Quasar Variability Research Articles

The Variability Structure Function of the Highest-Luminosity Quasars on Short Timescales

Abstract The stochastic photometric variability of quasars is known to follow a random-walk phenomenology on emission timescales of months to years. Some high-cadence restframe optical monitoring in the past has hinted at a suppression of variability amplitudes on shorter timescales of a few days or weeks, opening the question of what drives the suppression and how it might scale with quasar properties. Here, we study a few thousand of the highest-luminosity quasars in the sky, mostly in the luminosity range of Lbol=[46.4, 47.3] and redshift range of z = [0.7, 2.4]. We use a dataset from the NASA/ATLAS facility with nightly cadence, weather permitting, which has been used before to quantify strong regularity in longer-term restframe-UV variability. As we focus on a careful treatment of short timescales across the sample, we find that a linear function is sufficient to describe the UV variability structure function. Although the result can not rule out the existence of breaks in some groups completely, a simpler model is usually favoured under this circumstance. In conclusion, the data is consistent with a single-slope random walk across restframe timescales of Δt = [10, 250] days.

Optical Variability of Gaia CRF3 Sources with Robust Statistics and the 5000 Most Variable Quasars

Using the light-curve time-series data for more than 11.7 million variable sources published in the Gaia Data Release 3, the average magnitudes, colors, and variability parameters have been computed for 0.836 million Gaia CRF objects, which are mostly quasars and active galactic nuclei (AGNs). To mitigate the effects of occasional flukes in the data, robust statistical measures have been employed: namely, the median, median absolute deviation, and Spearman correlation. We find that the majority of the CRF sources have moderate amplitudes of variability in the Gaia G band just below 0.1 mag. The heavy-tailed distribution of variability amplitudes (quantified as robust standard deviations) does not find a single analytical form, but is closer to Maxwell distribution with a scale of 0.078 mag. The majority of CRF sources have positive correlations between G magnitude and G BP−G RP colors, meaning that these quasars and AGNs become bluer when they are brighter. The variations in the G BP and G RP bands are also mostly positively correlated. Dependencies of all variability parameters with cosmological redshift are fairly flat for the more accurate estimates above redshift 0.7, while the median color shows strong systematic variations with redshift. Using a robust normalized score of magnitude deviations, a sample of the 5000 most variable quasars is selected and published. The intersection of this sample with the ICRF3 catalog shows a much higher rate of strongly variable quasars (mostly blazars) in ICRF3.

Identifying the electromagnetic counterparts of LISA massive black hole binaries in archival LSST data

ABSTRACT LSST will catalogue the light curves of up to 100 million quasars. Among these there can be $\sim$100 ultra-compact massive black hole (MBH) binaries, whose gravitational waves (GWs) can be detected 5–15 yr later by LISA. Here, we assume such a LISA detection occurred, and assess whether or not its electromagnetic (EM) counterpart can be identified as a periodic quasar in archival LSST data. We use the binary’s properties derived from the LISA waveform, including the evolution of its orbital frequency, its total mass, distance, and sky localization, to predict the redshift, magnitude, and historical periodicity of the quasar expected in the LSST data. We then use Monte Carlo simulations to compute the false alarm probability (FAP), i.e. the number of quasars in the LSST catalogue matching these properties by chance, based on the (extrapolated) quasar luminosity function, the cadence of LSST, and intrinsic ‘damped random walk’ quasar variability. We analyse four fiducial LISA binaries, with masses and redshifts of $(M_{\rm bin}/{\rm M_{\odot }},z) = (3\times 10^5,0.3)$, $(3\times 10^6,0.3)$, $(10^7,0.3)$, and $(10^7,1)$. While noise and aliasing due to LSST’s cadence produces false periodicities by chance, we find that the frequency chirp of the LISA source during the LSST observations washes out these noise peaks and allows the genuine source to stand out in appropriately scaled Lomb–Scargle periodograms. We find that all four fiducial binaries can be uniquely identified, with ${\rm FAP}\lt 10^{-5}$, a week or more before merger. This should enable follow-up EM observations targeting individual EM counterparts during their inspiral stage.

