Intensive broadband reverberation mapping of Fairall 9 with 1.8 years of daily Swift monitoring

TL;DR

This study presents an extensive 1.8-year Swift monitoring campaign of the AGN Fairall 9, revealing detailed UV/optical reverberation lags, complex X-ray/UV relationships, and disk structure insights consistent with the BLR edge.

Contribution

First large-scale daily monitoring campaign of Fairall 9, providing detailed reverberation mapping and insights into disk and BLR structure with novel findings on lag stability and X-ray/UV complexity.

Findings

Clear detection of interband UV lags consistent with thin disk models

X-ray to UV lags are unstable and vary in magnitude and direction

Disk thickening at ~10 light-days and ~8000K temperature, matching BLR inner edge

Abstract

We present 1.8 years of near-daily Swift monitoring of the bright, strongly variable Type 1 AGN Fairall 9. Totaling 575 successful visits, this is the largest such campaign reported to date. Variations within the UV/optical are well-correlated, with longer wavelengths lagging shorter wavelengths in the direction predicted by thin disk/lamp-post models. The correlations are improved by detrending; subtracting a second-order polynomial fit to the UV/optical light curves to remove long-term trends that are not of interest to this study. Extensive testing indicates detrending with higher-order polynomials removes too much intrinsic variability signal on reverberation timescales. These data provide the clearest detection to date of interband lags within the UV, indicating that neither emission from a large disk nor diffuse continuum emission from the broad-line region can independently explain the full observed lag spectrum. The observed X-ray flux variations are poorly correlated with those in the UV/optical. Further, subdivision of the data into four ~160 day light curves shows that the UV/optical lag spectrum is highly stable throughout the four periods, but the X-ray to UV lags are unstable, significantly changing magnitude and even direction from one period to the next. This indicates the X-ray to UV relationship is more complex than predicted by the simple reprocessing model often adopted for AGN. A bowl model (lamp-post irradiation and blackbody reprocessing on a disk with a steep rim) fit suggests the disk thickens at a distance (~10 lt-day) and temperature (~8000K) consistent with the inner edge of the BLR.