-11 erg cm-2s-1. A comparison between the two states shows aenergy-dependent flux variation of about a factor of three and a factorof two in the LECS (0.15-3 keV) and MECS (1.5-10 keV), respectively,while in the PDS (13-200 keV) the difference is marginal. A soft excess,a narrow iron line and a Compton reflection hump above 10 keV areclearly apparent in the residuals after fitting the spectra with asimple power law. We tested two alternative models. In the first theiron line and the high energy bump are well reproduced by reprocessingin a cold and Compton thick material. The intensity of the iron line(also consistent with a Chandra measurement) as well as thenormalization of the reflection hump are consistent with a constant inthe two epochs: this, combined with the fact that the line is narrow asobserved by Chandra, suggests a common origin from distant and opticallythick matter. This model further requires a component to model the softexcess: the empirical choice of two black bodies accounts well for theexcess in both observations; their combined strength was a factor ofabout three higher in the high than in the low flux state defined above.However, the relative contribution of the soft excess is higher in thelow flux state. In the second model we attempted to reproduce allspectral features, except for the narrow cold line, with reflection froman ionized disc. This model is successful only in the high flux state,but it fails in the low flux state, when the soft excess is onlypartially accounted for. In either model, the slope of the power law isgreater in the high than in the low flux state, (ΔΓ0.2), in agreement with a behaviour known to be shared by severalobjects of the same type.
| Timescales of Soft X-Ray Variability and Physical Constraints in Active Galactic Nuclei
We present soft X-ray variability timescales for 65 active galacticnuclei (AGNs) derived from ROSAT Position Sensitive Proportional Counterpointing data. For these 65 objects with obvious exponential increasingor decreasing patterns in their light curves, we use the exponentialtimescales and find they are more suitable for analyzing ROSATlight-curve data. We also discuss some physical constraints on thecentral engine implied by our results. Assuming that this soft X-rayvariability exponential timescale is approximately equal to the thermaltimescale of the standard accretion disk, we obtain the accretion rate,the size of the soft X-ray radiation region, and the compactnessparameter for 37 AGNs, using their recently estimated central black holemasses. For 12 of these 37 AGNs, the radii of the gravitationalinstability in the standard thin accretion disks are obtained using thecentral black hole masses and the calculated accretion rates. These areconsistent with the results from the reverberation mapping method. Theseresults provide supporting evidence that such gravitationalinstabilities contribute to the formation of the broad-line regions inAGNs.
| 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).
| Motion of charged particles around a rotating black hole in a magnetic field
We study the effects of an external magnetic field, which is assumed tobe uniform at infinity, on the marginally stable circular motion ofcharged particles in the equatorial plane of a rotating black hole. Weshow that the magnetic field has its greatest effect in enlarging theregion of stability towards the event horizon of the black hole. Usingthe Hamilton-Jacobi formalism we obtain the basic equations governingthe marginal stability of the circular orbits and their associatedenergies and angular momenta. As instructive examples, we review thecase of the marginal stability of the circular orbits in the Kerrmetric, as well as around a Schwarzschild black hole in a magneticfield. For large enough values of the magnetic field around a maximallyrotating black hole we find the limiting analytical solutions to theequations governing the radii of marginal stability. We also show thatthe presence of a strong magnetic field provides the possibility ofrelativistic motions in both direct and retrograde innermost stablecircular orbits around a Kerr black hole.
| Detection of a high frequency break in the X-ray power spectrum of Ark 564
We present a power spectrum analysis of the long ASCA observation of Ark564 in June/July 2001. The observed power spectrum covers a frequencyrange of ~ 3.5 decades. We detect a high frequency break at ~ 2x10-3 Hz. The power spectrum has an rms of ~ 30% and a slopeof ~ -1 and ~ -2 below and above the break frequency. When combined withthe results from a long RXTE observation (Pounds et al.\cite{Pounds2001}), the observed power spectra of Ark 564 and Cyg X-1(in the low/hard state) are almost identical, showing a similar shapeand rms amplitude. However, the ratio of the high frequency breaks isvery small ( ~ 103-4), implying that these characteristicfrequencies are not indicative of the black hole mass. This resultsupports the idea of a small black hole mass/high accretion rate in Ark564.
