The fast UV variability of the active galactic nucleus in Fairall 9

TL;DR

This study presents multi-wavelength monitoring of Fairall 9, revealing rapid UV variability and potential links to X-ray flares, supporting models of disk irradiation and heating in active galactic nuclei.

Contribution

First detailed simultaneous optical/UV/X-ray variability analysis of Fairall 9, highlighting rapid UV flares and their possible connection to X-ray microflares.

Findings

Correlated optical/UV variability observed across all time scales.

A 20% UV flux dip in 4 days not seen in optical or X-ray bands.

Rapid (<10 ks) UV flares suggest a hot, compact emission region possibly heated by X-ray flares.

Abstract

We present results from a new optical/UV/X-ray monitoring campaign of the luminous Seyfert galaxy Fairall 9 using the Swift satellite. Using the UV-Optical Telescope (UVOT) on Swift, we find correlated optical/UV variability on all time scales ranging from the sampling time (4-days) to the length of the campaign (2.5 months). In one noteworthy event, the UW2-band flux dips by 20% in 4-days, and then recovers equally quickly; this event is not seen in either the optical or the X-ray bands. We argue that this event provides further evidence that a significant fraction of the UV-emission must be driven by irradiation/reprocessing of emission from the central disk. We also use an archival XMM-Newton observation to examine shorter time scale UV/X-ray variability. We find very rapid (<10 ks) UV flares of small amplitude. We show that, unless this emission is non-thermal, we must be seeing the Wien tail from a compact (<3 light hours), hot (T > $8\times10^4$ K) region. The possible association with X-ray microflares suggests that we may be seeing the UV signatures of direct X-ray flare heating of the innermost disk.