Constraints on the temperature inhomogeneity in quasar accretion discs from the ultraviolet-optical spectral variability

Kokubo, Mitsuru

doi:10.1093/mnras/stv241

Cited by 32 publications

(38 citation statements)

References 120 publications

(219 reference statements)

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“…That AGNs exhibit strong variability in a wide spectral range, covering radio, UV/optical, X-ray, and γ-ray wavelengths (Ulrich et al 1997) allows identification of AGNs on the basis of their variability properties. The onset of wide-area massive time-domain optical imaging surveys triggered a multitude of AGN variability studies with the aim of characterizing the optical variability and establishing a method for quasar selection (Kelly et al 2009(Kelly et al , 2011(Kelly et al , 2013Kozłowski et al 2010Kozłowski et al , 2011Kozłowski et al , 2012Kozłowski et al , 2013MacLeod et al 2010MacLeod et al , 2011MacLeod et al , 2012Schmidt et al 2010Schmidt et al , 2012Palanque-Delabrouille et al 2011;Butler & Bloom 2011;Kim et al 2011;Ruan et al 2012;Zuo et al 2012;Andrae et al 2013;Zu et al 2013;Morganson et al 2014;Sun et al 2014;Graham et al 2014;De Cicco et al 2015;Falocco et al 2015;Kokubo et al 2014;Kokubo 2015). These investigations confirmed the general picture that AGNs show non-periodic, stochastic flux variability occurring on timescales of several months to several years with a fractional amplitude of typically ∼10−20%.…”

Section: Introductionsupporting

confidence: 59%

Pan-STARRS1 variability of XMM-COSMOS AGN

Simm

Saglia

Salvato

et al. 2015

A&A

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Aims. Upcoming large area sky surveys like Euclid and eROSITA, which are dedicated to studying the role of dark energy in the expansion history of the Universe and the three-dimensional mass distribution of matter, crucially depend on accurate photometric redshifts. The identification of variable sources, such as active galactic nuclei (AGNs), and the achievable redshift accuracy for varying objects are important in view of the science goals of the Euclid and eROSITA missions. Methods. We probe AGN optical variability for a large sample of X-ray-selected AGNs in the XMM-COSMOS field, using the multiepoch light curves provided by the Pan-STARRS1 (PS1) 3π and Medium Deep Field surveys. To quantify variability we employed a simple statistic to estimate the probability of variability and the normalized excess variance to measure the variability amplitude. Utilizing these two variability parameters, we defined a sample of varying AGNs for every PS1 band. We investigated the influence of variability on the calculation of photometric redshifts by applying three different input photometry sets for our fitting procedure. For each of the five PS1 bands g P1 , r P1 , i P1 , z P1 , and y P1 , we chose either the epochs minimizing the interval in observing time, the median magnitude values, or randomly drawn light curve points to compute the redshift. In addition, we derived photometric redshifts using PS1 photometry extended by GALEX/IRAC bands. Results. We find that the photometry produced by the 3π survey is sufficient to reliably detect variable sources provided that the fractional variability amplitude is at least ∼3%. Considering the photometric redshifts of variable AGNs, we observe that minimizing the time spacing of the chosen points yields superior photometric redshifts in terms of the percentage of outliers (33%) and accuracy (0.07), outperforming the other two approaches. Drawing random points from the light curve gives rise to typically 57% of outliers and an accuracy of ∼0.4. Adding GALEX/IRAC bands for the redshift determination weakens the influence of variability. Although the redshift quality generally improves when adding these bands, we still obtain not less than 26% of outliers and an accuracy of 0.05 at best, therefore variable sources should receive a flag stating that their photometric redshifts may be low quality.

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Section: Introductionsupporting

confidence: 59%

Pan-STARRS1 variability of XMM-COSMOS AGN

Simm

Saglia

Salvato

et al. 2015

A&A

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“…Although it is possible to predict correlations between the variability amplitude and the AGN parameters (also an anticorrelation with λ Edd ) within the standard α-disk prescription (Shakura & Sunyaev 1973) by assuming global fluctuations ofṀ (e.g., Zuo et al 2012;Meusinger & Weiss 2013), the resulting scaling relations are much flatter than the observed ones. What is more, the typical timescales of optical variability are much shorter than the time needed for global changes of the mass accretion rate in the entire disk, associated with the sound crossing or viscous timescale (Courvoisier & Clavel 1991;Uttley & Casella 2014;Ruan et al 2014;Kokubo 2015). For this reason it is unlikely that global accretion rate changes are the sole driver of optical variability.…”

Section: Discussionmentioning

confidence: 99%

“…Since the autocovariance function is coupled to the PSD by a Fourier transform, the latter can be expressed as a weighted sum of Lorentzian functions, which are known to provide a good description of the PSDs of X-ray binaries and AGNs (Nowak 2000;Belloni et al 2002;Belloni 2010;De Marco et al 2013, 2015. The CARMA model includes the Ornstein-Uhlenbeck process or the "damped random walk", which is depicted in detail in Kelly et al (2009) and was found to accurately describe quasar light curves in many subsequent works, as the special case of p = 1 and q = 0.…”