| Study of the Long-Term X-Ray Variability of a Possible Quasar RX J0957.9+6903 with ASCA
The long-term variability and spectral properties of a possible quasar,RX J0957.9+6903, were studied utilizing 16 ASCA observations spanning5.5 years. The average 0.7-10 keV spectrum of RX J0957.9+6903 is wellrepresented by a power-law continuum having a photon index of 1.58 +/-0.03 and an absorption column of sim 1 ×1021cm-2. The 2-10 keV flux of RX J0957.9+6903varied by a factor of four over a period of six years, around a mean ofsim 8.8 × 10-12ergs-1cm-2.Peak-to-peak variability within each observation was less than 25% onsim 1 day time scale. These properties support the classification of RXJ0957.9+6903 as a quasar. The power spectrum density (PSD) was estimatedin a ``forward'' manner over a frequency range of 10-8.2 -10-4.3Hz by utilizing the structure function method and aMonte Carlo simulation assuming a broken power-law type PSD. Then, thebreak frequency fb of the PSD of RX J0957.9+6903 has beenconstrained as 1/fb =1600+ &infy;-1100 days, and the logarithmic slope of thehigh-frequency region of the PSD as α = -1.55 +/- 0.2 . Acomparison of the estimated PSDs is made between RX J0957.9+6903 and theM81 nucleus, observed in the same field of view.
| Seeing Galaxies through Thick and Thin. I. Optical Opacity Measures in Overlapping Galaxies
We describe the use of partially overlapping galaxies to provide directmeasurements of the effective absorption in galaxy disks, independent ofassumptions about internal disk structure. The nonoverlapping parts ofthe galaxies and symmetry considerations are used to reconstruct, viadifferential photometry, how much background galaxy light is lost inpassing through the foreground disks. Extensive catalog searches andfollow-up imaging yield ~15-25 nearby galaxy pairs suitable for varyingdegrees of our analysis; 11 of the best such examples are presentedhere. From these pairs, we find that interarm extinction is modest,declining from AB~1 mag at 0.3RB25 toessentially zero by RB25; the interarm dust has ascale length consistent with that of the disk starlight. In contrast,dust in spiral arms and resonance rings may be optically thick(AB>2) at virtually any radius. Some disks have flatterextinction curves than the Galaxy, with AB/AI~1.6this is probably the signature of clumpy dust distributions. Even thoughtypical spirals are not optically thick throughout their disks, wherethey are optically thick is correlated with where they are mostluminous: in spiral arms and inner disks. This correlation betweenabsorption and emission regions may account for their apparent surfacebrightness being only mildly dependent on inclination, erroneouslyindicating that spirals are generally optically thick. Taken as anensemble, the opacities of spiral galaxies may be just great enough tosignificantly affect QSO counts, though not enough to cause theirhigh-redshift cutoff. Based in part on archival observations with theNASA/ESA Hubble Space Telescope (HST) obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS5-26555.
| Dynamics of Warm Absorbing Gas in Seyfert Galaxies: NGC 5548
A magnetohydromagnetic (MHD) wind from a clumpy molecular accretion diskis invoked to explain observations of warm absorbing gas at UV and X-rayenergies in Seyfert 1 galaxies. This paper focuses on two importantissues: (1) compatibility of kinematics and dynamics of an MHD wind withthe observed properties of warm absorbers, and (2) the relationshipbetween the UV and X-ray absorbing gases. We provide an in-depthcomparison between the MHD wind model and the well-studied Seyfert 1galaxy NGC 5548, which at high spectral resolution exhibits a number ofdiscrete UV absorption components. Model parameters used in this studyhave been fixed by our previous work on the variability of broad-lineregion in this object. The detailed UV-X-ray (UVX) absorption structurein NGC 5548 is modeled in order to infer the position, size, density andtemperature of each component, as well as the overall dynamics of theabsorbing gas. We find that for NGC 5548 (1) the total column densitiesof O VII, O VIII, and H inferred from X-ray observations are reproducedby constraining the UV ion column densities of C IV and N V in eachcomponent to lie within a factor of 2 of their observed values andoptimizing over the possible sets of component ionization states and CIV column densities; (2) the warm absorbing gas exists in the outer partof the wind and is not a continuation of the flow in the broademission-line region; and (3) the warm absorber extends both in radialand polar directions and is ionization stratified. X-ray absorption isfound to be heavily biased toward smaller radii, and UV absorptionoriginates at larger distances from the central continuum source. Ouranalysis shows that the discrete absorption components along the line ofsight are intrinsically clumpy. Density differences between kinematiccomponents result in a range of ionization and recombination timescales.We further test the applicability of the MHD wind to warm absorbers ingeneral by constructing a quasi-continuous flow model and extending itto arbitrary aspect angles. Constraining the ionization and volumefilling factor for a generic case, we estimate the fraction of Seyfert 1galaxies having detectable warm absorbers with larger O VII than O VIIIcolumn densities, and the range of total hydrogen column densities. Wealso find that the ratio of O VII to O VIII optical depths can serve asa new diagnostic of active galactic nucleus aspect angle. Finally, thethermal stability of the UVX absorption model is tested. We find thatall kinematic components in NGC 5548 are thermally stable to isobaricperturbations. In a more general case, we show that the magnetic fieldis crucial in order to stabilize the warm absorber gas over a wide rangeof incident continuum spectral energy distribution and gas metallicity.