Nonlinear Characteristics of Radio Variability in Quasar 3C 446

Quasars are characterized by violent and large amplitude variability in all the observation wavelengths, and the analysis of optical variability is useful for developing theoretical models. Long-term optical variability data of quasar 3C 446 were collected from the University of Michigan Radio Astronomy Observatory database in the 4.8, 8.0, and 14.5 GHz radio bands from 1976 to 2012. Due to the complexity of the variability data, it's hard to study by the linear time series analysis methods. For learning more about non-linear characteristics of the temporal evolution of quasar variability, the Ensemble Empirical Mode Decomposition and nonlinear analysis are used to analyse chaotic dynamics, fractal properties, and periodicity. This paper focuses on whether there is a significant difference between the periodic and non-linear properties of the quasar variability before and after the removal of the periodic or chaotic components. It turns out that the variability of quasar 3C 446 in the radio bands consists of periodic, trend, and chaotic components, and the periodic and trend components are dominant. The periods of the variability after removing the chaotic and trend components are exactly the same as the periods of the original variability data, but the chaotic and fractal characteristics of the two are significantly different. The saturated correlation dimension indicates that the reconstruction of the dynamical system requires more independent parameters than the original optical variables after the removal of the periodic and trend components. The Kolmogorov entropy indicates that the loss of information is greater for the former than the latter, and the system is more chaotic and more complex. The Hurst value indicates that the self-similarity and long-range correlation are slightly stronger for the latter than the former.

Quasars as standard candles

A characteristic feature of quasars is the observed non-linear relationship between their monochromatic luminosities at rest-frame 2500 Å and 2 keV. This relationship is evident across all redshifts and luminosities and, due to its non-linearity, can be implemented to estimate quasar distances and construct a Hubble Diagram for quasars. Historically, a significant challenge in the cosmological application of this relation has been its high observed dispersion. Recent studies have demonstrated that this dispersion can be reduced by excluding biased objects from the sample. Nevertheless, the dispersion remains considerable (δ ∼ 0.20 dex), especially when compared to the Phillips relation for supernovae Ia. Given the absence of a comprehensive physical model for the relation, it remains unclear how much of the remaining dispersion is tied to the physical mechanism behind the relation itself and how much can be attributed to other factors, not addressed by the sample selection and by the choice of X-ray and UV indicators. Potential contributing factors include (i) the scatter produced by using X-ray photometric results instead of spectroscopic ones, (ii) the intrinsic variability of quasars, and (iii) the inclination of the accretion disc relative to our line of sight. In this study, we thoroughly examine these three factors and quantify their individual contributions to the observed dispersion. Based on our findings, we argue that the characteristic dispersion of the X-ray–UV luminosity relationship (which is attributable to the physical mechanism behind it) is likely below 0.06 dex. This result reinforces the validity of using quasars as standard candles and offers valuable insights for developing physical models of the X-ray/UV relation. Achieving such a low dispersion on large observed data sets is hardly feasible, due to the complexity of removing all the empirical contributions to the scatter. Nevertheless, we argue that high-redshift subsamples can show a significantly lower dispersion than the average subsample. This aspect is particularly significant for cosmological applications, indicating that targeted observations of select high-redshift objects can enhance the cosmological power of quasars in constraining the shape of the Hubble Diagram at high redshift.

Results of a long-term optical variability study of 11 quasars and VRI photometry of comparison stars

ABSTRACT We present the results of a 15-yr long-term optical monitoring of 11 quasars conducted with the 2-m Ritchey–Chretien–Coude and the 50/70 cm Schmidt telescopes at the Rozhen National Astronomical Observatory, Bulgaria. Our observations are performed with standard Johnson-Cousins $VRI$ band filters and for each quasar we present a set of comparison standard stars that can be used for monitoring of objects in these fields (including finding charts for the stars identification). The variability and periodicity of each quasar are analysed individually and discussed. The physical properties of each quasar, such as their classification, redshift, and radio structures, are also discussed based on previous literature. Damped random walk model shows the best or the second best fit to the light curves of all objects. However, in six cases periodic models provide comparably good fits and make these six objects a valuable addition to the growing sample of quasars with periodic flux variation. They will be suitable for further investigation of the hitherto unclear mechanisms that give rise to this variability pattern. Our results provide important insights into the long-term variability and physical properties of quasars, which can further deepen our understanding of the nature and evolution of active galaxy nuclei.

