Intensive disc-reverberation mapping of Fairall 9: 1st year of Swift & LCO monitoring

TL;DR

This study presents a detailed time-series analysis of Fairall 9, using Swift and LCO data, confirming accretion disc models and revealing complex reprocessing signals across multiple wavelengths.

Contribution

First-year intensive disc-reverberation monitoring of Fairall 9 with multi-wavelength data, providing new insights into accretion disc structure and reprocessing mechanisms.

Findings

Lag spectrum consistent with $ au\,\propto\,\lambda^{4/3}$ scaling

Variable component's spectrum slightly steeper than standard model

Evidence for additional bound-free continuum from broad-line region

Abstract

We present results of time-series analysis of the first year of the Fairall 9 intensive disc-reverberation campaign. We used Swift and the Las Cumbres Observatory global telescope network to continuously monitor Fairall 9 from X-rays to near-infrared at a daily to sub-daily cadence. The cross-correlation function between bands provides evidence for a lag spectrum consistent with the $\tau\propto\lambda^{4/3}$ scaling expected for an optically thick, geometrically thin blackbody accretion disc. Decomposing the flux into constant and variable components, the variable component's spectral energy distribution is slightly steeper than the standard accretion disc prediction. We find evidence at the Balmer edge in both the lag and flux spectra for an additional bound-free continuum contribution that may arise from reprocessing in the broad-line region. The inferred driving light curve suggests two distinct components, a rapidly variable ($<4$ days) component arising from X-ray reprocessing, and a more slowly varying ($>100$ days) component with an opposite lag to the reverberation signal.