What Drives the Variability in AGN? Explaining the UV-Xray Disconnect Through Propagating Fluctuations
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arXiv
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| Format: | Preprint |
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2024
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| _version_ | 1866910429841719296 |
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| author | Hagen, Scott Done, Chris Edelson, Rick |
| author_facet | Hagen, Scott Done, Chris Edelson, Rick |
| contents | Intensive broadband 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 timescales are longer than expected. Worse, the UV/optical lightcurves 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 $L \sim 0.1L_{\rm{Edd}}$, where the spectrum is dominated by the UV/EUV. Our model produces lightcurves where the X-rays and UV have very different fast variability, yet are well correlated on longer timescales, 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 which 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2401_03452 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | What Drives the Variability in AGN? Explaining the UV-Xray Disconnect Through Propagating Fluctuations Hagen, Scott Done, Chris Edelson, Rick High Energy Astrophysical Phenomena Astrophysics of Galaxies Intensive broadband 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 timescales are longer than expected. Worse, the UV/optical lightcurves 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 $L \sim 0.1L_{\rm{Edd}}$, where the spectrum is dominated by the UV/EUV. Our model produces lightcurves where the X-rays and UV have very different fast variability, yet are well correlated on longer timescales, 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 which 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. |
| title | What Drives the Variability in AGN? Explaining the UV-Xray Disconnect Through Propagating Fluctuations |
| topic | High Energy Astrophysical Phenomena Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2401.03452 |