AGN Light Curves Research Articles

Investigating the variability of Swift-BAT blazars with NICER

ABSTRACT We present results of X-ray spectral and time-domain variability analyses of four faint, ‘quiescent’ blazars from the Swift-BAT 105-month catalogue. We use observations from a recent, 5-month long NICER campaign, as well as archival BAT data. Variations in the 0.3–2 keV flux are detected on minute, ∼weekly, and monthly time-scales, but we find that the fractional variability Fvar on these time-scales is <25 per cent and decreases on longer time-scales, implying generally low-amplitude variability across all sources and showing very low variability on monthly time-scales ($F_{\rm var}\lesssim 13{{\ \rm per\ cent}}$), which is at odds with previous studies that show that blazars are highly variable in the X-rays on a wide range of time-scales. Moreover, we find that the flux variability on very short time-scales appears to be characterized by long periods of relative quiescence accompanied by occasional short bursts, against the relatively time-stationary nature of the variability of most other AGN light curves. Our analysis also shows that the broadband X-ray spectra (0.3–195 keV) of our sources can be described with different power-law models. As is the case with most blazars, we find that two sources (2MASS J09343014-1721215 and PKS 0312-770) are well-modelled with a simple power law, while the remaining two (1RXS J225146.9-320614 and PKS 2126-15) exhibit curvature in the form of a log-parabolic power law. We also find that, in addition to the continuum, PKS 2126-15 requires significant absorption at the soft X-rays (≲1 keV) to fully describe the observed curvature, possibly due to absorption from the intergalactic medium.

Using AGN light curves to map accretion disc temperature fluctuations

ABSTRACT We introduce a new model for understanding AGN continuum variability. We start from a Shakura–Sunyaev thin accretion disc with a steady-state radial temperature profile T(R) and assume that the variable flux is due to axisymmetric temperature perturbations δT(R, t). After linearizing the equations, we fit UV–optical AGN light curves to determine δT(R, t) for a sample of seven AGNs. We see a diversity of |δT/T| ∼ 0.1 fluctuation patterns which are not dominated by outgoing waves travelling at the speed of light as expected for the ‘lamppost’ model used to interpret disc reverberation mapping studies. Rather, the most common pattern resembles slow (v ≪ c) ingoing waves. An explanation for our findings is that these ingoing waves trigger central temperature fluctuations that act as a lamppost, producing lower amplitude temperature fluctuations moving outwards at the speed of light. The light curves are dominated by the lamppost signal – even though the temperature fluctuations are dominated by other structures with similar variability time-scales – because the discs exponentially smooth the contributions from the slower moving (v ≪ c) fluctuations to the observed light curves. This leads to light curves that closely resemble the expectations for a lamppost model but with the slow variability time-scales of the ingoing waves. This also implies that longer time-scale variability signals will increasingly diverge from lamppost models because the smoothing of slower moving waves steadily decreases as their period or spatial wavelength increases.

On the Role of the Hot Feedback Mode in Active Galactic Nuclei Feedback in an Elliptical Galaxy

Depending on the value of the accretion rate, black hole accretion is divided into cold and hot modes. The two modes have distinctly different physics and correspond to two feedback modes. Most previous feedback works either focus only on one mode, or the accretion physics is not always properly adopted. Here, by performing hydrodynamical numerical simulations of AGN feedback in an elliptical galaxy, we show that including both is important, and gives different results from including just one or the other. We specifically focus on the wind and radiation (but neglecting the jet) feedback in the hot mode, to explore whether this particular mode of feedback can play any important feedback role. For this aim, we have run two test models. In one model, we always adopt the cold mode no matter what value the accretion rate is; in another model, we turn off the AGN once it enters into the hot mode. We have calculated the AGN light curves, black hole growth, star formation, and AGN energy duty-cycle; and compared the results with the model in which the two modes are correctly included. Important differences are found. For example, if we were to adopt only the cold mode, the total mass of newly formed stars would become two orders of magnitude smaller, and the fraction of energy ejected within the high accretion regime (i.e., $L_{\rm BH} > 2\% L_{\rm Edd}$) would be too small to be consistent with observations. We have also investigated the respective roles of wind and radiation in the hot mode.

