Continuum Reverberation Mapping of Mrk 876 over Three Years with Remote Robotic Observatories

Abstract

Continuum reverberation mapping probes the size scale of the optical continuum-emitting region in active galactic nuclei (AGN). Through 3 yr of multiwavelength photometric monitoring in the optical with robotic observatories, we perform continuum reverberation mapping on Mrk 876. All wave bands show large-amplitude variability and are well correlated. Slow variations in the light curves broaden the cross-correlation function (CCF) significantly, requiring detrending in order to robustly recover interband lags. We measure consistent interband lags using three techniques (CCF, JAVELIN, and PyROA), with a lag of around 13 days from u to z. These lags are longer than the expected radius of 12 days for the self-gravitating radius of the disk. The lags increase with wavelength roughly following λ 4/3, as would be expected from thin disk theory, but the lag normalization is approximately a factor of 3 longer than expected, as has also been observed in other AGN. The lag in the i band shows an excess that we attribute to variable Hα broad-line emission. A flux–flux analysis shows a variable spectrum that follows f ν ∝ λ −1/3, as expected for a disk, and an excess in the i band that also points to strong variable Hα emission in that band.

AGN STORM 2. VI. Mapping Temperature Fluctuations in the Accretion Disk of Mrk 817

“…A key insight from this exercise is that the temperature fluctuations produce effects on the lightcurves that exist over relatively long timescales (>56 days). These effects are often removed by detrending the lightcurves in order to more cleanly detect the reverberation signal (e.g., McHardy et al 2014McHardy et al , 2018Pahari et al 2020;Cackett et al 2023;Miller et al 2023). In other words, the signal that is excluded by detrending is being produced by the slow-moving temperature fluctuations that we see in our maps.…”

“…In our analysis, the component that is excluded by the detrending corresponds to the slow-moving temperature fluctuations. This is further evidenced by studies of the reverberation lags calculated from long-term (1 yr) lightcurves, where detrending is sometimes required to calculate the reverberation lag (see, e.g., Miller et al 2023). Similarly, in Homayouni et al (2023b), the mismatch between the UV emission lines and the 1180 Å lightcurves is solved by adding a slowly varying component to the emission line lightcurves, which implies a discrepancy between the 1180 Å continuum and the extreme-UV (EUV) continuum that is driving the UV emission lines.…”

Spectral Variability Studies in Active Galactic Nuclei: Exploring Continuum and Emission Line Regions in the Age of LSST and JWST

“…The reverberation technique has been employed to measure the size of the accretion disk in AGNs using continuum time delays (see, e.g., [40,[183][184][185][186][187][188][189][190][191][192][193]). Initially proposed by Collier et al [194] (with a torus-based variation by Oknyanskij [195]), this approach involves evaluating the size of the accretion disk across different wavelengths and then comparing it to the classical accretion disk model proposed by Shakura and Sunyaev [89].…”

Latent Stochastic Differential Equations for Modeling Quasar Variability and Inferring Black Hole Properties

“…The reprocessing of the driving variability in the UV/optical regions of the accretion disk leads to wavelength-dependent time lags that can be measured through continuum reverberation mapping to probe the relative size scales of the emitting regions. This also makes the UV/ optical variability highly correlated, since these regions are driven by the same X-ray source (e.g., Miller et al 2023). The time delays between bandpasses are related to properties of the accretion disk and black hole (Cackett et al 2021;Jha et al 2022;Wang et al 2023).…”