Dependence of the Optical/Ultraviolet Variability on the Emission-Line Properties and Eddington Ratio in Active Galactic Nuclei

YL, Ai; Yuan, Weihua; Zhou, Hao; Wang, TG; Dong, Xiao-Bo; Wang, JG; Lu, H.

doi:10.1088/2041-8205/716/1/l31

Cited by 90 publications

(106 citation statements)

References 31 publications

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“…If L bol /L Edd is an (at least crude) estimate of the accretion rate, both weak emission lines (but strong Fe ii pseudo-continuum) and small variability amplitudes are the consequence of strong accretion. An anticorrelation with variability is plausible because, if the accretion rate is high, the region emitting the continuum observed in a given passband is larger and the variability amplitudes are consequently smaller than in the case of a low accretion rate (Ai et al 2010). Wilhite et al (2008) proposed that the accretion rate is high and the variability is small for optically young quasars, i.e., when they become visible in the optical.…”

Section: Special Quasar Typesmentioning

confidence: 99%

“…Numerous studies have shown that the optical/UV continuum variability of quasars correlates with various physical parameters such as time-lag, rest-frame wavelength, luminosity, radio properties, emission line properties, and the Eddington ratio (e.g., Ulrich et al 1997;Giveon 1999;Helfand 2001;Gaskell & Klimek 2003;Vanden Berk et al 2004;Rengstorf et al 2006;Wold et al 2007;Wilhite et al 2008;Bauer et al 2009;Ai et al 2010;MacLeod et al 2010;Schmidt et al 2010). Despite considerable progress in the description of quasar variability, there are still conflicting claims about these correlations and their physical interpretation is fraught with problems.…”

Section: Introductionmentioning

confidence: 99%

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Spectral variability of quasars from multi-epoch photometric data in the Sloan Digital Sky Survey Stripe 82

Meusinger

Hinze

Hoon

2010

A&A

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Aims. The study of the ensemble properties of the UV/optical broadband variability of quasars is hampered by the combined effects of the dependence of variability on timescale, rest-frame wavelength, and luminosity. Here, we present a new approach to analysing the dependence of quasar variability on rest-frame wavelengths. Methods. We exploited the spectral archive of the Sloan Digital Sky Survey (SDSS) to create a sample of over 9000 quasars in the Stripe 82. The quasar catalogue was matched with the Light Motion Curve Catalogue for SDSS Stripe 82 and first-order structure functions were computed from the lightcurves. The structure functions are used to create a variability indicator that is related to the same intrinsic timescales for all quasars (about 1 to 2 yr in the rest-frame). We study the variability ratios for adjacent SDSS filter bands as a function of redshift. A quantitative interpretation of these relations is provided by comparing with the results of simple Monte Carlo simulations of variable quasar spectra. Results. We confirm the well-known dependence of variability on time-lag; the best power-law fit of the sample-averaged structure function has a slope β = 0.31 ± 0.03. We also confirm that anti-correlations exist with luminosity, wavelength, and redshift, where the latter can be fully explained as a consequence of the former two dependencies. The variability ratios as a function of redshift resemble the corresponding colour index-redshift relations. While variability is almost always stronger in the bluer passband than in the redder, the variability ratio depends on whether strong emission lines contribute to either one band or the other. We find that the observed variability ratio-redshift relations are described well assuming that (a) the r.m.s. fluctuation of the quasar continuum flux follows a power law σ( f λ ) ∝ λ −2 (i.e., is bluer when brighter) and (b) the variability of the emission line flux is only ∼10% of that of the underlying continuum. These results, based upon the photometry of more than 8000 quasars, confirm the previous findings by Wilhite and collaborators for 315 quasars with repeated SDSS spectroscopy. Finally, we find that quasars with unusual spectra and weak emission lines tend to have less variability than conventional quasars. This trend is the opposite of that expected from the dilution effect of variability due to line emission and may be indicative of high Eddington ratios in these unusual quasars.

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Section: Special Quasar Typesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Spectral variability of quasars from multi-epoch photometric data in the Sloan Digital Sky Survey Stripe 82

Meusinger

Hinze

Hoon

2010

A&A

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“…The BWB trend has also been found in radio-quiet AGNs, for which the variability was found to be anti-correlated with the Eddington ratio (Pu et al 2006;Ai et al 2010). In radioquiet AGNs, the optical variability could be due to the variation in the accretion process, for example, the variation in the accretion rate (e.g., Li & Cao 2008), since the jet is either weak or absent.…”

