EUCLIA—Exploring the UV/Optical Continuum Lag in Active Galactic Nuclei. I. A Model without Light Echoing

Cai, Zhen-Yi; Wang, Junxian; Zhu, Fei-Fan; Sun, Mouyuan; Gu, Wei‐Min; Cao, Xinwu; Yuan, Feng

doi:10.3847/1538-4357/aab091

Cited by 39 publications

(32 citation statements)

References 110 publications

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“…As the intrinsic luminosity changes, the color would also change in response and is typically observed as being "bluerwhen-brighter" that has been widely found in quasar variabilities (e.g., Ruan et al 2014;Guo & Gu 2016), which could be well explained by the stochastic temperature fluctuation in an inhomogeneous accretion disk (e.g., Dexter & Agol 2011;Cai et al 2016Cai et al , 2018. Alternatively, the flux variability caused by variable extinction also displays a similar trend, since extinctions are more significant at shorter wavelengths (e.g., Calzetti et al 2000).…”

Section: Color Variabilitymentioning

confidence: 99%

On the origin of the dramatic spectral variability of WPVS 007

Wang

et al. 2019

Monthly Notices of the Royal Astronomical Society

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We report the discovery of large-amplitude mid-infrared variabilities (MIR; ∼ 0.3 mag) in the Wide-field Infrared Survey Explorer W1 and W2 bands of the low-luminosity narrow-line Seyfert 1 galaxy WPVS 007, which exhibits prominent and varying broadabsorption lines (BALs) with blueshifted velocity up to ∼ 14000 km s −1 . The observed significant MIR variability, the UV to optical color variabilities in the Swift bands that deviate from the predictions of pure dust attenuation models, and the fact that Swift light curves can be well fitted by the stochastic AGN variability model suggest that its observed flux variabilities in UV-optical-MIR bands should be intrinsic, rather than owing to variable dust extinction. Furthermore, the variations of BAL features (i.e., trough strength and maximum velocity) and continuum luminosity are concordant. Therefore, we propose that the BAL variability observed in WPVS 007 is likely induced by the intrinsic ionizing continuum variation, alternative to the rotating-torus model proposed in a previous work. The BAL gas in WPVS 007 might be in the lowionization state as traced by its weak N v BAL feature; as the ionizing continuum strengthens, the C iv and Si iv column densities also increase, resulting in stronger BALs and the emergence of high-velocity components of the outflow. The outflow launch radius might be as small as ∼ 8 × 10 −4 pc under the assumption of being radiatively driven, but a large-scale origin (e.g., torus) cannot be fully excluded because of the unknown effects from additional factors, e.g., the magnetic field.

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Section: Color Variabilitymentioning

confidence: 99%

On the origin of the dramatic spectral variability of WPVS 007

Wang

et al. 2019

Monthly Notices of the Royal Astronomical Society

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“…The original model proposed by Dexter & Agol (2011) is able to match the disk size with micro-lensing observations but larger than thin disk theory prediction. More excitingly, the revised inhomogeneous disk models by Cai et al (2016Cai et al ( , 2018 can further explain the observed timescale dependent color variations, and inter-band coordinations/lags without light echoing.…”

Section: The Underlying Physicsmentioning

confidence: 91%

An Intrinsic Link between Long-term UV/Optical Variations and X-Ray Loudness in Quasars

Kang

Wang

Cai

et al. 2018

ApJ

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Observations have shown that UV/optical variation amplitude of quasars depend on several physical parameters including luminosity, Eddington ratio, and likely also black hole mass. Identifying new factors which correlate with the variation is essential to probe the underlying physical processes. Combining ∼ ten years long quasar light curves from SDSS stripe 82 and X-ray data from Stripe 82X, we build a sample of X-ray detected quasars to investigate the relation between UV/optical variation amplitude (σ r ms ) and X-ray loudness. We find that quasars with more intense X-ray radiation (compared to bolometric luminosity) are more variable in UV/optical. Such correlation remains highly significant after excluding the effect of other parameters including luminosity, black hole mass, Eddington ratio, redshift, rest-frame wavelength (i.e., through partial correlation analyses). We further find the intrinsic link between X-ray loudness and UV/optical variation is gradually more prominent on longer timescales (up to 10 years in the observed frame), but tends to disappear at timescales < 100 days. This suggests a slow and long-term underlying physical process. The X-ray reprocessing paradigm, in which UV/optical variation is produced by a variable central X-ray emission illuminating the accretion disk, is thus disfavored. The discovery points to an interesting scheme that both the X-ray corona heating and UV/optical variation is quasars are closely associated with magnetic disc turbulence, and the innermost disc turbulence (where corona heating occurs) correlates with the slow turbulence at larger radii (where UV/optical emission is produced).

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“…Theoretically, disc thermal fluctuating models (Dexter & Agol 2011;Cai et al 2016Cai et al , 2018Cai et al , 2019Cai et al , 2020 associate the multi-wavelength variability to magnetic turbulence in the accretion disc. Note such fluctuation models indeed predict bluer averaged EUV SEDs than the standard thin disk model without temperature fluctuation, and the stronger turbulence the bluer the mean SED (see Fig.…”

Section: Discussionmentioning

confidence: 99%

More variable quasars have stronger emission lines

Kang,

Wang,

Cai

et al. 2021

Preprint

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The UV/optical variation, likely driven by accretion disc turbulence, is a defining characteristic of type 1 active galactic nuclei (AGNs) and quasars. In this work we investigate an interesting consequence of such turbulence using quasars in SDSS Stripe 82 for which the measurements of the UV/optical variability amplitude are available from ∼ 10 years long light curves. We discover positive correlations between UV/optical variability amplitude σ rms and equivalent widths of CIV, Mg II and [OIII]5007 emission lines. Such correlations remain statistically robust through partial correlation analyses, i.e., after controlling the effects of other variables including bolometric luminosity, central supermassive black hole mass, Eddington ratio and redshift. This, for the first time, indicates a causal link between disc turbulence and emission line production. We propose two potential underlying mechanisms both of which may be involved: 1) quasars with stronger disc turbulence have on average bluer/harder broadband SED, an expected effect of the disc thermal fluctuation model; 2) stronger disc turbulence could lead to launch of emission line regions with larger covering factors.

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EUCLIA—Exploring the UV/Optical Continuum Lag in Active Galactic Nuclei. I. A Model without Light Echoing

Cited by 39 publications

References 110 publications

On the origin of the dramatic spectral variability of WPVS 007

On the origin of the dramatic spectral variability of WPVS 007

An Intrinsic Link between Long-term UV/Optical Variations and X-Ray Loudness in Quasars

More variable quasars have stronger emission lines

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