arXiv subjects

Gordon T. Richards

Publications and source records attributed to Gordon T. Richards.

At least 73 records · Page 4Linked to original sources

arXiv2018

Winds as the origin of radio emission in $z=2.5$ radio-quiet extremely red quasars

Most active galactic nuclei (AGNs) are radio-quiet, and the origin of their radio emission is not well-understood. One hypothesis is that this radio emission is a by-product of quasar-driven winds. In this paper, we present the radio properties of 108 extremely red quasars (ERQs) at $z=2-4$. ERQs are among the most luminous quasars ($L_{bol} \sim 10^{47-48}$ erg/s) in the Universe, with signatures of extreme ($\gg 1000$ km/s) outflows in their [OIII]$λ$5007 Å emission, making them the best subjects to seek the connection between radio and outflow activity. All ERQs but one are unresolved in the radio on $\sim 10$ kpc scales, and the median radio luminosity of ERQs is $νL_ν[{\rm 6\,GHz}] = 10^{41.0}$ erg/s, in the radio-quiet regime, but one to two orders of magnitude higher than that of other quasar samples. The radio spectra are steep, with a mean spectral index $\langle α\rangle = -1.0$. In addition, ERQs neatly follow the extrapolation of the low-redshift correlation between radio luminosity and the velocity dispersion of [OIII]-emitting ionized gas. Uncollimated winds, with a power of one per cent of the bolometric luminosity, can account for all these observations. Such winds would interact with and shock the gas around the quasar and in the host galaxy, resulting in acceleration of relativistic particles and the consequent synchrotron emission observed in the radio. Our observations support the picture in which ERQs are signposts of extremely powerful episodes of quasar feedback, and quasar-driven winds as a contributor of the radio emission in the intermediate regime of radio luminosity $νL_ν= 10^{39}-10^{42}$ erg/s.

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arXiv2018

The Sloan Digital Sky Survey Reverberation Mapping Project: the CIV Blueshift, Its Variability, and Its Dependence Upon Quasar Properties

We use the multi-epoch spectra of 362 quasars from the Sloan Digital Sky Survey Reverberation Mapping project to investigate the dependence of the blueshift of CIV relative to MgII on quasar properties. We confirm that high-blueshift sources tend to have low CIV equivalent widths (EWs), and that the low-EW sources span a range of blueshift. Other high-ionization lines, such as HeII, also show similar blueshift properties. The ratio of the line width (measured as both the full-width at half maximum and the velocity dispersion) of CIV to that of MgII increases with blueshift. Quasar variability might enhance the connection between the CIV blueshift and quasar properties (e.g., EW). The variability of the MgII line center (i.e., the wavelength that bisects the cumulative line flux) increases with blueshift. In contrast, the CIV line center shows weaker variability at the extreme blueshifts. Quasars with the high-blueshift CIV lines tend to have less variable continuum emission, when controlling for EW, luminosity, and redshift. Our results support the scenario that high-blueshift sources tend to have large Eddington ratios.

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arXiv2017

Extracting Information from AGN Variability

AGN exhibit rapid, high amplitude stochastic flux variations across the entire electromagnetic spectrum on timescales ranging from hours to years. The cause of this variability is poorly understood. We present a Green's Function-based method for using variability to (1) measure the time-scales on which flux perturbations evolve and (2) characterize the driving flux perturbations. We model the observed light curve of an AGN as a linear differential equation driven by stochastic impulses. We analyze the light curve of the Kepler AGN Zw 229-15 and find that the observed variability behavior can be modeled as a damped harmonic oscillator perturbed by a colored noise process. The model powerspectrum turns over on time-scale $385$~d. On shorter time-scales, the log-powerspectrum slope varies between $2$ and $4$, explaining the behavior noted by previous studies. We recover and identify both the $5.6$~d and $67$~d timescales reported by previous work using the Green's Function of the C-ARMA equation rather than by directly fitting the powerspectrum of the light curve. These are the timescales on which flux perturbations grow, and on which flux perturbations decay back to the steady-state flux level respectively. We make the software package KALI used to study light curves using our method available to the community.