| X-ray emission/absorption mechanisms of 4 NLSy-1-like AGN and a radio quasar. QSO 0117-2837, RX J0134.3-4258, NGC 4051, Mrk 1298, 4C +74.26
We present a study of the X-ray variability properties and spectralshapes of five active galaxies all of which show extreme or enigmaticX-ray properties. We focus on QSO 0117-2837, RX J0134-4258, and NGC4051, and briefly comment on Mrk 1298 and 4C +74.26. The individualobjects were originally partly selected as candidates to host warmabsorbers on the basis of (i) characteristic X-ray absorption features(NGC 4051, Mrk 1298, 4C +74.26), (ii) extreme X-ray spectral steepness(QSO 0117-2837; this object is found to be located in the `zone ofavoidance' when plotted in the Gamma_x-FWHM_Hβ diagram), and (iii)drastic spectral variability (RX J0134-4258). The temporal analysisreveals large-amplitude variability by a factor \sim$30 in the long-termX-ray lightcurve of NGC 4051, very rapid variability of Mrk 1298,constant X-ray flux of the NLSy1 galaxy QSO 0117-2837, and constant meancountrate of RX J0134-4258 despite huge spectral changes. Besides thewarm absorber, several further mechanisms and their merits/shortcomingsare investigated to explain the spectral characteristics of theindividual objects. Different models are favored for different sources.Consequences for Narrow-line Seyfert 1s in general are discussed and wepresent results from photoionization models to distinguish betweendifferent suggested NLSy1 scenarios.
| X-Ray Variability of an Illuminated Irregular Accretion Disk around a Black Hole
Models for active galactic nuclei and X-ray binaries where ``cold''rotating matter is illuminated by a nonthermal source above it have beenwidely studied, and they successfully explain the main spectral featuresof black hole candidates, including reflection humps above 10 keV andthe iron line at 6.4 keV. Making the reasonable assumptions that theinnermost part of an accretion disk around a black hole is unstable andclumpy and that it is illuminated by X-ray sources originating above it,we compute the X-ray variability induced by both a clump of mattermoving relativistically around the black hole and the nonthermal sourcemoving above the disk. One interesting new result we find is that evenfor a face-on geometry, rotation-induced variability can still beobserved as long as the nonthermal source is not exactly at the symmetryaxis; this is true even if the accretion disk is smooth. We also showthat the reflected X-ray component, peaked around 10 or several tens ofkeV, can vary more than other spectral components, such as the thermalemission from the disk and the external power-law flux, and that itcarries richer information on the nonthermal sources. This is becausethe reflected component experiences a double path through thegravitational field of the central mass, i.e., from the source to thedisk and then from the disk to the observer. Thus our study is mostrelevant to X-ray variability of black hole candidates whererelativistic effects are strong. These results may provide a partialexplanation of why some Seyfert 1 galaxies exhibit rapid X-rayvariability with timescales comparable to the fastest orbital periods,and why soft X-rays and UV emission generally show slower variabilitythan do the hard X-rays.
