English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

Intensive broadband reverberation mapping (IBRM) campaigns have shown thatAGN variability is significantly more complex than expected from discreverberation of the variable X-ray illumination. The UV/optical variability ishighly correlated and lagged, with longer lag at longer wavelength, but thetimescale is longer than expected. More challenging though is that theUV/optical lightcurves are not well correlated with the X-rays which were meantto be driving them. Instead, we consider an intrinsically variable accretiondisc, where mass accretion rate fluctuations propagate in through the flow,modulating the intrinsically faster X-ray variability from the central regions.We match our model to the parameters of Fairall 9, a well studied AGN with$L\sim 0.1L_{\mathrm{Edd}}$, where the spectrum is dominated by the UV/EUVrather than the X-rays. We show that intrinsic variability and propagationgives X-ray and UV/optical light-curves that are dominated by variability ontwo different time-scales, yet are correlated on long time-scales. We includereprocessing of the X-rays from the disc but this has negligible impact on thelightcurves for spectra where the EUV dominates the bolometric power. We alsoinclude reverberation of the total (EUV plux X-ray) variable spectrum off awind. This results in a bound-free component which predominantly follows theslow variable EUV, but lagged and smoothed on the light-travel time. Thisspectrum is redder than the EUV disc emission, so it contributes more at longerwavelengths giving the apparent rise in lag time with wavelength from aconstant lag component. We conclude that contrary to the original motivationfor IBRM campaigns, AGN variability is likely driven by intrinsic fluctuationswithin the disc, not X-ray reprocessing, and that the observed lags areproduced by the EUV illumination of the wind not the X-ray illumination of thedisc.

What Drives the Variability in AGN: Explaining the UV-Xray Disconnect Through Propagating Fluctuations