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arXiv2017

Correcting CIV-Based Virial Black Hole Masses

The CIV broad emission line is visible in optical spectra to redshifts exceeding z~5. CIV has long been known to exhibit significant displacements to the blue and these `blueshifts' almost certainly signal the presence of strong outflows. As a consequence, single-epoch virial black hole (BH) mass estimates derived from CIV velocity-widths are known to be systematically biased compared to masses from the hydrogen Balmer lines. Using a large sample of 230 high-luminosity (log $L_{\rm Bol}$ = 45.5-48 erg/s), redshift 1.5 5000 km/s. Using the monotonically increasing relationship between the CIV blueshift and the mass ratio BH(CIV)/BH(H$α$) we derive an empirical correction to all CIV BH-masses. The scatter between the corrected CIV masses and the Balmer masses is 0.24 dex at low CIV blueshifts (~0 km/s) and just 0.10 dex at high blueshifts (~3000 km/s), compared to 0.40 dex before the correction. The correction depends only on the CIV line properties - i.e. full-width at half maximum and blueshift - and can therefore be applied to all quasars where CIV emission line properties have been measured, enabling the derivation of un-biased virial BH mass estimates for the majority of high-luminosity, high-redshift, spectroscopically confirmed quasars in the literature.

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arXiv2016

The Sloan Digital Sky Survey Reverberation Mapping Project: Velocity Shifts of Quasar Emission Lines

Quasar emission lines are often shifted from the systemic velocity due to various dynamical and radiative processes in the line-emitting region. The level of these velocity shifts depends both on the line species and on quasar properties. We study velocity shifts for the line peaks of various narrow and broad quasar emission lines relative to systemic using a sample of 849 quasars from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. The coadded (from 32 epochs) spectra of individual quasars have sufficient signal-to-noise ratio (SNR) to measure stellar absorption lines to provide reliable systemic velocity estimates, as well as weak narrow emission lines. The sample also covers a large dynamic range in quasar luminosity (~2 dex), allowing us to explore potential luminosity dependence of the velocity shifts. We derive average line peak velocity shifts as a function of quasar luminosity for different lines, and quantify their intrinsic scatter. We further quantify how well the peak velocity can be measured for various lines as a function of continuum SNR, and demonstrate there is no systematic bias in the line peak measurements when the spectral quality is degraded to as low as SNR~3 per SDSS pixel. Based on the observed line shifts, we provide empirical guidelines on redshift estimation from [OII]3728, [OIII]5008, [NeV]3426, MgII, CIII], HeII1640, broad Hbeta, CIV, and SiIV, which are calibrated to provide unbiased systemic redshifts in the mean, but with increasing intrinsic uncertainties of 46, 56, 119, 205, 233, 242, 400, 415, and 477 km/s, in addition to the measurement uncertainties. These more realistic redshift uncertainties are generally much larger than the formal uncertainties reported by the redshift pipelines for spectroscopic quasar surveys, and demonstrate the infeasibility of measuring quasar redshifts to better than ~200 km/s with only broad lines.

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arXiv2016

Extremely Red Quasars in BOSS

Red quasars are candidate young objects in an early transition stage of massive galaxy evolution. Our team recently discovered a population of extremely red quasars (ERQs) in the Baryon Oscillation Spectroscopic Survey (BOSS) that has a suite of peculiar emission-line properties including large rest equivalent widths (REWs), unusual "wingless" line profiles, large NV/Lya, NV/CIV, SiIV/CIV and other flux ratios, and very broad and blueshifted [OIII] 5007. Here we present a new catalog of CIV and NV emission-line data for 216,188 BOSS quasars to characterize the ERQ line properties further. We show that they depend sharply on UV-to-mid-IR color, secondarily on REW(CIV), and not at all on luminosity or the Baldwin Effect. We identify a "core" sample of 97 ERQs with nearly uniform peculiar properties selected via i-W3 > 4.6 (AB) and REW(CIV) > 100 A at redshifts 2.0-3.4. A broader search finds 235 more red quasars with similar unusual characteristics. The core ERQs have median luminosity log L (ergs/s) ~ 47.1, sky density 0.010 deg^-2, surprisingly flat/blue UV spectra given their red UV-to-mid-IR colors, and common outflow signatures including BALs or BAL-like features and large CIV emission-line blueshifts. Their SEDs and line properties are inconsistent with normal quasars behind a dust reddening screen. We argue that the core ERQs are a unique obscured quasar population with extreme physical conditions related to powerful outflows across the line-forming regions. Patchy obscuration by small dusty clouds could produce the observed UV extinctions without substantial UV reddening.