Section: Measuring σ 2 Rms On Different Timescalesmentioning

confidence: 99%

Pan-STARRS1 variability of XMM-COSMOS AGN

Simm

Salvato

Saglia

et al. 2016

A&A

130

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Aims. The goal of this work is to better understand the correlations between the rest-frame UV/optical variability amplitude of quasistellar objects (QSOs) and physical quantities such as redshift, luminosity, black hole mass, and Eddington ratio. Previous analyses of the same type found evidence for correlations between the variability amplitude and these active galactic nucleus (AGN) parameters. However, most of the relations exhibit considerable scatter, and the trends obtained by various authors are often contradictory. Moreover, the shape of the optical power spectral density (PSD) is currently available for only a handful of objects. Methods. We searched for scaling relations between the fundamental AGN parameters and rest-frame UV/optical variability properties for a sample of ∼90 X-ray selected AGNs covering a wide redshift range from the XMM-COSMOS survey, with optical light curves in four bands (g P1 , r P1 , i P1 , z P1 ) provided by the Pan-STARRS1 (PS1) Medium Deep Field 04 survey. To estimate the variability amplitude, we used the normalized excess variance (σ 2 rms ) and probed variability on rest-frame timescales of several months and years by calculating σ 2 rms from different parts of our light curves. In addition, we derived the rest-frame optical PSD for our sources using continuous-time autoregressive moving average (CARMA) models. Results. We observe that the excess variance and the PSD amplitude are strongly anticorrelated with wavelength, bolometric luminosity, and Eddington ratio. There is no evidence for a dependency of the variability amplitude on black hole mass and redshift. These results suggest that the accretion rate is the fundamental physical quantity determining the rest-frame UV/optical variability amplitude of quasars on timescales of months and years. The optical PSD of all of our sources is consistent with a broken power law showing a characteristic bend at rest-frame timescales ranging between ∼100 and ∼300 days. The break timescale exhibits no significant correlation with any of the fundamental AGN parameters. The low-frequency slope of the PSD is consistent with a value of −1 for most of our objects, whereas the high-frequency slope is characterized by a broad distribution of values between ∼-2 and ∼-4. These findings unveil significant deviations from the simple damped random walk model that has frequently been used in previous optical variability studies. We find a weak tendency for AGNs with higher black hole mass to have steeper high-frequency PSD slopes.

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“…While an inhomogeneous disk has been demonstrated to fit composite difference spectra (Ruan et al 2014) and color variability of quasars (Schmidt et al 2012), Hung et al (2016) find a simple disk model adequate to fit differenceflux UV/optical spectral energy distributions of individual quasars. Moreover, Kokubo (2015) argues that the tight interband correlations observed in SDSS quasar light curves are inconsistent with the inhomogeneous disk model. However, whether or not these thermal fluctuations can be coherent enough to produce a large-amplitude outburst is still to be determined.…”

Section: Accretion Disk Instabilitiesmentioning

confidence: 99%

iPTF Discovery of the Rapid “Turn-on” of a Luminous Quasar

Gezari

Hung

Cenko

et al. 2017

ApJ

137

175

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We present a radio-quiet quasar at z=0.237 discovered "turning on" by the intermediate Palomar Transient Factory (iPTF). The transient, iPTF 16bco, was detected by iPTF in the nucleus of a galaxy with an archival Sloan Digital Sky Survey spectrum with weak narrow-line emission characteristic of a low-ionization nuclear emissionline region (LINER). Our follow-up spectra show the dramatic appearance of broad Balmer lines and a power-law continuum characteristic of a luminous ( » L 10 bol 45 erg s −1 ) type 1 quasar 12 yr later. Our photometric monitoring with PTF from 2009-2012 and serendipitous X-ray observations from the XMM-Newton Slew Survey in 2011 and 2015 constrain the change of state to have occurred less than 500 days before the iPTF detection. An enhanced broad Hα/[O III] λ5007 line ratio in the type 1 state relative to other changing-look quasars also is suggestive of the most rapid change of state yet observed in a quasar. We argue that the >10 increase in Eddington ratio inferred from the brightening in UV and X-ray continuum flux is more likely due to an intrinsic change in the accretion rate of a preexisting accretion disk than an external mechanism such as variable obscuration, microlensing, or the tidal disruption of a star. However, further monitoring will be helpful in better constraining the mechanism driving this change of state. The rapid "turn-on" of the quasar is much shorter than the viscous infall timescale of an accretion disk and requires a disk instability that can develop around a  M 10 8 black hole on timescales less than 1 yr.

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Constraints on the temperature inhomogeneity in quasar accretion discs from the ultraviolet-optical spectral variability

Cited by 32 publications

References 120 publications

Pan-STARRS1 variability of XMM-COSMOS AGN

Pan-STARRS1 variability of XMM-COSMOS AGN

Pan-STARRS1 variability of XMM-COSMOS AGN

iPTF Discovery of the Rapid “Turn-on” of a Luminous Quasar

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