Connecting the X-ray/UV variability of Fairall 9 with NICER: A Possible Warm Corona

TL;DR

This study uses NICER observations of Fairall 9 to explore the connection between X-ray and UV variability, revealing a warm corona's role and its dynamic behavior in the AGN's central environment.

Contribution

It provides new evidence for a warm Comptonized corona in Fairall 9 and links UV/X-ray variability to the corona's structure and dynamics.

Findings

Detection of a strong UV and soft X-ray flux correlation.

Evidence for a warm corona lying above the accretion disk.

X-ray variability insufficient to drive long-term UV changes.

Abstract

The Seyfert 1 AGN Fairall 9 was targeted by NICER, Swift, and ground-based observatories for a $\sim$1000-day long reverberation mapping campaign. The following analysis of NICER spectra taken at a two-day cadence provides new insights into the structure and heating mechanisms of the central black hole environment. Observations of Fairall 9 with NICER and Swift revealed a strong relationship between the flux of the UV continuum and the X-ray soft excess, indicating the presence of a "warm" Comptonized corona which likely lies in the upper layers of the innermost accretion flow, serving as a second reprocessor between the "hot" X-ray corona and the accretion disk. The X-ray emission from the hot corona lacks sufficient energy and variability to power slow changes in the UV light curve on timescales of 30 days or longer, suggesting an intrinsic disk-driven variability process in the UV and soft X-rays. Fast variability in the UV on timescales shorter than 30 days can be explained through X-ray reprocessing, and the observed weak X-ray/UV correlation suggests that the corona changes dynamically throughout the campaign.