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arXiv2016

CIV Emission Line Properties and Systematic Trends in Quasar Black Hole Mass Estimates

Black-hole masses are crucial to understanding the physics of the connection between quasars and their host galaxies and measuring cosmic black hole-growth. At high redshift, z > 2.1, black hole masses are normally derived using the velocity-width of the CIV broad emission line, based on the assumption that the observed velocity-widths arise from virial-induced motions. In many quasars, the CIV-emission line exhibits significant blue asymmetries (`blueshifts') with the line centroid displaced by up to thousands of km/s to the blue. These blueshifts almost certainly signal the presence of strong outflows, most likely originating in a disc wind. We have obtained near-infrared spectra, including the H$α$ emission line, for 19 luminous ($L_{Bol}$ = 46.5-47.5 erg/s) Sloan Digital Sky Survey quasars, at redshifts 2 < z < 2.7, with CIV emission lines spanning the full-range of blueshifts present in the population. A strong correlation between CIV-velocity width and blueshift is found and, at large blueshifts, > 2000 km/s, the velocity-widths appear to be dominated by non-virial motions. Black-hole masses, based on the full width at half maximum of the CIV-emission line, can be overestimated by a factor of five at large blueshifts. A larger sample of quasar spectra with both CIV and H$β$, or H$α$, emission lines will allow quantitative corrections to CIV-based black-hole masses as a function of blueshift to be derived. We find that quasars with large CIV blueshifts possess high Eddington luminosity ratios and that the fraction of high-blueshift quasars in a flux-limited sample is enhanced by a factor of approximately four relative to a sample limited by black hole mass.

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arXiv2016

SpIES: The Spitzer IRAC Equatorial Survey

We describe the first data release from the Spitzer-IRAC Equatorial Survey (SpIES); a large-area survey of 115 deg^2 in the Equatorial SDSS Stripe 82 field using Spitzer during its 'warm' mission phase. SpIES was designed to probe sufficient volume to perform measurements of quasar clustering and the luminosity function at z > 3 to test various models for "feedback" from active galactic nuclei (AGN). Additionally, the wide range of available multi-wavelength, multi-epoch ancillary data enables SpIES to identify both high-redshift (z > 5) quasars as well as obscured quasars missed by optical surveys. SpIES achieves 5σ depths of 6.13 μJy (21.93 AB magnitude) and 5.75 μJy (22.0 AB magnitude) at 3.6 and 4.5 microns, respectively - depths significantly fainter than WISE. We show that the SpIES survey recovers a much larger fraction of spectroscopically-confirmed quasars (98%) in Stripe 82 than are recovered by WISE (55%). This depth is especially powerful at high-redshift (z > 3.5), where SpIES recovers 94% of confirmed quasars, whereas WISE only recovers 25%. Here we define the SpIES survey parameters and describe the image processing, source extraction, and catalog production methods used to analyze the SpIES data. In addition to this survey paper, we release 234 images created by the SpIES team and three detection catalogs: a 3.6 μm-only detection catalog containing 6.1 million sources, a 4.5 μm-only detection catalog containing 6.5 million sources, and a dual-band detection catalog containing 5.4 million sources.