BLACK HOLE AND GALAXY COEVOLUTION FROM CONTINUITY EQUATION AND ABUNDANCE MATCHING

[abridged] We investigate the coevolution of galaxies and hosted supermassive black holes throughout the history of the Universe by a statistical approach based on the continuity equation and the abundance matching technique. Specifically, we present analytical solutions of the continuity equation without source term to reconstruct the supermassive black hole (BH) mass function from the AGN luminosity functions. Such an approach includes physically-motivated AGN lightcurves tested on independent datasets, which describe the evolution of the Eddington ratio and radiative efficiency from slim- to thin-disc conditions. We nicely reproduce the local estimates of the BH mass function, the AGN duty cycle as a function of mass and redshift, along with the Eddington ratio function and the fraction of galaxies with given stellar mass hosting an AGN with given Eddington ratio. We exploit the same approach to reconstruct the observed stellar mass function at different redshift from the UV and far-IR luminosity functions associated to star formation in galaxies. These results imply that the buildup of stars and BHs in galaxies occurs via in-situ processes, with dry mergers playing a marginal role at least for stellar masses < 3 10^11 M_sun and BH masses < 10^9 M_sun, where the statistical data are more secure and less biased by systematic errors. In addition, we develop an improved abundance matching technique to link the stellar and BH content of galaxies to the gravitationally dominant dark matter component. The resulting relationships constitute a testbed for galaxy evolution models, highlighting the complementary role of stellar and AGN feedback in the star formation process. Finally, the clustering properties of BHs and galaxies are found to be in full agreement with current observations, so further validating our results from the continuity equation.

Do theKeplerAGN light curves need reprocessing?

We gauge the impact of spacecraft-induced effects on the inferred variability properties of the light curve of the Seyfert 1 AGN Zw 229-15 observed by \Kepler. We compare the light curve of Zw 229-15 obtained from the Kepler MAST database with a re-processed light curve constructed from raw pixel data (Williams & Carini, 2015). We use the first-order structure function, $SF(\delta t)$, to fit both light curves to the damped power-law PSD of Kasliwal, Vogeley & Richards, 2015. On short timescales, we find a steeper log-PSD slope ($\gamma = 2.90$ to within $10$ percent) for the re-processed light curve as compared to the light curve found on MAST ($\gamma = 2.65$ to within $10$ percent)---both inconsistent with a damped random walk which requires $\gamma = 2$. The log-PSD slope inferred for the re-processed light curve is consistent with previous results (Carini & Ryle, 2012, Williams & Carini, 2015) that study the same re-processed light curve. The turnover timescale is almost identical for both light curves ($27.1$ and $27.5$~d for the reprocessed and MAST database light curves). Based on the obvious visual difference between the two versions of the light curve and on the PSD model fits, we conclude that there remain significant levels of spacecraft-induced effects in the standard pipeline reduction of the Kepler data. Re-processing the light curves will change the model inferenced from the data but is unlikely to change the overall scientific conclusion reached by Kasliwal et al. 2015---not all AGN light curves are consistent with the DRW.

KEPLERPHOTOMETRY OF FOUR RADIO-LOUD ACTIVE GALACTIC NUCLEI IN 2010-2012

We have used Kepler photometry to characterize variability in four radio-loud active galactic nuclei (three quasars and one object tentatively identified as a Seyfert 1.5 galaxy) on timescales from minutes to months, comparable to the light crossing time of the accretion disk around the central supermassive black hole or the base of the relativistic jet. Kepler's almost continuous observations provide much better temporal coverage than is possible from ground-based observations. We report the first such data analyzed for quasars. We have constructed power spectral densities using 8 Kepler quarters of long-cadence (30-minute) data for three AGN, 6 quarters for one AGN and 2 quarters of short-cadence (1-minute) data for all four AGN. On timescales longer than about 0.2-0.6 day, we find red noise with mean power-law slopes ranging from -1.8 to -1.2, consistent with the variability originating in turbulence either behind a shock or within an accretion disk. Each AGN has a range of red noise slopes which vary slightly by month and quarter of observation. No quasi-periodic oscillations of astrophysical origin were detected. We detected several days-long flares when brightness increased by 3% - 7% in two objects. No flares on timescales of minutes to hours were detected. Our observations imply that the duty cycle for enhanced activity in these radio-loud AGN is small. These well-sampled AGN light curves provide an impetus to develop more detailed models of turbulence in jets and instabilities in accretion disks.