Section: Ssrqs and Fsrqsmentioning

confidence: 67%

“…Both the long-term or short-term variabilities of radio-quiet AGNs have also been explored (e.g., Stalin et al 2004Stalin et al , 2005Gupta & Joshi 2005;Ai et al 2010). The accretion disk instabilities might explain the optical microvariability of radio quiet AGNs (e.g., Gopal-Krishna et al 1995; see also Gopal-Krishna et al 2003, for alternative scenario).…”

Section: Introductionmentioning

confidence: 99%

The optical variability of steep-spectrum radio quasars in the SDSS stripe 82 region

Gu¹,

2011

A&A

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Context. There have been many investigations of the optical variability of flat-spectrum radio quasars (FSRQs), but the optical variability of steep-spectrum radio quasars (SSRQs) has been poorly studied. Aims. We investigate the optical variability of SSRQs, comparing this with the optical variability of FSRQs. Methods. We measure the optical variability of 18 SSRQs and 15 FSRQs in the SDSS Stripe 82 region using SDSS DR7 released multi-epoch data covering about nine years. We determine the spectral index by fitting a power law to SDSS ugriz photometric data, and explore the relationship between this spectral index and source brightness. Results. For all SSRQs studied, we detect variations in r band flux of overall amplitude between 0.22 mag and 0.92 mag in different sources. Eight of 18 SSRQs display a bluer-when-brighter (BWB) trend. In contrast, the variability amplitude of 15 FSRQs in r band ranges from 0.18 to 0.97 mag, and 11 of 15 sources follow a BWB trend. For all SSRQs studied, we found an anti-correlation between the Eddington ratio and the variability amplitude in r band, which is similar to that in radio-quiet AGNs. This implies that the thermal emission from the accretion disk may be responsible for the variability in SSRQs, although the jet nonthermal emission cannot be excluded. In contrast, there is no correlation found for FSRQs, which implies that the mechanisms of variability in FSRQs may be different from that in SSRQs. The optical continuum variability of radio-loud broad absorption-line quasars (BALQs) are investigated for the first time here for two sources with steep radio spectra. Both radio-loud BALQs display variations with amplitudes of about 0.3 mag at r band. Their spectral variability all follow a BWB trend. In our previous analysis of a combined sample of 18 SSRQs and 44 FSRQs, we found that the line width of the broad Mg II emission line is broader for steeper radio spectral indices. It implies a disk-like broad line region (BLR) geometry may present in these quasars. In these 62 quasars, we found a ∼57% source percentage showing BWB trend in FSRQs, whereas it is ∼44% in SSRQs.

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“…To measure the variability amplitude in each filter band for each quasar, we adopt the formalism similar to that used in Ai et al (2010) and Sesar et al (2007):…”

Section: Variability Estimationmentioning

confidence: 99%

The correlations between optical variability and physical parameters of quasars in SDSS Stripe 82

Zuo¹,

Wu²,

Liu³

et al. 2012

Proc. IAU

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We investigate the optical variability of 7658 quasars from SDSS Stripe 82. Taking advantage of a larger sample and relatively more data points for each quasar, we estimate variability amplitudes and divide the sample into small bins of redshift, rest-frame wavelength, black hole mass, Eddington ratio and bolometric luminosity respectively, to investigate the relationships between variability and these parameters. An anti-correlation between variability and rest-frame wavelength is found. The variability amplitude of radio-quiet quasars shows almost no cosmological evolution, but that of radio-loud ones may weakly anticorrelate with redshift. In addition, variability increases as either luminosity or Eddington ratio decreases. However, the relationship between variability and black hole mass is uncertain; it is negative when the influence of Eddington ratio is excluded, but positive when the influence of luminosity is excluded. The intrinsic distribution of variability amplitudes for radio-loud and radio-quiet quasars are different. Both radio-loud and radio-quiet quasars exhibit a bluer-whenbrighter chromatism. Assuming that quasar variability is caused by variations of accretion rate, the Shakura-Sunyaev disk model can reproduce the tendencies of observed correlations between variability and rest-frame wavelength, luminosity as well as Eddington ratio, supporting that changes of accretion rate plays an important role in producing the observed optical variability. However, the predicted positive correlation between variability and black hole mass seems to be inconsistent with the observed negative correlation between them in small bins of Eddington ratio, which suggests that other physical mechanisms may still need to be considered in modifying the simple accretion disk model.

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Dependence of the Optical/Ultraviolet Variability on the Emission-Line Properties and Eddington Ratio in Active Galactic Nuclei

Cited by 90 publications

References 31 publications

Spectral variability of quasars from multi-epoch photometric data in the Sloan Digital Sky Survey Stripe 82

Spectral variability of quasars from multi-epoch photometric data in the Sloan Digital Sky Survey Stripe 82

The optical variability of steep-spectrum radio quasars in the SDSS stripe 82 region

The correlations between optical variability and physical parameters of quasars in SDSS Stripe 82

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