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arXiv2016

Insights into Quasar UV Spectra Using Unsupervised Clustering Analysis

Machine learning can provide powerful tools to detect patterns in multi-dimensional parameter space. We use K-means -a simple yet powerful unsupervised clustering algorithm which picks out structure in unlabeled data- to study a sample of quasar UV spectra from the Quasar Catalog of the 10th Data Release of the Sloan Digital Sky Survey of Paris et al. (2014). Detecting patterns in large datasets helps us gain insights into the physical conditions and processes giving rise to the observed properties of quasars. We use K-means to find clusters in the parameter space of the equivalent width (EW), the blue- and red-half-width at half-maximum (HWHM) of the Mg II 2800 A line, the C IV 1549 A line, and the C III] 1908 A blend in samples of Broad Absorption-Line (BAL) and non-BAL quasars at redshift 1.6-2.1. Using this method, we successfully recover correlations well-known in the UV regime such as the anti-correlation between the EW and blueshift of the C IV emission line and the shape of the ionizing Spectra Energy distribution (SED) probed by the strength of He II and the Si III]/C III] ratio. We find this to be particularly evident when the properties of C III] are used to find the clusters, while those of Mg II proved to be less strongly correlated with the properties of the other lines in the spectra such as the width of C IV or the Si III]/C III] ratio. We conclude that unsupervised clustering methods (such as K-means) are powerful methods for finding "natural" binning boundaries in multidimensional datasets and discuss caveats and future work.

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arXiv2016

Extremely Red Quasars from SDSS, BOSS and WISE: Classification of Optical Spectra

Quasars with extremely red infrared-to-optical colours are an interesting population that can test ideas about quasar evolution as well as orientation, obscuration and geometric effects in the so-called AGN unified model. To identify such a population we match the quasar catalogues of the Sloan Digital Sky Survey (SDSS), the Baryon Oscillation Spectroscopic Survey (BOSS) to the Wide-Field Infrared Survey Explorer (WISE) to identify quasars with extremely high infrared-to-optical ratios. We identify 65 objects with r(AB)-W4(Vega)>14 mag (i.e., F_nu(22um)/F_nu(r) > ~1000). This sample spans a redshift range of 0.28 2.6 objects that are detected in the W4-band but not W1 or W2 (i.e., W1W2-dropouts). The SDSS/BOSS spectra show that the majority of the objects are reddened Type 1 quasars, Type 2 quasars (both at low and high redshift) or objects with deep low-ionization broad absorption lines (BALs) that suppress the observed r-band flux. In addition, we identify a class of Type 1 permitted broad-emission line objects at z~2-3 which are characterized by emission line rest-frame equivalent widths (REWs) of >~150Ang , much larger than those of typical quasars. In particular, 55% (45%) of the non-BAL Type 1s with measurable CIV in our sample have REW(CIV) > 100 (150)Ang, compared to only 5.8% (1.3%) for non-BAL quasars in BOSS. These objects often also have unusual line ratios, such as very high NV/Ly-alpha ratios. These large REWs might be caused by suppressed continuum emission analogous to Type 2 quasars; however, there is no obvious mechanism in standard Unified Models to suppress the continuum without also obscuring the broad emission lines.

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arXiv2016

The Sloan Digital Sky Survey Reverberation Mapping Project: An Investigation of Biases in CIV Emission-Line Properties

We investigate the dependence on data quality of quasar properties measured from the CIV emission line region at high redshifts. Our measurements come from 32 epochs of Sloan Digital Sky Survey (SDSS) Reverberation Mapping Project spectroscopic observations of 482 z>1.46 quasars. We compare the differences between measurements made from the single-epoch and coadded spectra, focusing on the CIV emission line because of its importance for studies of high-redshift quasar demographics and physical properties, including black hole masses. In addition to statistical errors increasing (by factors of ~2-4), we find increasing systematic offsets with decreasing S/N. The systematic difference (measurement uncertainty) in our lowest S/N (<5) subsample between the single-epoch and coadded spectrum (i) CIV equivalent width is 17A (31A), (ii) centroid wavelength is <1A (2A), and fractional velocity widths, Delta V/V, characterized by (iii) the line dispersion, sigma_l, is 0.104 (0.12), and (iv) the mean absolute deviation (MAD) is 0.072 (0.11). These remain smaller than the 1-sigma measurement uncertainties for all subsamples considered. The MAD is found to be the most robust line-width characterization. Offsets in the CIV full-width at half maximum (FWHM) velocity width and the CIV profile characterized by FWHM/sigma_l are only smaller than the statistical uncertainties when S/N>10, although offsets in lower S/N spectra exceed the statistical uncertainties by only a factor of ~1.5. Characterizing the CIV line profile by the kurtosis is the least robust property investigated, as the median systematic coadded--single-epoch measurement differences are larger than the statistical uncertainties for all S/N subsamples.