Ensemble power spectral density of SDSS quasars in UV/optical bands

Context. Quasar variability has proven to be a powerful tool to constrain the properties of their inner engine and the accretion process onto supermassive black holes. Correlations between UV/optical variability and physical properties have been long studied with a plethora of different approaches and time-domain surveys, although the detailed picture is not yet clear. Aims. We analysed archival data from the SDSS Stripe-82 region to study how the quasar power spectral density (PSD) depends on the black hole mass, bolometric luminosity, accretion rate, redshift, and rest-frame wavelength. We developed a model-independent analysis framework that could be easily applied to upcoming large surveys such as the Legacy Survey of Space and Time (LSST). Methods. We used light curves of 8042 spectroscopically confirmed quasars, observed in at least six yearly seasons in five filters ugriz. We split the sample into bins of similar physical properties containing at least 50 sources, and we measured the ensemble PSD in each of them. Results. We find that a simple power law is a good fit to the power spectra in the frequency range explored. Variability does not depend on the redshift at a fixed wavelength. Instead, both PSD amplitude and slope depend on the black hole mass, accretion rate, and rest-frame wavelength. We provide scaling relations to model the observed variability as a function of the physical properties, and discuss the possibility of a universal PSD shape for all quasars, where frequencies scale with the black hole mass, while normalization and slope(s) are fixed (at any given wavelength and accretion rate).

Reliable Identification of Binary Supermassive Black Holes from Rubin Observatory Time-domain Monitoring

Periodic signatures in time-domain observations of quasars have been used to search for binary supermassive black holes (SMBHs). These searches, across existing time-domain surveys, have produced several hundred candidates. The general stochastic variability of quasars, however, can masquerade as a false-positive periodic signal, especially when monitoring cadence and duration are limited. In this work, we predict the detectability of binary SMBHs in the upcoming Rubin Observatory Legacy Survey of Space and Time (LSST). We apply computationally inexpensive sinusoidal curve fits to millions of simulated LSST Deep Drilling Field light curves of both single, isolated quasars and binary quasars. The period and phase of simulated binary signals can generally be disentangled from quasar variability. Binary amplitude is overestimated and poorly recovered for two-thirds of potential binaries due to quasar accretion variability. Quasars with strong intrinsic variability can obscure a binary signal too much for recovery. We also find that the most luminous quasars mimic current binary candidate light curves and their properties: The false-positive rates are 60% for these quasars. The reliable recovery of binary period and phase for a wide range of input binary LSST light curves is promising for multi-messenger characterization of binary SMBHs. However, pure electromagnetic detections of binaries using photometric periodicity with amplitude greater than 0.1 mag will result in samples that are overwhelmed by false positives. This paper represents an important and computationally inexpensive way forward for understanding the true and false-positive rates for binary candidates identified by Rubin.

The universal power spectrum of quasars in optical wavelengths

Context. The optical variability of quasars is one of the few windows through which we can explore the behaviour of accretion discs around supermassive black holes. Aims. We aim to establish the dependence of variability properties, such as characteristic timescales and the variability amplitude, on basic quasar parameters such as black hole mass and the accretion rate, controlling for the rest-frame wavelength of emission. Methods. Using large catalogues of quasars, we selected the g-band light curves for 4770 objects from the Zwicky Transient Facility archive. All the selected objects fall into a narrow redshift bin, 0.6 < z < 0.7, but cover a wide range of accretion rates in Eddington units (REdd) and black hole masses (M). We grouped these objects into 26 independent bins according to these parameters, calculated low-resolution g-band variability power spectra for each of these bins, and approximated the power spectra with a simple analytic model that features a break at a timescale, tb. Results. We find a clear dependence of the break timescale, tb, on REdd, on top of the known dependence of tb on the black hole mass, M. In our fits, tb ∝ M0.65 − 0.55REdd0.35−0.3, where the ranges in the exponents correspond to the best-fitting parameters of different power spectrum models. This mass dependence is slightly steeper than that found in other studies. Scaling tb to the orbital timescale of the innermost stable circular orbit (ISCO), tISCO, results approximately in tb/tISCO ∝ (REdd/M)0.35. In the standard thin disc model, (REdd/M) ∝ Tmax4, where Tmax is the maximum disc temperature, so that tb/tISCO appears to scale approximately with the maximum temperature of the disc to a small power. The observed values of tb are ∼10 longer than the orbital timescale at the light-weighted average radius of the disc region emitting in the (observer frame) g-band. The different scaling of the break frequency with M and REdd shows that the shape of the variability power spectrum cannot be solely a function of the quasar luminosity, even for a single rest-frame wavelength. Finally, the best-fitting models have slopes above the break in the range between −2.5 and −3. A slope of −2, as in the damped random walk models, fits the data significantly worse.

