Using AGN lightcurves to map accretion disc temperature fluctuations

Abstract

We introduce a new model for understanding AGN continuum variability. We start from a Shakura--Sunyaev thin accretion disc with a steady-state radial temperature profile $T(R)$ and assume that the variable flux is due to axisymmetric temperature perturbations $\delta T(R,t)$. After linearizing the equations, we fit UV-optical AGN lightcurves to determine $\delta T(R,t)$ for a sample of seven AGNs. We see a diversity of $|\delta T/T|\sim 0.1$ fluctuation patterns which are not dominated by outgoing waves traveling at the speed of light as expected for the "lamppost" model used to interpret disc reverberation mapping studies. Rather, the most common pattern resembles slow ($v\ll c$) ingoing waves. An explanation for our findings is that these ingoing waves trigger central temperature fluctuations that act as a lamppost, producing lower amplitude temperature fluctuations moving outwards at the speed of light. The lightcurves are dominated by the lamppost signal -- even though the temperature fluctuations are dominated by other structures with similar variability time-scales -- because the discs exponentially smooth the contributions from the slower moving ($v\ll c$) fluctuations to the observed lightcurves. This leads to lightcurves that closely resemble the expectations for a lamppost model but with the slow variability time-scales of the ingoing waves. This also implies that longer time-scale variability signals will increasingly diverge from lamppost models because the smoothing of slower-moving waves steadily decreases as their period or spatial wavelength increases.

18 pages, 25 figures. Published by MNRAS. Animated versions of select figures (see Section 8) can be found at https://github.com/jackneustadt/disc-gifs

References in corpus (13)

• Modeling the Time Variability of SDSS Stripe 82 Quasars as a Damped Random Walk
• Simultaneous X-ray/optical/UV snapshots of active galactic nuclei from XMM-Newton: spectral energy distributions for the reverberation mapped sample
• Is Quasar Optical Variability a Damped Random Walk?
• Space Telescope and Optical Reverberation Mapping Project. II. Swift and HST Reverberation Mapping of the Accretion Disk of NGC 5548
• Testing thermal reprocessing in AGN accretion discs
• Quasar Accretion Disks Are Strongly Inhomogeneous
• Space Telescope and Optical Reverberation Mapping Project. I. Ultraviolet Observations of the Seyfert 1 Galaxy NGC 5548 with the Cosmic Origins Spectrograph on Hubble Space Telescope
• A characteristic optical variability timescale in astrophysical accretion disks
• Intensive disc-reverberation mapping of Fairall 9: 1st year of Swift & LCO monitoring
• Space Telescope and Optical Reverberation Mapping Project VI: reverberating Disk Models for NGC 5548
• Supermassive black holes with high accretion rates in active galactic nuclei. XI. Accretion disk reverberation mapping of Mrk 142
• On the multi-wavelength variability of Mrk 110: Two components acting at different timescales
• Opacity Driven Convection and Variability in Accretion Disks around Supermassive Black Holes