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Register a new userAccretion Disc Time Lag Distributions: Applying CREAM to Simulated AGN Light Curves
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Active Galactic Nuclei (AGN) vary in their brightness across all wavelengths. Moreover, longer wavelength ultraviolet - optical continuum light curves appear to be delayed with respect to shorter wavelength light curves. A simple way to model these delays is by assuming thermal reprocessing of a variable point source (a lamp post) by a blackbody accretion disc. We introduce a new method, CREAM (textbf{C}ontinuum textbf{RE}processed textbf{A}GN textbf{M}arkov Chain Monte Carlo), that models continuum variations using this lamp post model. The disc light curves lag the lamp post emission with a time delay distribution sensitive to the disc temperature-radius profile and inclination. We test CREAMs ability to recover both inclination and product of black hole mass and accretion rate $mmdot$, and show that the code is also able to infer the shape of the driving light curve. CREAM is applied to synthetic light curves expected from 1000 second exposures of a 17th magnitude AGN with a 2m telescope in Sloan g and i bands with signal to noise of 500 - 900 depending on the filter and lunar phase. We also tests CREAM on poorer quality g and i light curves with SNR = 100. We find in the high SNR case that CREAM can recover the accretion disc inclination to within an uncertainty of 5 degrees and an $mmdot$ to within 0.04 dex.
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We performed the observation of the flux densities of SgrA* at 90 and 102GHz in order to detect the time lag between these frequencies using the Nobeyama Millimeter Array, which was previously reported at lower frequencies. We detected a radio flare during the observation period on 6 April 2005 and calculated the z-transformed discrete correlation function between the light curves. The time lag between these frequencies was not detected. If the expanding plasma model which explains the time lag at lower frequencies is valid, the light curve at 90GHz would be delayed with respect to the one at 102GHz. This result suggests that the plasma blobs ejected near the Galactic Center black hole may be widely diverse especially in optical thickness. Another possibility is that the major portion of the flux above 100GHz does not originate from the blobs.
We performed the observation of the flux densities of Sgr A* at 90 and 102 GHz on 6 April 2005 using the Nobeyama Millimeter Array in order to detect the time lag between these frequencies. We constructed light curves covering a few hour with 1 min bin, and the Intra-Day Variability, which had a rising phase and intensity peak, of Sgr A* is clearly seen at both frequencies. We calculated the z-transformed discrete correlation function between the light curves of Sgr A* at 90 and 102 GHz. The derived time lag of the flares at these frequencies was approximately zero, contrary to our expectations based on the previously reported time lag at lower frequencies. If the radio flares of Sgr A* are explained by the expanding plasma model, the light curve at 90 GHz would be delayed with respect to the one at 102 GHz. However, we could not find such a delay with statistical significance in our data.
In this work, which is a continuation of Castello-Mor et al. (2016), we present new X-ray and infrared (IR) data for a sample of active galactic nuclei (AGN) covering a wide range in Eddington ratio over a small luminosity range. In particular, we rigorously explore the dependence of the optical-to-X-ray spectral index $alpha_{OX}$ and the IR-to-optical spectral index on the dimensionless accretion rate, $dot{mathcal{M}}=dot{m}/eta$ where $dot{m}=L_{AGN}/L_{Edd}$ and $eta$ is the mass-to-radiation conversion efficiency, in low and high accretion rate sources. We find that the SED of the faster accreting sources are surprisingly similar to those from the comparison sample of sources with lower accretion rate. In particular: i) the optical-to-UV AGN SED of slow and fast accreting AGN can be fitted with thin AD models. ii) The value of $alpha_{OX}$ is very similar in slow and fast accreting systems up to a dimensionless accretion rate $dot{mathcal{M}}_{c}sim$10. We only find a correlation between $alpha_{OX}$ and $dot{mathcal{M}}$ for sources with $dot{mathcal{M}} > dot{mathcal{M}}_{c}$. In such cases, the faster accreting sources appear to have systematically larger $alpha_{OX}$ values. iii) We also find that the torus in the faster accreting systems seems to be less efficient in reprocessing the primary AGN radiation having lower IR-to-optical spectral slopes. These findings, failing to recover the predicted differences between the SEDs of slim and thin ADs within the observed spectral window, suggest that additional physical processes or very special geometry act to reduce the extreme UV radiation in fast accreting AGN. This may be related to photon trapping, strong winds, and perhaps other yet unknown physical processes.
With the advent of high-cadence and multi-band photometric monitoring facilities, continuum reverberation mapping is becoming of increasing importance to measure the physical size of quasar accretion disks. The method is based on the measurement of the time it takes for a signal to propagate from the center to the outer parts of the central engine, assuming the continuum light curve at a given wavelength has a time shift of the order of a few days with respect to light curves obtained at shorter wavelengths. We show that with high-quality light curves, this assumption is not valid anymore and that light curves at different wavelengths are not only shifted in time but also distorted: in the context of the lamp-post model and thin-disk geometry, the multi-band light curves are in fact convolved by a transfer function whose size increase with wavelength. We illustrate the effect with simulated light curves in the LSST ugrizy bands and examine the impact on the delay measurements when using three different methods, namely JAVELIN, CREAM, and PyCS. We find that current accretion disk sizes estimated from JAVELIN and PyCS are underestimated by $sim30%$ and that unbiased measurement are only obtained with methods that properly take the skewed transfer functions into account, as the CREAM code does. With the LSST-like light curves, we expect to achieve measurement errors below $5%$ with typical 2-day photometric cadence.
Motivated by upcoming photometric and spectroscopic surveys (Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST), Manuakea Spectroscopic Explorer), we design the statistical proxies to measure the cadence effects on active galactic nuclei (AGN) variability-observables (time-lags, periodicity, and structure-function (SF)). We constructed a multiple-regression model to statistically identify the cadence-formal error pattern knowing AGN time-lags and periodicity from different surveys. We defined the simple metric for the SFs properties, accounting for the observed SFs deviation relative to those obtained from the homogenously-sampled light curves. We tested the regression models on different observing strategies: the optical dataset of long light-curves of eight AGN with peculiarities and the artificial datasets based on several idealized and LSST-like cadences. The SFs metric is assessed on synthetic datasets. The regression models (for both data types) predict similar cadences for time-lags and oscillation detection, whereas for light curves with low variability ($sim 10%$), cadences for oscillation detection differ. For higher variability ($sim20%$), predicted cadences are larger than for $F_{var}sim 10%$. The predicted cadences are decreasing with redshift. SFs with dense and homogenous cadences are more likely to behave similarly. SFs with oscillatory signals are sensitive to the cadences, possibly impacting LSST-like operation strategy. The proposed proxies can help to select spectroscopic and photometric-surveys cadence strategies, and they will be tested further in larger samples of objects.
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