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arXiv2015

The Sloan Digital Sky Survey Reverberation Mapping Project: First Broad-line Hbeta and MgII Lags at z>~0.3 from six-Month Spectroscopy

Reverberation mapping (RM) measurements of broad-line region (BLR) lags in z>0.3 quasars are important for directly measuring black hole masses in these distant objects, but so far there have been limited attempts and success given the practical difficulties of RM in this regime. Here we report preliminary results of 15 BLR lag measurements from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project, a dedicated RM program with multi-object spectroscopy designed for RM over a wide redshift range. The lags are based on the 2014 spectroscopic light curves alone (32 epochs over 6 months) and focus on the Hbeta and MgII broad lines in the 100 lowest-redshift (z<0.8) quasars included in SDSS-RM; they represent a small subset of the lags that SDSS-RM (including 849 quasars to z~4.5) is expected to deliver. The reported preliminary lag measurements are for intermediate-luminosity quasars at 0.3<~z<0.8, including 9 Hbeta lags and 6 MgII lags, for the first time extending RM results to this redshift-luminosity regime and providing direct quasar black hole mass estimates over ~ half of cosmic time. The MgII lags also increase the number of known MgII lags by several-fold, and start to explore the utility of MgII for RM at high redshift. The location of these new lags at higher redshifts on the observed BLR size-luminosity relationship is statistically consistent with previous Hbeta results at z<0.3. However, an independent constraint on the relationship slope at z>0.3 is not yet possible due to the limitations in our current sample. Our results demonstrate the general feasibility and potential of multi-object RM for z>0.3 quasars.

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arXiv2015

Quasar Classification Using Color and Variability

We conduct a pilot investigation to determine the optimal combination of color and variability information to identify quasars in current and future multi-epoch optical surveys. We use a Bayesian quasar selection algorithm (Richards et al. 2004) to identify 35,820 type 1 quasar candidates in a 239 square degree field of the Sloan Digital Sky Survey (SDSS) Stripe 82, using a combination of optical photometry and variability. Color analysis is performed on 5-band single- and multi-epoch SDSS optical photometry to a depth of r ~22.4. From these data, variability parameters are calculated by fitting the structure function of each object in each band with a power law model using 10 to >100 observations over timescales from ~1 day to ~8 years. Selection was based on a training sample of 13,221 spectroscopically-confirmed type-1 quasars, largely from the SDSS. Using variability alone, colors alone, and combining variability and colors we achieve 91%, 93%, and 97% quasar completeness and 98%, 98%, and 97% efficiency respectively, with particular improvement in the selection of quasars at 2.7<z<3.5 where quasars and stars have similar optical colors. The 22,867 quasar candidates that are not spectroscopically confirmed reach a depth of i ~22.0; 21,876 (95.7%) are dimmer than coadded i-band magnitude of 19.9, the cut off for spectroscopic follow-up for SDSS on Stripe 82. Brighter than 19.9, we find 5.7% more quasar candidates without confirming spectra in sky regions otherwise considered complete. The resulting quasar sample has sufficient purity (and statistically correctable incompleteness) to produce a luminosity function comparable to those determined by spectroscopic investigations. We discuss improvements that can be made to the process in preparation for performing similar photometric selection and science on data from post-SDSS sky surveys.

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arXiv2015

The Sloan Digital Sky Survey Reverberation Mapping Project: Ensemble Spectroscopic Variability of Quasar Broad Emission Lines

We explore the variability of quasars in the MgII and Hbeta broad emission lines and UV/optical continuum emission using the Sloan Digital Sky Survey Reverberation Mapping project (SDSS-RM). This is the largest spectroscopic study of quasar variability to date: our study includes 29 spectroscopic epochs from SDSS-RM over $6$ months, containing 357 quasars with MgII and 41 quasars with Hbeta . On longer timescales, the study is also supplemented with two-epoch data from SDSS-I/II. The SDSS-I/II data include an additional $2854$ quasars with MgII and 572 quasars with Hbeta. The MgII emission line is significantly variable ($Δf/f$ 10% on 100-day timescales), a necessary prerequisite for its use for reverberation mapping studies. The data also confirm that continuum variability increases with timescale and decreases with luminosity, and the continuum light curves are consistent with a damped random-walk model on rest-frame timescales of $\gtrsim 5$ days. We compare the emission-line and continuum variability to investigate the structure of the broad-line region. Broad-line variability shows a shallower increase with timescale compared to the continuum emission, demonstrating that the broad-line transfer function is not a $δ$-function. Hbeta is more variable than MgII (roughly by a factor of $1.5$), suggesting different excitation mechanisms, optical depths and/or geometrical configuration for each emission line. The ensemble spectroscopic variability measurements enabled by the SDSS-RM project have important consequences for future studies of reverberation mapping and black hole mass estimation of $1<z<2$ quasars.