Latent Stochastic Differential Equations for Modeling Quasar Variability and Inferring Black Hole Properties

Quasars are bright and unobscured active galactic nuclei (AGN) thought to be powered by the accretion of matter around supermassive black holes at the centers of galaxies. The temporal variability of a quasar’s brightness contains valuable information about its physical properties. The UV/optical variability is thought to be a stochastic process, often represented as a damped random walk described by a stochastic differential equation (SDE). Upcoming wide-field telescopes such as the Rubin Observatory Legacy Survey of Space and Time (LSST) are expected to observe tens of millions of AGN in multiple filters over a ten year period, so there is a need for efficient and automated modeling techniques that can handle the large volume of data. Latent SDEs are machine learning models well suited for modeling quasar variability, as they can explicitly capture the underlying stochastic dynamics. In this work, we adapt latent SDEs to jointly reconstruct multivariate quasar light curves and infer their physical properties such as the black hole mass, inclination angle, and temperature slope. Our model is trained on realistic simulations of LSST ten year quasar light curves, and we demonstrate its ability to reconstruct quasar light curves even in the presence of long seasonal gaps and irregular sampling across different bands, outperforming a multioutput Gaussian process regression baseline. Our method has the potential to provide a deeper understanding of the physical properties of quasars and is applicable to a wide range of other multivariate time series with missing data and irregular sampling.

Extreme Variability Quasars in Their Various States. II. Spectral Variation Revealed with Multiepoch Spectra

We previously built a sample of 14,012 extremely variable quasars (EVQs) based on Sloan Digital Sky Survey (SDSS) and Pan-STARRS1 photometric observations. In this work we present the spectral fitting to their SDSS spectra and study the spectral variation in 1259 EVQs with multiepoch SDSS spectra (after prudently excluding spectra with potentially unreliable spectroscopic photometry). We find a clear “bluer-when-brighter” trend in EVQs, consistent with previous findings of normal quasars and active galactic nuclei. We detect significant intrinsic Baldwin effect (iBeff, i.e., smaller line equivalent width at higher continuum flux in individual active galactic nuclei) in the broad Mg ii and C iv lines of EVQs. Meanwhile, no systematical iBeff is found for the broad Hβ line, which could be attributed to strong host contamination at longer wavelengths. Remarkably, by comparing the iBeff slope of EVQs with archived changing-look quasars, we show that the changing-look quasars identified in the literature are most likely a biased (due to its definition) subpopulation of EVQs, rather than a distinct population of quasars. We also found no significant broad line breathing of Hβ, Mg ii, or C iv, suggesting the broad line breathing in quasars may disappear at longer timescales (∼3000 days).

Probing the origin of the two-component structure of broad-line region by reverberation mapping of an extremely variable quasar

ABSTRACT The physical origins of quasar components, such as the broad-line region (BLR) and dust torus, remain under debate. To gain insights into them, we focused on changing-state quasars (CSQs) which provide a unique perspective through structural changes associated with accretion disc state transitions. We targeted SDSS J125809.31+351943.0, an extremely variable CSQ, to study its central core structure and kinematics. We conducted reverberation mapping with optical spectroscopy to explore the structure of the BLR and estimate the black hole mass. The results from Hβ reverberation mapping indicated a black hole mass of $10^{9.64^{+0.11}_{-0.20}}\rm {M_\odot }$. Additionally, we analysed variations in the optical to X-ray spectral indices, αox, before and after the state transition, to investigate the accretion disc. These variations in αox and the Eddington ratio (from 0.4 per cent to 2.4 per cent) exhibitied behaviour similar to state transitions observed in X-ray binary systems. Spectral analysis of Hβ revealed a predominantly double-peaked profile during dim periods, transitioning to include a single-peaked component as the quasar brightened, suggesting that Hβ contains a mixture of two components. Each of these components has its distinct characteristics: the first is a double-peaked profile that remains stable despite changes in the accretion rate, while the second is a variable single-peaked profile. Using time-lags from reverberation mapping, we estimated the spatial relationships between these BLR components, the accretion disc, and the dust torus. Our results suggest that the BLR consists of two distinct components, each differing in location and origin.

