X-Ray/UVOIR Frequency-resolved Time Lag Analysis of Mrk 335 Reveals Accretion Disk Reprocessing

Abstract

UV and optical continuum reverberation mapping is a powerful tool for probing the accretion disk and inner broad-line region. However, recent reverberation mapping campaigns in the X-ray, UV, and optical have found lags consistently longer than those expected from the standard disk reprocessing picture. The largest discrepancy to date was recently reported in Mrk 335, where UV/optical lags are up to 12 times longer than expected. Here, we perform a frequency-resolved time lag analysis of Mrk 335, using Gaussian processes to account for irregular sampling. For the first time, we compare the Fourier frequency-resolved lags directly to those computed using the popular interpolated cross-correlation function method applied to both the original and detrended light curves. We show that the anticipated disk reverberation lags are recovered by the Fourier lags when zeroing in on the short-timescale variability. This suggests that a separate variability component is present on long timescales. If this separate component is modeled as reverberation from another region beyond the accretion disk, we constrain a size scale of roughly 15 lt-days from the central black hole. This is consistent with the size of the broad-line region inferred from Hβ reverberation lags. We also find tentative evidence for a soft X-ray lag, which we propose may be due to light travel time delays between the hard X-ray corona and distant photoionized gas that dominates the soft X-ray spectrum below 2 keV.

Corona-heated Accretion-disk Reprocessing: Frequency-resolved Lag Predictions for UV/Optical Reverberation Mapping of Active Galactic Nuclei

“…The frequency range is 0.02-0.9 days −1 . The black dots with error bars are the frequency-resolved lags and their 1σ uncertainties from Lewin et al (2023). The simulated frequency-resolved lags for Mrk 335 from the CHAR model are broadly consistent with observations.…”

“…Alternatively, the CHAR model has predicted a similar trend of the frequency-resolved time lags (see Figure 7 of Sun et al 2020). Recently, the frequency-resolved UV/optical time lags of a second source, Mrk 335, were presented by Lewin et al (2023). Hence, we are motivated to quantitatively compare the CHAR model with the frequency-resolved time lags of NGC 5548 and Mrk 335.…”

“…Very recently, the frequency-resolved lags in Mrk 335 were obtained by Lewin et al (2023). Here we run 512 CHAR model simulations for Mrk 335 with M BH = 1.69 × 10 7 M e (Grier et al 2012), L/L Edd = 0.07 (Tripathi et al 2020), and redshift z = 0.025785 (Huchra et al 1999).…”

“…To the best of our knowledge, the Fourier technique has been used to measure the frequency-resolved time lags of the AGN STORM I target NGC 5548 (Cackett et al 2022) and other sources, e.g., Mrk 335 (Lewin et al 2023) and Fairall 9 (but with large uncertainties; Yao et al 2023). The frequencyresolved time lags of NGC 5548 from Cackett et al (2022) show that the X-ray reprocessing of a large accretion disk produces excessive lags in high frequency and less lags in low frequency.…”