| Gamma-Ray Burst Counterparts: Optical Data
The surest solution of the gamma-ray burst (GRB) mystery is to find anunambiguous low-energy quiescent counterpart. To this end, we haveintensively searched within the smallest GRB error boxes for anycounterpart candidates. This paper reports on 255 hr of exposure withground-based telescopes since 1981. We report our results in the U, B,V, R. I, J, H, and K bands. We find the usual array of mildly unusualsources in the boxes, but none is sufficiently unusual to suggest acausal connection. We find that the smallest boxes are empty to fairlydeep magnitudes. This fact can be of significance since virtually allcosmological models place bright bursters inside normal host galaxies atmoderate distances. To allow for quantitative evaluation of thepredictions of these models, we have compiled a list of limits on thebrighest galaxy inside each of the 26 regions in various bands. Thislist was compiled from our own results as well as from the publishedliterature. The limits on host galaxy luminosities from these data aresubstantially more restrictive than the limits from recent opticaltransients because the bursts we report on are much brighter than thebursts with optical transients.
| Dynamics of cD Clusters of Galaxies. III. Redshift Data for 11 Abell Clusters
We present the final observational data for a spectroscopic study of asample of cD galaxy clusters. The goal of this program has been to studythe dynamics of the clusters, with emphasis on determining the natureand frequency of peculiar-velocity cD galaxies. In this paper we presentredshifts for 762 galaxies in the fields of the rich Abell clustersA779, A1691, A1749, A1767, A1837, A1927, A2067, A2079, A2089, A2199, andA2666. We also present preliminary dynamical properties for theseclusters using our measured redshifts.
| Properties of dusty warm absorbers and the case of IRAS17020+4544
We present a study of the properties of dusty warm absorbers and pointto some important consequences for the resulting X-ray absorptionspectra of AGN. Pronounced effects of the presence of dust in warmmaterial are (i) an apparent `flattening' of the observed X-ray spectrumin the ROSAT band due to a sequence of absorption edges and a shift tolower gas ionization, (ii) the presence of a strong carbon edge at 0.28keV, and (iii) the expectation of increased time variability of the warmabsorber parameters. The first two effects can be drastic and maycompletely hamper an X-ray spectral fit with a dusty warm absorber evenif a dust-free one was successfully applied to the data. In order todemonstrate facets of the dusty warm absorber model and test therecently reported important, albeit indirect, evidence for dusty warmmaterial in the Narrow-Line Seyfert 1 galaxy IRAS 17020+4544 we haveanalyzed ROSAT PSPC and HRI observations of this galaxy. The X-rayspectrum can be successfully described by a single powerlaw with indexGamma_x =-2.4 plus small excess cold absorption, or alternatively by asteeper intrinsic powerlaw (Gamma_x =~ -2.8) absorbed by a dusty warmabsorber. The findings are discussed in light of the NLSy1 character ofIRAS 17020 and consequences for NLSy1s in general are pointed out. Inparticular, the presence of dusty warm gas results in a steeperintrinsic powerlaw than observed, thus exaggerating the `FeII problem'.It also implies weaker potential warm-absorber contribution tohigh-ionization Fe coronal lines.
| The Starburst Model for Agn: Past, Present, and Future
It is now eleven years since Terlevich \& Melnick first proposed an`AGN without black-holes' model, an idea which since then evolved intowhat is now called the starburst model for AGN. This model has been thesubject of much debate in the last decade, with observational evidenceboth for and against it further fuelling the controversy. Can we afterall these years reach a veredictum on whether starbursts can power AGN?This contribution tries to answer this question reviewing the mainachievements of the starburst model, its current status and futureprospects.
| Testing the X-ray variability of active galactic nuclei with the non-linear prediction method
The analysis of eight EXOSAT X-ray light curves of six active galacticnuclei by non-linear prediction methods indicates that the observedvariability is truly stochastic and is not caused by deterministicchaos. This result favours X-ray emission models with multiple centresof production of hot, possibly relativistic, electrons.
| First results from the MECS onboard BeppoSAX:
In this contribution we discuss briefly a few calibration items relevantto the data analysis and present some preliminary scientific results.The discussion on instrumental topics focuses on the response matrix andPoint Spread Function (PSF). In the scientific results section wediscuss a first analysis of the two Seyferts MCG 6-30-15 and NGC 4151and of the Cosmic X-ray Background.