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arXiv2015

Bayesian High-Redshift Quasar Classification from Optical and Mid-IR Photometry

We identify 885,503 type 1 quasar candidates to i<22 using the combination of optical and mid-IR photometry. Optical photometry is taken from the Sloan Digital Sky Survey-III: Baryon Oscillation Spectroscopic Survey (SDSS-III/BOSS), while mid-IR photometry comes from a combination of data from the Wide-Field Infrared Survey Explorer (WISE) "ALLWISE" data release and several large-area Spitzer Space Telescope fields. Selection is based on a Bayesian kernel density algorithm with a training sample of 157,701 spectroscopically-confirmed type-1 quasars with both optical and mid-IR data. Of the quasar candidates, 733,713 lack spectroscopic confirmation (and 305,623 are objects that we have not previously classified as photometric quasar candidates). These candidates include 7874 objects targeted as high probability potential quasars with 3.5 3.5 than the traditional mid-IR selection "wedges" and to 2.2 3. This catalog paves the way for luminosity-dependent clustering investigations of large numbers of faint, high-redshift quasars and for further machine learning quasar selection using Spitzer and WISE data combined with other large-area optical imaging surveys.

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arXiv2015

Do the Kepler AGN Light Curves Need Re-processing?

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, $SF(δt)$, 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 ($γ= 2.90$ to within $10$ percent) for the re-processed light curve as compared to the light curve found on MAST ($γ= 2.65$ to within $10$ percent)---both inconsistent with a damped random walk which requires $γ= 2$. 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.

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arXiv2015

Mining for Dust in Type 1 Quasars

We explore the extinction/reddening of ~35,000 uniformly selected quasars with 0 0.1 and 0.1% (1.3%) with E(B-V)>0.2. Simulations show both populations of quasars are intrinsically bluer than the mean composite, with a mean spectral index ($α_λ$) of -1.79 (-1.83). The emission and absorption-line properties of both samples reveal that quasars with intrinsically red continua have narrower Balmer lines and stronger ionizing spectral lines, the latter indicating a harder continuum in the extreme-UV and the former pointing to differences in black hole mass and/or orientation.

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arXiv2015

Are the Variability Properties of the Kepler AGN Light Curves Consistent with a Damped Random Walk?

We test the consistency of active galactic nuclei (AGN) optical flux variability with the $\textit{damped random walk}$ (DRW) model. Our sample consists of 20 multi-quarter $\textit{Kepler}$ AGN light curves including both Type 1 and 2 Seyferts, radio-loud and -quiet AGN, quasars, and blazars. $\textit{Kepler}$ observations of AGN light curves offer a unique insight into the variability properties of AGN light curves because of the very rapid ($11.6-28.6$ min) and highly uniform rest-frame sampling combined with a photometric precision of $1$ part in $10^{5}$ over a period of 3.5 yr. We categorize the light curves of all 20 objects based on visual similarities and find that the light curves fall into 5 broad categories. We measure the first order structure function of these light curves and model the observed light curve with a general broken power-law PSD characterized by a short-timescale power-law index $γ$ and turnover timescale $τ$. We find that less than half the objects are consistent with a DRW and observe variability on short timescales ($\sim 2$ h). The turnover timescale $τ$ ranges from $\sim 10-135$ d. Interesting structure function features include pronounced dips on rest-frame timescales ranging from $10-100$ d and varying slopes on different timescales. The range of observed short-timescale PSD slopes and the presence of dip and varying slope features suggests that the DRW model may not be appropriate for all AGN. We conclude that AGN variability is a complex phenomenon that requires a more sophisticated statistical treatment.

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