The origin of the UV/optical lags in NGC 5548

Abstract

The new multi-wavelength monitoring campaign on NGC 5548 shows clearly that the variability of the UV/optical lightcurves lags by progressively longer times at longer wavelengths, as expected from reprocessing of an optically thick disk, but that the timescales are longer than expected for a standard Shakura-Sunyaev accretion disc. We build a full spectraltiming reprocessing model to simulate the UV/optical lightcurves of NGC 5548. We show that disc reprocessing of the observed hard X-ray lightcurve produces optical lightcurves with too much fast variability as well as too short a lag time. Supressing the fast variability requires an intervening structure preventing the hard X-rays from illuminating the disc. We propose this is the disc itself, perhaps due to atomic processes in the UV lifting the photosphere, increasing the scale-height, making it less dense and less able to thermalise, so that it radiates low temperature Comptonised emission as required to produce the soft X-ray excess. The outer edge of the puffed-up Comptonised disc region emits FUV flux, and can illuminate the outer thin blackbody disc but while this gives reprocessed variable emission which is much closer to the observed UV and optical lightcurves, the light travel lags are still too short to match the data. We reverse engineer a solution to match the observations and find that the luminosity and temperature of the lagged emission is not consistent with material at the light travel lag distance responding to the irradiating flux (either FUV or X-ray) from the AGN. We conclude that the UV/optical lags of NGC 5548 are not the light travel time from X-ray reprocessing, nor the light travel time from FUV reprocessing, but instead could be the timescale for the outer blackbody disc vertical structure to respond to the changing FUV illumination.

Accretion Disk Reverberation with Hubble Space Telescope Observations of NGC 4593: Evidence for Diffuse Continuum Lags

“…Starkey et al (2017) showed that for a disk reprocessing model the driving light curve needed to fit the UV and optical variations does not resemble a blurred version of the X-ray light curve. Gardner & Done (2017) argued that the observed X-ray light curve cannot be the driver, based on energetic considerations as well as on the light curve shapes, and invoked a 2-stage reprocessing model in which X-rays heated a vertically-extended torus producing EUV radiation that subsequently irradiates the disk. NGC4151 shows a good X-ray/UV/optical correlation, but also shows a much larger X-ray-to-UV lag than expected based on extrapolating the UV/optical lags (Edelson et al 2017).…”

“…However, the driving light curve that is recovered from this fitting does not match the observed X-ray light curve (Starkey et al 2017). Gardner & Done (2017) also showed that blurring the hard X-ray light curve gives too much fast variability, and suggested that there is an intervening puffed-up Comptonized disk region that blocks the X-rays from illuminating the disk directly.…”

“…This disconnect between the X-ray-to-UV and UV-tooptical lags is even more strongly in conflict with the standard model. One possible explanation could be that an additional component shields the disk from the corona and reprocesses the energy on a longer (e.g., dynamical) timescale, increasing the lag between the X-rays and optical/UV (Edelson et al 2017;Gardner & Done 2017).…”

Corona-heated Accretion-disk Reprocessing: A Physical Model to Decipher the Melody of AGN UV/Optical Twinkling

“…However, the expected inter-band time lags are too small to be consistent with observations (see observational fact # 6) although this discrepancy could be resolved by adding an additional ingredient, e.g., non-blackbody emission (Hall et al 2018) or strong winds (Sun et al 2019). Moreover, the simplest X-ray reprocessing model also predicts too much shortterm variability (note that this inconsistency can be solved by replacing the X-ray corona with a UV torus; Gardner & Done 2017). There are additional fundamental observational challenges.…”

Continuum Reverberation Mapping of the Accretion Disks in Two Seyfert 1 Galaxies

“…A possible explanation is that the driving light curve is in the extreme UV (Shakura & Sunyaev 1973;Gardner & Done 2017). On the other hand, although the X-ray light curve has additional structure compared to the UV/optical light Note.…”