VaDAR: Varstrometry for Dual AGN Using Radio Interferometry

Binary and dual active galactic nuclei (AGNs) are an important observational tool for studying the formation and dynamical evolution of galaxies and supermassive black holes. An entirely new method for identifying possible AGN pairs makes use of the exquisite positional accuracy of Gaia to detect astrometrically variable quasars, in tandem with the high spatial resolution of the Karl G. Jansky Very Large Array (VLA). We present a new pilot study of radio observations of 18 quasars (0.8 ≤ z ≤ 2.9), selected from the Sloan Digital Sky Survey DR16Q and matched with the Gaia DR3. All 18 targets are identified by their excess astrometric noise in Gaia. We targeted these 18 quasars with the VLA at 2–4 GHz (S band) and 8–12 GHz (X band), providing resolutions of 0.″65 and 0.″2, respectively, in order to constrain the origin of this variability. We combine these data with ancillary radio survey data and perform radio spectral modeling. The new observations are used to constrain the driver of the excess astrometric noise. We find that ∼44% of the target sample is likely to be either candidate dual AGN or gravitationally lensed quasars. Ultimately, we use this new strategy to help identify and understand this sample of astrometrically variable quasars, demonstrating the potential of this method for systematically identifying kiloparsec-scale dual quasars.

Free-floating ‘planets’ in the macrolensed quasar Q2237+0305

ABSTRACT It has been claimed that the variability of field quasars resembles gravitational lensing by a large cosmological population of free-floating planets with mass $\sim\!\! 10\ {\rm M}_{\oplus }$. However, Galactic photometric monitoring experiments, on the other hand, exclude a large population of such planetary-mass gravitational lenses. These apparently contradictory pieces of evidence can be reconciled if the objects under consideration have a mean column density that lies between the critical column densities for gravitational lensing in these two contexts. Dark matter in that form is known to be weakly collisional, so a core develops in galaxy halo density profiles, and a preferred model has already been established. Here, we consider what such a model implies for Q2237+0305, which is the best-studied example of a quasar that is strongly lensed by an intervening galaxy. We construct microlensing magnification maps appropriate to the four macro-images of the quasar – all of which are seen through the bulge of the galaxy. Each of these maps exhibits a caustic network arising from the stars, plus many small, isolated caustics arising from the free-floating ‘planets’ in the lens galaxy. The ‘planets’ have little influence on the magnification histograms but a large effect on the statistics of the magnification gradients. We compare our predictions to the published Optical Gravitational Lensing Experiment (OGLE) photometry of Q2237+0305 and find that these data are consistent with the presence of the hypothetical ‘planets’. However, the evidence is relatively weak because the OGLE data set is not well suited to testing our predictions and requires low-pass filtering for this application. New data from a large, space-based telescope are desirable to address this issue.

SunPhot: Preparations for an Upcoming Quasar Variability Survey with the International Liquid Mirror Telescope

Recent research suggests a correlation between the variability and intrinsic brightness of quasars. If calibrated, this could lead to the use of quasars on the cosmic distance ladder, but this work is currently limited by lack of quasar light curve data with high cadence and precision. The Python photometric data pipeline SunPhot is being developed as part of preparations for an upcoming quasar variability survey with the International Liquid Mirror Telescope (ILMT). SunPhot uses aperture photometry to directly extract light curves for a catalogue of sources from calibrated ILMT images. SunPhot version 2.1 is operational, but the project is awaiting completion of ILMT commissioning.

Modeling quasar variability through self-organizing map-based neural process

Conditional Neural Process (QNPy) has shown to be a good tool for modeling quasar light curves. However, given the complex nature of the source and hence the data represented by light curves, processing could be time-consuming. In some cases, accuracy is not good enough for further analysis. In an attempt to upgrade QNPy, we examine the effect of the prepossessing quasar light curves via the Self-Organizing Map (SOM) algorithm on modeling a large number of quasar light curves. After applying SOM on the SWIFT/BAT data and modeling curves from several clusters, results show the Conditional Neural Process performs better after the SOM clustering. We conclude that the SOM clustering of quasar light curves could be a beneficial prepossessing method for QNPy.

NuSTAR Hard X-Ray Monitoring of Gravitationally Lensed Quasar RX J1131–1231

The X-ray emission from active galactic nuclei is believed to come from a combination of inverse Compton scattering of photons from the accretion disk and reprocessing of the direct X-ray emission by reflection. We present hard (10–80 keV) and soft (0.5–8 keV) X-ray monitoring of a gravitationally lensed quasar RX J1131−1231 (hereafter RXJ1131) with NuSTAR, Swift, and XMM-Newton between 2016 June 10 and 2020 November 30. Comparing the amplitude of quasar microlensing variability at the hard and soft bands allows a size comparison, where larger sources lead to smaller microlensing variability. During the period between 2018 June 6 and 2020 November 30, where both the hard and soft light curves are available, the hard and soft bands varied by factors of 3.7 and 5.5, respectively, with rms variability of 0.40 ± 0.05 and 0.57 ± 0.02. Both the variability amplitude and rms are moderately smaller for the hard X-ray emission, indicating that the hard X-ray emission is moderately larger than the soft X-ray emission region. We found the reflection fraction from seven joint hard and soft X-ray monitoring epochs is effectively consistent with a constant with low significance variability. After decomposing the total X-ray flux into direct and reprocessed components, we find a smaller variability amplitude for the reprocessed flux compared to the direct emission. The power-law cutoff energy is constrained at keV, which positions the system in the allowable parameter space due to the pair production limit.

