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

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journal contribution
posted on 2018-05-24, 10:35 authored by Edward M. Cackett, Chia-Ying Chiang, Ian McHardy, Rick Edelson, Michael R. Goad, Keith Horne, Kirk T. Korista
The Seyfert 1 galaxy NGC 4593 was monitored spectroscopically with the Hubble Space Telescope as part of a reverberation mapping campaign that also included Swift, Kepler, and ground-based photometric monitoring. During 2016 July 12–August 6, we obtained 26 spectra across a nearly continuous wavelength range of ∼1150–10000 Å. These were combined with Swift data to produce a UV/optical “lag spectrum,” which shows the interband lag relative to the Swift UVW2 band as a function of wavelength. The broad shape of the lag spectrum appears to follow the τ ∝ λ4/3 relation seen previously in photometric interband lag measurements of other active galactic nuclei (AGNs). This shape is consistent with the standard thin disk model, but the magnitude of the lags implies a disk that is a factor of ∼3 larger than predicted, again consistent with what has been previously seen in other AGNs. In all cases these large disk sizes, which are also implied by independent gravitational microlensing of higher-mass AGNs, cannot be simply reconciled with the standard model. However, the most striking feature in this higher-resolution lag spectrum is a clear excess around the 3646 Å Balmer jump. This strongly suggests that diffuse emission from gas in the much larger broad-line region (BLR) must also contribute significantly to the interband lags. While the relative contributions of the disk and BLR cannot be uniquely determined in these initial measurements, it is clear that both will need to be considered to comprehensively model and understand AGN lag spectra


We thank the Hubble and Swift teams for their hard work and efforts in successfully scheduling this monitoring campaign. Support for program number 14121 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. K.H. acknowledges support from STFC grant ST/M001296/1. E.M.C. thanks Kayhan Gültekin for helpful discussions on the 2D lag centroid distributions.



Astrophysical Journal, 2018, 857:53 (12pp)

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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