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The origin of the UV/optical lags in NGC 5548

Gardner, Emma; Done, Chris

The origin of the UV/optical lags in NGC 5548 Thumbnail


Authors

Emma Gardner



Abstract

The new multiwavelength monitoring campaign on NGC 5548 shows clearly that the variability of the ultraviolet (UV)/optical light curves lags by progressively longer times at longer wavelengths, as expected from reprocessing off an optically thick disc, but that the time-scales are longer than expected for a standard Shakura–Sunyaev accretion disc. We build a full spectral-timing reprocessing model to simulate the UV/optical light curves of NGC 5548. We show that disc reprocessing of the observed hard X-ray light curve produces optical light curves with too much fast variability as well as too short a lag time. Suppressing 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 scaleheight, making it less dense and less able to thermalize, so that it radiates low-temperature Comptonized emission as required to produce the soft X-ray excess. The outer edge of the puffed-up Comptonized disc region emits far-UV (FUV) flux, and can illuminate the outer thin blackbody disc, but while this gives reprocessed variable emission that is much closer to the observed UV and optical light curves, 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 are not consistent with material at the light travel lag distance responding to the irradiating flux (either FUV or X-ray). We conclude that the UV/optical lags of NGC 5548 are neither the light travel time from X-ray reprocessing nor the light travel time from FUV reprocessing, but instead could be the time-scale for the outer blackbody disc vertical structure to respond to the changing FUV illumination.

Citation

Gardner, E., & Done, C. (2017). The origin of the UV/optical lags in NGC 5548. Monthly Notices of the Royal Astronomical Society, 470(3), 3591-3605. https://doi.org/10.1093/mnras/stx946

Journal Article Type Article
Acceptance Date Apr 14, 2017
Online Publication Date Apr 21, 2017
Publication Date Sep 21, 2017
Deposit Date Oct 4, 2017
Publicly Available Date Mar 29, 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 470
Issue 3
Pages 3591-3605
DOI https://doi.org/10.1093/mnras/stx946

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Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.






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