The first comprehensive study of a giant nebula around a radio-quiet quasar in the z < 1 Universe

ABSTRACT We present the first comprehensive study of a giant, ≈70 kpc-scale nebula around a radio-quiet quasar at z < 1. The analysis is based on deep integral field spectroscopy with Multi-Unit Spectroscopic Explorer of the field of HE 0238−1904, a luminous quasar at z = 0.6282. The nebula emits strongly in [O ii], $\rm H \beta$, and [O iii], and the quasar resides in an unusually overdense environment for a radio-quiet system. The environment likely consists of two groups which may be merging, and in total have an estimated dynamical mass of Mdyn ≈ 4 × 1013 to 1014 M⊙. The nebula exhibits largely quiescent kinematics and irregular morphology. The nebula may arise primarily through interaction-related stripping of circumgalactic and interstellar medium (CGM/ISM) of group members, with some potential contributions from quasar outflows. The simultaneous presence of the giant nebula and a radio-quiet quasar in a rich environment suggests a correlation between such circum-quasar nebulae and environmental effects. This possibility can be tested with larger samples. The upper limits on the electron number density implied by the [O ii] doublet ratio range from $\log (n_{\rm e, [O\,{\small II}]} /\mathrm{cm}^{-3})<1.2$ to 2.8. However, assuming a constant quasar luminosity and negligible projection effects, the densities implied from the measured line ratios between different ions (e.g. [O ii], [O iii], and [Ne v]) and photoionization simulations are often 10−400 times larger. This large discrepancy can be explained by quasar variability on a time-scale of ≈104−105 yr.

A search for extragalactic fast optical transients in the Tomo-e Gozen high-cadence survey

ABSTRACT The population of optical transients evolving within a time-scale of a few hours or a day (so-called fast optical transients, FOTs) has recently been debated extensively. In particular, our understanding of extragalactic FOTs and their rates is limited. We present a search for extragalactic FOTs with the Tomo-e Gozen high-cadence survey. Using the data taken from 2019 August to 2022 June, we obtain 113 FOT candidates. Through light curve analysis and cross-matching with other survey data, we find that most of these candidates are in fact supernovae, variable quasars, and Galactic dwarf novae that were partially observed around their peak brightness. We find no promising candidate of extragalactic FOTs. From this non-detection, we obtain upper limits on the event rate of extragalactic FOTs as a function of their time-scale. For a very luminous event (absolute magnitude M < −26 mag), we obtain the upper limits of 4.4 × 10−9 Mpc−3 yr−1 for a time-scale of 4 h, and 7.4 × 10−10 Mpc−3 yr−1 for a time-scale of 1 d. Thanks to our wide (although shallow) surveying strategy, our data are less affected by the cosmological effects, and thus, give one of the more stringent limits to the event rate of intrinsically luminous transients with a time-scale of <1 d.

Optical variability in quasars: scalings with black hole mass and Eddington ratio depend on the observed time-scales

ABSTRACT Quasars emission is highly variable, and this variability gives us clues to understand the accretion process onto supermassive black holes. We can expect variability properties to correlate with the main physical properties of the accreting black hole, i.e. its mass and accretion rate. It has been established that the relative amplitude of variability anticorrelates with the accretion rate. The dependence of the variance on black hole mass has remained elusive, and contradicting results, including positive, negative, or no correlation, have been reported. In this work, we show that the key to these contradictions lies in the times-cales of variability studied (e.g. the length of the light curves available). By isolating the variance on different time-scales in well-defined mass and accretion rate bins we show that there is indeed a negative correlation between black hole mass and variance and that this anticorrelation is stronger for shorter time-scale fluctuations. The behaviour can be explained in terms of a universal variability power spectrum for all quasars, resembling a broken power law where the variance is constant at low temporal frequencies and then drops continuously for frequencies higher than a characteristic (break) frequency fb, where fb correlates with the black hole mass. Furthermore, to explain all the variance results presented here, not only the normalization of this power spectrum must anticorrelate with the accretion rate, but also the shape of the power spectra at short time-scales must depend on this parameter as well.