The spatial distribution of X-ray selected AGN in theChandradeep fields: a theoretical perspective

We study the spatial distribution of X-ray selected AGN in the framework of hierarchical co-evolution of supermassive black holes and their host galaxies and dark matter haloes. To this end, we have applied the model developed by Croton et al.(2006), De Lucia & Blaizot(2007) and Marulli et al.(2008) to the output of the Millennium Run and obtained hundreds of realizations of past light-cones from which we have extracted realistic mock AGN catalogues that mimic the Chandra deep fields. We find that the model AGN number counts are in fair agreement with observations, except at fluxes <1e-15 erg/cm^2/s. The spatial two-point correlation function predicted by the model is well described by a power-law relation out to 20 Mpc/h, in close agreement with observations. Our model matches the correlation length r_0 of AGN in the Chandra Deep Field North but underestimates it in the Chandra Deep Field South. When fixing the slope to gamma = 1.4, as in Gilli et al. (2005), the statistical significance of the mismatch is 2-2.5 sigma, suggesting that the predicted cosmic variance, which dominates the error budget, may not account for the different correlation length of the AGN in the two fields. While our results are robust to changes in the model prescriptions for the AGN lightcurves, the luminosity dependence of the clustering is sensitive to the different lightcurve models adopted. However, irrespective of the model considered, the luminosity dependence of the AGN clustering in our mock fields seems to be weaker than in the real Chandra fields. The significance of this mismatch needs to be confirmed using larger datasets.

QUASARS ARE NOT LIGHT BULBS: TESTING MODELS OF QUASAR LIFETIMES WITH THE OBSERVED EDDINGTON RATIO DISTRIBUTION

We use the observed distribution of Eddington ratios as a function of supermassive black hole (BH) mass to constrain models of quasar/active galactic nucleus (AGN) lifetimes and light curves. Given the observed (well constrained) AGN luminosity function, a particular model for AGN light curves L(t) or, equivalently, the distribution of AGN lifetimes (time above a given luminosity t(>L)) translates directly and uniquely (without further assumptions) to a predicted distribution of Eddington ratios at each BH mass. Models for self-regulated BH growth, in which feedback produces a self-regulating decay or blowout phase after the AGN reaches some peak luminosity/BH mass and begins to expel gas and shut down accretion, make specific predictions for the light curves/lifetimes, distinct from, e.g., the expected distribution if AGN simply shut down by gas starvation (without feedback) and very different from the prediction of simple phenomenological light bulb scenarios. We show that the present observations of the Eddington ratio distribution, spanning nearly 5 orders of magnitude in Eddington ratio, 3 orders of magnitude in BH mass, and redshifts z = 0-1, agree well with the predictions of self-regulated models, and rule out phenomenological light bulb or pure exponential models, as well as gas starvation models, at high significance (~5?). We also compare with observations of the distribution of Eddington ratios at a given AGN luminosity, and find similar good agreement (but show that these observations are much less constraining). We fit the functional form of the quasar lifetime distribution and provide these fits for use, and show how the Eddington ratio distributions place precise, tight limits on the AGN lifetimes at various luminosities, in agreement with model predictions. We compare with independent estimates of episodic lifetimes and use this to constrain the shape of the typical AGN light curve, and provide simple analytic fits to these for use in other analyses. Given these constraints, the average local BH must have gained its mass in no more than a couple of bright, near peak-luminosity episodes, in agreement with models of accretion triggering in interactions and mergers.

A NEAR-INFRARED VARIABILITY STUDY OF THE GALACTIC BLACK HOLE: A RED NOISE SOURCE WITH NO DETECTED PERIODICITY

We present the results of near-infrared (2 and 3 microns) monitoring of Sgr A*-IR with 1 min time sampling using the natural and laser guide star adaptive optics (LGS AO) system at the Keck II telescope. Sgr A*-IR was observed continuously for up to three hours on each of seven nights, between 2005 July and 2007 August. Sgr A*-IR is detected at all times and is continuously variable, with a median observed 2 micron flux density of 0.192 mJy, corresponding to 16.3 magnitude at K'. These observations allow us to investigate Nyquist sampled periods ranging from about 2 minutes to an hour. Using Monte Carlo simulations, we find that the variability of Sgr A* in this data set is consistent with models based on correlated noise with power spectra having frequency dependent power law slopes between 2.0 to 3.0, consistent with those reported for AGN light curves. Of particular interest are periods of ~20 min, corresponding to a quasi-periodic signal claimed based upon previous near-infrared observations and interpreted as the orbit of a 'hot spot' at or near the last stable orbit of a spinning black hole. We find no significant periodicity at any time scale probed in these new observations for periodic signals. This study is sensitive to periodic signals with amplitudes greater than 20% of the maximum amplitude of the underlying red noise component for light curves with duration greater than ~2 hours at a 98% confidence limit.