| Optical and Far-Infrared Emission of IRAS Seyfert Galaxies
This paper presents an analysis of moderately large samples of type 1and 2 Seyfert galaxies through optical observations and far-infraredIRAS data, also taking into account theoretical color indices derivedfrom dust emission models. The galaxies in the samples cover a ratherlarge interval in far-infrared luminosity, i.e., 7.6 <= log(LIR/Lȯ) <= 12.6. We show that both types of Seyferts haveapproximately the same distribution of number of objects with a givenLIR. Galaxies with similar far-infrared color indices alpha (100, 60)are grouped together, and the corresponding average color indices areinterpreted in terms of a simple model in which the observed colorsresult from the combination of dust directly heated by the activegalactic nucleus with a component from the host galaxy represented bythe emission of cool dust. On the basis of the average IRAS colors ofthe derived groups, we show that type 1 and 2 Seyfert galaxies areundistinguishable from each other. From the luminosity ratios LIR/LHalpha and LIR/L[O III], we show that basically the same model can beapplied to both types of Seyfert, only allowing for the variation ofmodel conditions: type 2 Seyferts would be like type 1 Seyferts but withthe Seyfert nucleus and broad line region more effectively "hidden" bydust.
| Soft excesses of hard X-ray selected Seyfert 1 galaxies studied with the ROSAT-PSPC
The nature of the soft X-ray excess in Seyfert 1 galaxies is still anopen issue. In order to derive some general properties we analysed theROSAT PSPC archive observations of the Seyfert galaxies in the\cite[Nandra & Pounds (1994)]{Nan94} Ginga sample. The use of thissample ensures a reliable estimate (under the assumption of lack ofintrinsic spectral variability) of the slope of the underlying powerlaw, which instead cannot be well measured using ROSAT data alone. Wetested three different models for the soft excess: emission fromaccretion disk (described by a black body), absorption from ionized gas(``warm absorber'', described by an absorption edge), and reflectionfrom mildly ionized material (described by a step function). The mainresult is that no model gives a satisfactory fit for all sources.Furthermore for two sources none of these models provides an acceptablefit. In these cases a physically meaningful combination of twocomponents (e.g. warm absorber plus black body) gives a good fit. Weconclude that several different effects concur to the soft excessphenomenon in Seyfert galaxies.
| NGC 4151 - A unified active galactic nucleus.
We present a unified picture of active galactic nuclei which weconstruct from a detailed model of line emission in the active source inNGC 4151. This source provides us with an opportunity to explore thevariation of structure with luminosity in an object which, in the model,derives its unusual properties as a consequence of the angle of theaccretion disc to the line of sight. The key features that emerge fromthe model are (i) a non-spherical broad line region (BLR); (ii)short-lived BLR clouds; (iii) a luminosity-dependent structure for theBLR; (iv) a luminosity-dependent flared accretion disc extending beyondthe BLR and (v) a separate intermediate line region between the BLR andNLR. The special orientation turns out to enable us to fix many of theparameters of the model for this source. It is then natural to ask howthis model would appear, in its various luminosity states, at otherorientations. To make contact with observations we need to include dustobscuration over a range of angles near to the plane of the disc. Wethen obtain the spread of observed types of radio-quiet active nucleiand we propose an extension of the model to a unification of radio-loudactive galaxies. Thus, in this scheme NGC 4151 can be regarded as atypical active nucleus, special only in its orientation. We shall findthat this alleviates a number of problems with a unified picture basedon a dusty molecular torus with a fixed structure (to which NGC 4151appears as an anomaly). In the proper sense of the expression, NGC 4151may be the exception that proves the rule.
| Astrophysics in 1995
Ap95 differs from Ap91 to Ap94 primarily in Ap number. That is, it onceagain attempts to highlight some areas of our mutual science where amajor event has occurred during the year (a possible solution of theType II supernova problem, the faintness of quasar host galaxies, andthe collision of Shoemaker-Levy 9 with Jupiter), but also to focus onsome topics where dogged determination is producing more gradualadvances (dense star clusters, the second parameter problem, inventoriesof nearby objects and dark matter candidates). In moderate departurefrom the rules of Ap92-94, there has been some deliberate up-dating oftopics discussed previously. The topics are only partly ordered fromnearby to far away. (SECTION: Invited Review Papers)
| On the evolution of ejecta fragments in compact supernova remnants.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996MNRAS.283..419C&db_key=AST
| Soft excesses of hard X-ray selected Seyfert 1 galaxies.
Not Available
| An image database. II. Catalogue between δ=-30deg and δ=70deg.
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
