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X-ray Variability in Active Galactic Nuclei: Implications of Magnetic Flares |
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Goosmann, R. W.; Czerny, B.; Mouchet, M.; Karas, V.; Dovčiak, M.; Ponti, G.; Różańska, A.; Dumont, A.-M. |
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2006IAUS..238E..21G |
The last generation of X-ray satellites continues to reveal a complex nature of the X-ray variability in Active Galactic Nuclei (AGN) in the energy band of the iron Kalpha line. Simple expectations of radiation reprocessing by the disk do not explain the data. We address this issue in the frame work of the magnetic flare model. We model the observed energy-dependent fractional variability amplitude in the Seyfert galaxy MCG-6-30-15 using Monte-Carlo simulations of large flare distributions co-rotating with the accretion disk. Our modeling includes detailed radiative transfer computations for an accretion disk in hydrostatic equilibrium, which is locally illuminated by a point source, and a full-relativistic treatment of the reprocessed radiation. It turns out that the relative lack of variability at the rest frame position of the iron Kalpha line and the enhanced variability in its red wing can be reproduced if the effective energy dissipation by the flares is large in the central region of the disk. We also address the issue of the time delays between the hard and soft X-rays by considering in detail an individual strong flare event. The model considers an intrinsic delay between the primary radiation, which is thought to be emitted by an elevated source above the disk, and the reprocessed radiation coming from the disk surface. Both spectral components are simulated by time-modulated power-laws. The model reproduces the observed time lags well and enables to put constraints on the height of the primary flare source above the accretion disk.
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Finding an Evolutionary Link Between VLIRGs-ULIRGs, Radio Galaxies and QSOs |
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Gonzalez Delgado, R. M.; Tadhunter, C. N.; Perez, E.; Rodriguez, J.; Villar-Martin, M. |
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2006IAUS..235E.137G |
VLIRGs and ULIRGs are believed to be some of the most actively star forming systems in the local universe. Although AGN may contribute to the heating of the dust in some of them, especially those with warm far-IR colours, it is believed that much of the far-IR excess in these objects represents reprocessed starlight. Moreover, the VLIRGs-ULIRGs invariably show morphological features suggestive of mergers such as tidal tails and double nuclei. The fact that the luminous AGN are more prevalent in warm than in cool VLIRGs-ULIRGs has led Sanders et al. to suggest that cool VLIRGs-ULIRGs evolve into warm VLIRGs-ULIRGs on their way to becoming quasars (QSOs). We investigate the properties of the young stellar population (age, mass, reddening) in a sample of VLIRGs-ULIRGs, radio galaxies and QSOs in order to understand the link between star formation and AGN activity in the merger process. Using the starburst ages as a clock we determine: (a) whether VLIRGs-ULIRGs evolve into radio galaxies and quasars; (b) whether cool VLIRGs-ULIRGs evolve into warm VLIRGs-ULIRGs; and (c) whether the merger sequence deduced from morphological studies is reflected in the properties of the young stellar population. This investigation is crucial to find out whether an evolutionary link exists betweenVLIRGs-ULIRGs, radio galaxies and QSOs, and normal elliptical galaxies, and understanding the evolution of stellar populations as the mergers progress.
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Spectral line variability amplitudes in active galactic nuclei |
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2006A&A...454..459K |
We present the results of a long-term variability campaign of very broad-line AGNs with line widths broader than FWHM 〉 5000 km s-1. The main goal of our investigation was to study whether the widths of the optical broad emission lines are correlated with the optical intensity variations on timescales of years. Our AGN sample consisted of 10 objects. We detected a significant correlation between optical continuum variability amplitudes and Hβ emission line widths (FWHM) and, to a lesser degree, between Hβ line intensity variations and Hβ equivalent widths. We add the spectroscopic data of variable AGNs from the literature to supplement our sample. The AGNs from other optical variability campaigns with different line-widths helped to improve the statistical significance of our very broad-line AGN sample. After including the data on 35 additional galaxies, the correlation between optical continuum variability amplitudes and Hβ emission line widths becomes even more significant and the probability that this is a random correlation drops to 0.7 percent.
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Evolutionary unification in composite active galactic nuclei |
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2006MNRAS.368.1001L |
In this paper, we present an evolutionary unification scenario, involving supermassive black holes (SMBHs) and starbursts (SBs) with outflow (OF), that seems capable of explaining most of the observational properties (of at least part) of active galactic nuclei (AGN).
