A Negative Long Lag from the Optical to the UV Continuum in Fairall 9

TL;DR

This paper reports the discovery of a long-timescale negative lag in the AGN Fairall 9, indicating inward-traveling signals in the accretion disk, which enhances understanding of disk structure and accretion processes.

Contribution

It presents the first detection of a long negative lag in an AGN, revealing inward-propagating signals and providing insights into the disk's vertical structure.

Findings

Detection of a $ ext{~}-70$ day negative lag in Fairall 9

Confirmation of short reverberation lags (<10 days)

Evidence that disk scale height increases outward

Abstract

We report the detection of a long-timescale negative lag, where the blue bands lag the red bands, in the nearby Seyfert 1 galaxy Fairall 9. Active Galactic Nuclei (AGN) light curves show variability over a wide range of timescales. By measuring time lags between different wavelengths, the otherwise inaccessible structure and kinematics of the accretion disk can be studied. One common approach, reverberation mapping, quantifies the continuum and line lags moving outwards through the disk at the light-travel time, revealing the size and temperature profile of the disk. Inspired by numerical simulations, we expect longer lags to exist in AGN light curves that travel inward on longer timescales, tracing the accretion process itself. By analyzing AGN light curves in both temporal and frequency space, we report the detection of long-timescale lags ($\sim -70$ days) in Fairall 9 which propagate in the opposite direction to the reverberation lag. The short continuum lag ($<10$ days) is also detected and is consistent with reverberation lags reported in the literature. When fitting the longer lag as a function of frequency with a model motivated by the thin disk model, we find that the disk scale height likely increases outward in the disk. This detection raises the exciting prospect of mapping accretion disk structures across a wide range of AGN parameters.