Continuum Reverberation Mapping of Mrk 876 Over Three Years With Remote Robotic Observatories

TL;DR

This study uses three years of optical monitoring to measure continuum lags in Mrk 876, revealing disk sizes larger than expected and confirming wavelength-dependent lag behavior consistent with thin disk theory, with additional emission line contributions.

Contribution

First multi-year reverberation mapping of Mrk 876 using robotic observatories, demonstrating consistent lag measurements and detailed analysis of disk structure and emission features.

Findings

Interband lags around 13 days from u to z band.

Lag increase with wavelength following λ^{4/3}.

Lag normalization about three times larger than standard models.

Abstract

Continuum reverberation mapping probes the sizescale of the optical continuum-emitting region in active galactic nuclei (AGN). Through 3 years of multiwavelength photometric monitoring in the optical with robotic observatories, we perform continuum reverberation mapping on Mrk~876. All wavebands 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, 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 $\lambda^{4/3}$, 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 which we attribute to variable H$\alpha$ broad-line emission. A flux-flux analysis shows a variable spectrum that follows $f_\nu \propto \lambda^{-1/3}$ as expected for a disk, and an excess in the $i$ band that also points to strong variable H$\alpha$ emission in that band.