The scenario includes a nuclear/circumnuclear SB closely associated with the AGN where the narrow-line region (NLR), broad-line region (BLR) and broad absorption line (BAL) region are produced in part by the OF process 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 problem regarding the BLR of AGN is analysed. The correlations between the BAL, IR emission, FeII intensity and the intrinsic properties of the AGN are not clearly understood. We suggest here that the behaviour of the BAL, IR and FeII emission in AGN can be understood within an evolutionary and composite model for AGN.
In our model, strong BAL systems and FeII emission are present (and intense) in young IR objects. Parameters like the BALs, IR emission, FeII/Hβ intensity ratio, FeII equivalent width (EW), broad-line width, [OIII]λ5007-Å intensity and width, NLR size, X-ray spectral slope in radio quiet (RQ) AGN plus lobe separation, and lobe to core intensity ratio in radio loud (RL) AGN are proposed to be fundamentally time-dependent variables inside time-scales of the order of 108 yr. Orientation/obscuration effects take the role of a second parameter providing the segregation between Seyfert 1/Seyfert 2 galaxies (Sy1/Sy2) and broad-/narrow-line radio galaxies (BLRG/NLRG).
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Accretion Disks, Jets and Blazar Variability |
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2006ASPC..350..183W |
Although blazar variability is probably dominated by emission from relativistic jets, accretion disks should be present in all blazars. These disks produce emission over most of the electromagnetic spectrum; various unstable processes operate in those disks which will lead to variable emission. Here I summarize some of the most relevant disk mechanisms for AGN variability. I also discuss some aspects of jet variability, focusing on the possibility that ultrarelativistic jets of modest opening angle can reconcile TeV blazar emission with the many subluminal VLBI knots seen in those sources. Finally, I present recently illuminated characteristics of optical microvariability of different classes of AGN which have important implications for the dominant processes involved.
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Spectral Variability of Quasars in the Sloan Digital Sky Survey. II. The C IV Line |
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Wilhite, Brian C.; Vanden Berk, Daniel E.; Brunner, Robert J.; Brinkmann, Jonathan V. |
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2006ApJ...641...78W |
We examine the variability
of the high-ionization C IV λ1549 line in a sample of 105 quasars observed
at multiple epochs by the Sloan Digital Sky Survey. We find a strong
correlation between the change in the C IV line flux and the change in the line
width, but no correlations between the change in flux and changes in line
center and skewness. The relation between line flux change and line width
change is consistent with a model in which a broad line base varies with
greater amplitude than the line core. The objects studied here are more
luminous and at higher redshift than those normally studied for variability, ranging in redshift from 1.65 to 4.00 and
in absolute r-band magnitude from roughly -24 to -28. Using moment analysis
line-fitting techniques, we measure line fluxes, centers, widths, and
skewnesses for the C IV line at two epochs for each object. The well-known
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The DEEP Groth Strip Survey. VI. Spectroscopic, Variability, and X-Ray Detection of Active Galactic Nuclei |
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Sarajedini, Vicki L.; Koo, David C.; Phillips, Andrew C.; Kobulnicky, Henry A.; Gebhardt, Karl; Willmer, Christopher N. A.; Vogt, Nicole P.; Laird, Elise; Im, Myungshin; Iverson, Sarah; Mattos, Wanessa |
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2006ApJS..166...69S |
We identify active galactic nuclei (AGNs) in the Groth-Westphal Survey Strip (GSS) using the independent and complementary selection techniques of optical spectroscopy and photometric variability. We discuss the X-ray properties of these AGNs using Chandra/XMM-Newton data for this region. From a sample of 576 galaxies with high-quality spectra we identify 31 galaxies with AGN signatures. Seven of these have broad emission lines (Type 1 AGNs). We also identify 26 galaxies displaying nuclear variability in Hubble Space Telescope Wide Field Planetary Camera 2 (HST WFPC2) images of the GSS separated by ~7 yr. The primary overlap of the two selected AGN samples is the set of broad-line AGNs, of which 80% appear as variable. Only a few narrow-line AGNs approach the variability threshold. The broad-line AGNs have an average redshift of <z〉~=1.1, while the other spectroscopic AGNs have redshifts closer to the mean of the general galaxy population (<z〉~=0.7). Eighty percent of the identified broad-line AGNs are detected in X-rays, and these are among the most luminous X-ray sources in the GSS. Only one narrow-line AGN is X-ray detected. Of the variable nuclei galaxies within the X-ray survey, 27% are X-ray detected. We find that 1.9%+/-0.6% of GSS galaxies to Vgal=24 are broad-line AGNs, 1.4%+/-0.5% are narrow-line AGNs, and 4.5%+/-1.4% contain variable nuclei. The fraction of spectroscopically identified BLAGNs and NLAGNs at z~1 reveals a marginally significant increase of 1.3%+/-0.9% when compared to the local population.
