The Sloan Digital Sky Survey Reverberation Mapping Project: Accretion Disk Sizes from Continuum Lags

Abstract

We present accretion-disk structure measurements from continuum lags in the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. Lags are measured using the JAVELIN software from the first-year SDSS-RM g and i photometry, resulting in well-defined lags for 95 quasars, 33 of which have lag SNR > 2σ. We also estimate lags using the ICCF software and find consistent results, though with larger uncertainties. Accretion-disk structure is fit using a Markov Chain Monte Carlo approach, parameterizing the measured continuum lags as a function of disk size normalization, wavelength, black hole mass, and luminosity. In contrast with previous observations, our best-fit disk sizes and color profiles are consistent with the Shakura & Sunyaev (1973) analytic solution. We also find that more massive quasars have larger accretion disks, similarly consistent with the analytic accretion-disk model. The continuum lag fits have a large excess dispersion, indicating that our measured lag errors are underestimated and/or our best-fit model may be missing the effects of orientation, spin, and/or radiative efficiency. We demonstrate that fitting disk parameters using only the highest-SNR lag measurements biases best-fit disk sizes to be larger than the disk sizes recovered using a Bayesian approach on the full sample of well-defined lags. This work represents a significant advance in continuum reverberation mapping, providing a large number of accretion-disk size and structure measurements for quasars spanning a broad range of redshift, mass and luminosity.

Quasar Accretion Disk Sizes from Continuum Reverberation Mapping in the DES Standard-star Fields

“…In addition, an inhomogeneous disk with significant local temperature fluctuations may also explain the larger disk sizes from microlensing studies (Dexter & Agol 2011). However, Mudd et al (2018) and Homayouni et al (2019) did not find systematic trends of larger disk sizes than the prediction of the thin-disk model. To address this discrepancy, more disk size measurements are required, especially using high-cadence time series data.…”

“…The broad-line variability may consequently make the measured lag larger than the real continuum lag. We assess the contamination from broad emission lines as the ratio of the equivalent width of the emission line to the effective width of the broadband filter, referred to as f BLR , similar to Homayouni et al (2019). Those authors identified potential contamination if f BLR >12.5%.…”

“…The disk size at effective wavelength λ, defined as the position where kT(R λ )=hc/λ, scales with wavelength as R λ ∝λ β , where β=4/3 in the standard thindisk model. Accretion disks are too small to be spatially resolved, and current measurements of the size of accretion disks are mainly from microlensing (e.g., Morgan et al 2010) and continuum reverberation mapping (RM; e.g., Shappee et al 2014;Fausnaugh et al 2016;Jiang et al 2017;Mudd et al 2018;Homayouni et al 2019).…”

“…Mudd et al (2018) used the photometric data on the supernova fields of the Dark Energy Survey (DES) and obtained disk size measurements of 15 quasars. Homayouni et al (2019) presented the continuum lags of 95 quasars from the photometric data for the Sloan Digital Sky Survey (SDSS) RM project.…”

Corona-heated Accretion-disk Reprocessing: A Physical Model to Decipher the Melody of AGN UV/Optical Twinkling

“…Variations in different bands are well coordinated. Changes of short-wavelength emission lead those of long-wavelength emission (e.g., Sergeev et al 2005;Fausnaugh et al 2016;Jiang et al 2017;Homayouni et al 2019;Cackett et al 2018;Kokubo 2018;McHardy et al 2018;Mudd et al 2018;Yu et al 2018;Edelson et al 2019). Current observations have a broad diversity of measured time lags beyond the SSD theory: some AGNs have time lags that are about three times larger than the flux-weighted light-travel time delays of the SSD theory.…”