On the nature of X-ray variability in Ark 564

We use data from a recent long ASCA observation of the Narrow Line Seyfert 1 galaxy Ark 564 to investigate in detail its timing properties. We show that a thorough analysis of the time series, employing techniques not generally applied to AGN light curves, can provide useful information to characterize the engines of these powerful sources. We searched for signs of non-stationarity in the data, but did not find strong evidences for it. We find that the process causing the variability is very likely nonlinear, suggesting that variability models based on many active regions, as the shot noise model, may not be applicable to Ark 564. The complex light curve can be viewed, for a limited range of time scales (as indicated by the breaks in the structure and power density spectrum), as a fractal object with non-trivial fractal dimension and statistical self-similarity. Finally, using a nonlinear statistic based on the scaling index as a tool to discriminate time series, we demonstrate that the high and low count rate states, which are indistinguishable on the basis of their autocorrelation, structure and probability density functions, are intrinsically different, with the high state characterized by higher complexity.

Relativistic Beaming and Flux Variability in Active Galactic Nuclei

We discuss the impact of special relativistic effects on the observed light curves and variability duty cycles of AGNs. We model the properties of AGN light curves at radio wavelengths using a simulated shot noise process in which the occurrence of major flaring events in a relativistic jet is governed by Poisson statistics. We show that flaring sources whose radiation is highly beamed toward us are able to reach very high flux levels, but will in fact spend most of their time in relatively low flaring states due to relativistic contraction of flare time scales in the observer frame. The fact that highly beamed AGNs do not return to a steady-state quiescent level between flares implies that their weakly beamed counterparts should have highly stable flux densities that result from a superposition of many long-term, low-amplitude flares. The ``apparent'' quiescent flux levels of these weakly beamed AGNs (identified in many unified models as radio galaxies) will be significantly higher than their ''true'' quiescent (i.e., non-flaring) levels. We use Monte Carlo simulations to investigate flux variability bias in the selection statistics of flat-spectrum AGN samples. In the case of the Caltech-Jodrell Flat-spectrum survey, the predicted orientation bias towards jets seen end-on is weakened if the parent population is variable, since the highly beamed sources have a stronger tendency to be found in low flaring states. This effect is small, however, since highly beamed sources are relatively rare, and their fluxes tend to be boosted sufficiently above the survey limit such that they are selected regardless of their flaring level. We find that for larger flat-spectrum AGN surveys with fainter flux cutoffs, variability should not be an appreciable source of selection bias.

X‐Ray Iron Line Reverberation from Black Hole Accretion Disks

The relativistically broad X-ray iron line seen in many AGN spectra is thought to originate from the central regions of the putative black hole accretion disk. Both the line profile and strength will vary in response to rapid variability of the primary X-ray continuum source. The temporal response of the line contains information on the accretion disk structure, the X-ray source geometry, and the spin of the black hole. Since the X-ray source will have a size comparable to the fluorescing region of the accretion disk, the general reverberation problem is not invertible. However, progress can be made since, empirically, AGN light curves are seen to undergo dramatic short timescale variability which presumably corresponds to the creation of a single new active region within the distributed X-ray source. The iron line response to these individual events can be described using linear transfer theory. We consider the line response to the activation/flaring of a new X-ray emitting region. Most of our detailed calculations are performed for the case of an X-ray source on the symmetry axis and at some height above the disk plane around a Kerr black hole. We also present preliminary calculations for off-axis flares. We suggest ways in which future, high-throughput X-ray observatories such as XMM and the Constellation X-ray Mission may use these reverberation signatures to probe both the mass and spin of AGN black holes, as well as the X-ray source geometry.

Quasar Variability and Gravitational Microlensing

In this Letter we examine the claim by Baganoff & Malkan that the timescale of brightness variations of quasars increases with decreasing optical wavelength, and that this effect can offset a time dilation effect at high redshift. They base their case on a thin accretion disk model that implies that timescales become shorter as the observed passband becomes bluer. We show that light curves of both Seyfert galaxies and quasars conflict with the predictions of their model and that neither the timescale nor the structure of AGN light curves changes with color. We discuss the consequences of this result for accretion disk models of quasars and the relation of our result to the microlensing mechanism for quasar variability. We also look at the implications of quasar microlensing for the nature of dark matter, and use some recent results on quark masses to support the idea that the dark matter is in the form of primordial black holes.