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The Lack of Broad Line Regions in Unobscured Seyfert 2 Galaxies |
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2005AAS...20717406T |
NGC 3147, NGC 4698 and 1ES 1927+654 are active galaxies that are classified as Seyfert 2s, based on the line ratios and presence of strong narrow emission lines in their optical spectra. However, they exhibit rapid X-ray spectral variability and/or little indication of obscuration from Chandra spectra, contrary to expectation from the standard AGN unification model. Using optical spectropolarimetry with LRIS and near-infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory, we conducted a deep search for hidden polarized broad Hα and direct broad Paβ or Brγ emission lines in these objects. We found no evidence of any broad emission lines from the active nucleus of NGC 3147, NGC 4698 or 1ES 1927+654. These results provide support to the idea that these AGNs intrinsically lack broad-line regions, and "true" Seyfert 2 galaxies may exist in nature.
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The Nature of Composite Seyfert/Star-forming Galaxies Revealed by X-Ray Observations |
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Panessa, Francesca; Wolter, Anna; Pellegrini, Silvia; Fruscione, Antonella; Bassani, Loredana; Della Ceca, Roberto; Palumbo, Giorgio G. C.; Trinchieri, Ginevra |
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2005ApJ...631..707P |
This paper presents new Chandra and BeppoSAX observations aimed at investigating the optical-X-ray mismatch in the enigmatic class of the composite galaxies discovered by a cross-correlation of IRAS and ROSAT all-sky survey catalogs. These galaxies have been classified as star-forming objects on the basis of their optical spectra, while the detection of weak broad wings in the Hα emission in a few of them and their high X-ray luminosity in the ROSAT band indicated the presence of an active nucleus. The analysis of Chandra observations for four composite galaxies has revealed nuclear pointlike sources, with a typical AGN spectrum (Γ~1.7-1.9) and little intrinsic absorption. A strong flux variability has been observed on different timescales; in particular, most of the sources were brighter at the ROSAT epoch. Although it is of relatively low luminosity for the AGN class (L2-10keV ~3-60×1041 ergs s-1), the active nucleus is nevertheless dominant in the X-ray domain. At other wavelengths it appears to be overwhelmed by the starburst and/or host galaxy light, yielding the composite classification for these objects.
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AGN variability time scales and the discrete-event model |
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2005A&A...443..451F |
We analyse the ultraviolet variability time scales in a sample of 15 type 1 Active Galactic Nuclei (AGN) observed by IUE. Using a structure function analysis, we demonstrate the existence in most objects of a maximum variability time scale of the order of 0.02-1.00 year. We do not find any significant dependence of these maximum variability time scales on the wavelength, but we observe a weak correlation with the average luminosity of the objects. We also observe in several objects the existence of long-term variability, which seems decoupled from the short-term one. We interpret the existence of a maximum variability time scale as a possible evidence that the light curves of type 1 AGN are the result of the superimposition of independent events. In the framework of the so-called discrete-event model, we study the event energy and event rate as a function of the object properties. We confront our results to predictions from existing models based on discrete events. We show that models based on a fixed event energy, like supernova explosions, can be ruled out. In their present form, models based on magnetic blobs are also unable to account for the observed relations. Stellar collision models, while not completely satisfactory, cannot be excluded.
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A Brief Review of Long-Term X-Ray and Optical Variability in Radio-Quiet AGN |
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2004PThPS.155..170U |
Long-time-scale X-ray and optical variability is a key characteristic of AGN. Here, we summarise our current understanding of the X-ray and optical continuum variability of radio-quiet AGN and the relation between the two bands. We demonstrate the strong connection between the X-ray variability properties of AGN and the variability of stellar-mass black hole candidates on much shorter time-scales, and discuss the implications of this result for the origins of the variability. The relationship between optical and X-ray variability is complex, with some AGN showing strong X-ray/optical correlations while others show no obvious correlation. We suggest a possible explanation for this variety of behaviour.
