AGN variability time scales and the discrete-event model

Favre, Pascal; Courvoisier, T. J. L.; Paltani, S.

doi:10.1051/0004-6361:20041903

Cited by 14 publications

(16 citation statements)

References 29 publications

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“…Collier & Peterson (2001) studied 4 of the objects presented here and found a τ max significantly smaller in all cases for the optical and UV. The same applies for the AGN variability study performed by Favre et al (2005) using UV data, including 7 of the objects studied here. On the contrary, de Vries et al (2005) do not find a turn-over in optical lightcurves up to τ max ∼ 40 yr.…”

Section: Discussionmentioning

confidence: 99%

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Hard X-ray variability of active galactic nuclei

Beckmann

Barthelmy

Courvoisier

et al. 2007

A&A

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Aims. Active Galactic Nuclei are known to be variable throughout the electromagnetic spectrum. An energy domain poorly studied in this respect is the hard X-ray range above 20 keV. Methods. The first 9 months of the Swift/BAT all-sky survey are used to study the 14−195 keV variability of the 44 brightest AGN. The sources have been selected due to their detection significance of >10σ. We tested the variability using a maximum likelihood estimator and by analysing the structure function. Results. Probing different time scales, it appears that the absorbed AGN are more variable than the unabsorbed ones. The same applies for the comparison of Seyfert 2 and Seyfert 1 objects. As expected the blazars show stronger variability. 15% of the non-blazar AGN show variability of >20% compared to the average flux on time scales of 20 days, and 30% show at least 10% flux variation. All the non-blazar AGN which show strong variability are low-luminosity objects with L (14−195 keV) < 10 44 erg sConclusions. Concerning the variability pattern, there is a tendency of unabsorbed or type 1 galaxies being less variable than the absorbed or type 2 objects at hardest X-rays. A more solid anti-correlation is found between variability and luminosity, which has been previously observed in soft X-rays, in the UV, and in the optical domain.

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Section: Discussionmentioning

confidence: 99%

“…Applications to other data sets have been shown, e.g. by Hughes et al (1992), Paltani (1999), de Vries (2005, and Favre et al (2005). The structure function is a useful and simple to use tool in order to find characteristic time scales for the variations in a source.…”

Section: Structure Functionmentioning

confidence: 99%

Hard X-ray variability of active galactic nuclei

Beckmann

Barthelmy

Courvoisier

et al. 2007

A&A

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“…Two possibilities are envisaged: either the variability process does not follow a Poisson distribution or the individual events are not identical, with the latter hypothesis being more plausible. Indeed, Favre et al (2005) found a lack of correlation between event rate and object luminosity, suggesting that a larger total luminosity cannot only be explained by a greater number of identical events. This implies that the event energy cannot be the same for all events.…”

Section: Optical/uv Variabilitymentioning

confidence: 99%

“…Collier & Peterson (2001) derived a characteristic optical/UV time scale of ∼5-100 days in a sample of 10 AGNs, performing a structure function analysis. Studying the variability time scales in the framework of discrete-event models, Favre et al (2005) showed a lack of (strong) dependence of the event duration on the source luminosity, hence on central mass. In their sample, the average luminosity of the sources covered four orders of magnitude, while the event duration varied by only two orders of magnitude.…”

Section: Optical/uv Variabilitymentioning

confidence: 99%

X-ray variability time scales in active galactic nuclei

Ishibashi

Courvoisier

2009

A&A

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X-ray variability in Active Galactic Nuclei (AGN) is commonly analysed in terms of the Power Spectral Density (PSD). The break observed in the power spectrum can be interpreted as a characteristic X-ray variability time scale. Here we study variability properties within the framework of clumpy accretion flows, in which shocks between accreting elements account for the UV and X-ray emissions. We derive a characteristic X-ray time scale, τ X , and compare it with the measured PSD break time scale, T B . A quite good agreement is found in both magnitude and trend. In particular, the model dependence on black hole mass and accretion rate precisely reproduces the empirical relation obtained by McHardy et al. (2006). We suggest a possible physical interpretation of the break time scale and briefly discuss the related aspects of optical/UV variability and correlations between different wavelengths.

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“…In this case, the structure function is to be preferred over power spectral density (PSD) analysis (e.g. Hughes et al 1992;Collier & Peterson 2001;Favre et al 2005). The SF was first introduced by Simonetti et al (1985), and has since been used in various bands, including radio (e.g.…”

Section: The Structure Functionmentioning

confidence: 99%

Ensemble X-ray variability of active galactic nuclei from serendipitous source catalogues

Vagnetti

Turriziani

Trèvese

2011

A&A

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Context. The X-ray variability of the active galactic nuclei (AGN) has been most often investigated with studies of individual, nearby sources, and only a few ensemble analyses have been applied to large samples in wide ranges of luminosity and redshift. Aims. We aim to determine the ensemble variability properties of two serendipitously selected AGN samples extracted from the catalogues of XMM-Newton and Swift, with redshift between ∼0.2 and ∼4.5, and X-ray luminosities, in the 0.5-4.5 keV band, between ∼10 43 erg/s and ∼10 46 erg/s. Methods. We used the structure function (SF), which operates in the time domain, and allows for an ensemble analysis even when only a few observations are available for individual sources and the power spectral density (PSD) cannot be derived. The SF is also more appropriate than fractional variability and excess variance, because these parameters are biased by the duration of the monitoring time interval in the rest-frame, and therefore by cosmological time dilation. Results. We find statistically consistent results for the two samples, with the SF described by a power law of the time lag, approximately as SF ∝ τ 0.1 . We do not find evidence of the break in the SF, at variance with the case of lower luminosity AGNs. We confirm a strong anti-correlation of the variability with X-ray luminosity, accompanied by a change of the slope of the SF. We find evidence in support of a weak, intrinsic, average increase of X-ray variability with redshift. Conclusions. The change of amplitude and slope of the SF with X-ray luminosity provides new constraints on both single oscillator models and multiple subunit models of variability.

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AGN variability time scales and the discrete-event model

Cited by 14 publications

References 29 publications

Hard X-ray variability of active galactic nuclei

Hard X-ray variability of active galactic nuclei

X-ray variability time scales in active galactic nuclei

Ensemble X-ray variability of active galactic nuclei from serendipitous source catalogues

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