What drives the variability in AGN? Explaining the UV-Xray disconnect through propagating fluctuations

Hagen, Scott; Done, Chris; Edelson, Rick

Abstract

Intensive broad-band reverberation mapping campaigns have shown that AGN variability is significantly more complex than expected from disc reverberation of the variable X-ray illumination. The UV/optical variability is highly correlated and lagged, with longer lags at longer wavelengths as predicted, but the observed time-scales are longer than expected. Worse, the UV/optical light curves are not well correlated with the X-rays, which should drive them. Instead, we consider an intrinsically variable accretion disc, where slow mass accretion rate fluctuations are generated in the optical-UV disc, propagating down to modulate intrinsically faster X-ray variability from the central regions. We match our model to Fairall 9, a well-studied AGN with L ∼ 0.1LEdd, where the spectrum is dominated by the UV/EUV. Our model produces light curves where the X-rays and UV have very different fast variability, yet are well correlated on longer time-scales, as observed. It predicts that the intrinsic variability has optical/UV leading the X-rays, but including reverberation of the variable EUV from an inner wind produces a lagged bound-free continuum that matches the observed UV-optical lags. We conclude that optical/UV AGN variability is likely driven by intrinsic fluctuations within the disc, not X-ray reprocessing: the observed longer than expected lags are produced by reverberation of the EUV illuminating a wind, not by X-ray illumination of the disc: the increasing lag with increasing wavelength is produced by the increased contribution of the (constant lag) bound-free continuum to the spectrum, rather than indicating intrinsically larger reverberation distances for longer wavelengths.

Citation

Hagen, S., Done, C., & Edelson, R. (2024). What drives the variability in AGN? Explaining the UV-Xray disconnect through propagating fluctuations. Monthly Notices of the Royal Astronomical Society, 530(4), 4850-4867. https://doi.org/10.1093/mnras/stae1177

• Journal Article Type: Article
• Acceptance Date: Apr 30, 2024
• Online Publication Date: May 2, 2024
• Publication Date: 2024-06
• Deposit Date: May 21, 2024
• Publicly Available Date: May 21, 2024
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 530
• Issue: 4
• Pages: 4850-4867
• DOI: https://doi.org/10.1093/mnras/stae1177
• Public URL: https://durham-repository.worktribe.com/output/2455262