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AGN Variability: Models and Observations |
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2004BaltA..13..642H |
Although the variability of quasars played a crucial role in the initial understanding of active galactic nuclei, since then little progress has been made in understanding the nature of quasar variations. This is partly through lack of high quality data taken over sufficiently long periods of time, and partly lack of theoretical modeling of the processes taking place in the active nucleus in a form which would allow quantitative comparison with observations. In this paper we make the case that variability can provide an essential clue as to the structure and mechanism of the emitting region, and review what is known about AGN variations in optical passbands, especially on longer timescales where most of the power resides. We also use theoretical predictions from the literature for various models of variation and compare them with observations. Although the models are still in preliminary stages, it is already clear that observations have the power to discriminate between them. Another aspect of AGN variability is the possibility that quasars are being gravitationally microlensed. This has been well-established in multiply lensed quasar systems, and may well take place in most quasars. Where this can be convincingly established it can be used to constrain the structure of the accretion disk and throw light on the nature of the microlensing bodies. Finally, we make some remarks regarding what is needed for future observations, and the need for model predictions which can be compared with observations in such a way that they can be eliminated, or their parameters refined.
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Color Variability of AGNs |
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2003MmSAI..74..963V |
Optical spectral variability of quasars and BL Lac Objects is compared by means of the spectral variability parameter beta \citep{tre02}. Both kinds of objects change their spectral slopes alpha , becoming bluer when brighter, but BL Lac Objects have smaller beta values and are clearly separated from quasars in the alpha -beta plane. Models accounting for the origin of the variability are discussed for both classes of objects.
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Optical Variability of AGNs Monitored by the Faint Sky Variability Survey |
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Huber, M. E.; Clowes, R. G.; Soechting, I. K.; Howell, S. B. |
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2002AAS...20114602H |
Variability in active galactic nuclei (AGN) can provide a means of studying the unresolved central engine, but a well time-sampled dataset is essential to provide comparison to physical models. Using the Faint Sky Variability Survey (FSVS) dataset, we investigate the optical variability on short to intermediate timescales (10s of minutes to 3 years) in the present set of 60 matched AGN and newly discovered sources.
Photometric analysis of point-like AGNs comprising a predominately radio-quiet set of quasars spanning 0.3--3.0 revealed a small fraction of sources (order of 10%) showing short, intranight and day to day variability with amplitudes of a few hundredths of a magnitude. The majority of the sources (order 80% and greater) showed longer, month to year variability up to amplitudes of half a magnitude. Quasars with low redshifts (0.3--0.5) and classified as point sources possess variability correlated with changes in the seeing profile due to the faint host galaxy. Both aperture and variable PSF photometry showed significant correlated variability amplitudes of 0.04 and 0.02 mag/arcsec respectively.
The applicability of the FSVS to study variability in AGNs in general will be discussed. In particular, relationships in variability timescales, radio strength, luminosity, and redshift will be addressed. The issue of spurious variability correlated with the seeing profile in low redshift quasar point sources will be explored. Correlated variability is of critical concern to studies to detect actual microvariability in quasars, but also presents a signature from the unresolved host galaxy in seemingly stellar sources.
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Radio Variability in Radio-Quiet Quasars and Low-Luminosity AGN |
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Falcke, Heino; Lehár, Joseph; Barvainis, Richard; Nagar, Neil M.; Wilson, Andrew S. |
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2001ASPC..224..265F |
We report on two surveys of radio-weak AGN to look for radio variability. We find significant variability with an RMS of 10-20% on a timescale of months in radio-quiet and radio-intermediate quasars. This exceeds the variability of radio cores in radio-loud quasars (excluding blazars), which vary only on a few percent level. The variability in radio-quiet quasars confirms that the radio emission in these sources is indeed related to the AGN. The most extremely variable source is the radio-intermediate quasar III Zw 2 which was recently found to contain a relativistic jet. In addition we find large amplitude variabilities (up to 300% peak-to-peak) in a sample of nearby low-luminosity AGN, Liners and dwarf-Seyferts, on a timescale of 1.5 years. The variability could be related to the activity of nuclear jets responding to changing accretion rates. Simultaneous radio/optical/X-ray monitoring also for radio-weak AGN, and not just for blazars, is therefore a potentially powerful tool to study the link between jets and accretion flows.
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Variability in X-ray absorption: a universal feature in AGNs |
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2000AAS...197.3907R |
We present an analysis of the variation of X-ray absorbing column density in a large sample of Seyfert 2 galaxies. We find that almost all of the sources with multiple X-ray observations show variability in NH on time scales from months to a few years. The lack of correlation between NH and soft X-ray light curves flux suggests that variability of NH is not due to changes in the photoionizing central emission, but most likely is related to variations of the amount of obscuring gas along the line of sight. We discuss the consequences of these results for models of the structure of the circumnuclear regions in AGNs.