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Accretion disk reverberation with Hubble Space Telescope observations of NGC 4593: Evidence for diffuse continuum lags

Accretion disk reverberation with Hubble Space Telescope observations of NGC 4593: Evidence for diffuse continuum lags
Accretion disk reverberation with Hubble Space Telescope observations of NGC 4593: Evidence for diffuse continuum lags

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 A. 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 ∝ λ 4/3 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 A 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.

galaxies: active, galaxies: individual (NGC 4593), galaxies: nuclei, galaxies: Seyfert
0004-637X
Cackett, Edward M.
db500756-c971-444b-9677-4ee742b2dac5
Chiang, Chia Ying
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McHardy, Ian
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Edelson, Rick
322e69d7-f317-4551-9054-31235bca54bd
Goad, Michael R.
98d06542-81a7-4d7b-a0a5-c79079b9c662
Horne, Keith
ffcda7c7-9cd8-4043-be08-c5db52b80e71
Korista, Kirk T.
586b39a9-4942-4d95-9a13-7b922029864e
Cackett, Edward M.
db500756-c971-444b-9677-4ee742b2dac5
Chiang, Chia Ying
af0d88d9-e6df-47e1-a945-3339a7336b39
McHardy, Ian
4f215137-9cc4-4a08-982e-772a0b24c17e
Edelson, Rick
322e69d7-f317-4551-9054-31235bca54bd
Goad, Michael R.
98d06542-81a7-4d7b-a0a5-c79079b9c662
Horne, Keith
ffcda7c7-9cd8-4043-be08-c5db52b80e71
Korista, Kirk T.
586b39a9-4942-4d95-9a13-7b922029864e

Cackett, Edward M., Chiang, Chia Ying, McHardy, Ian, Edelson, Rick, Goad, Michael R., Horne, Keith and Korista, Kirk T. (2018) Accretion disk reverberation with Hubble Space Telescope observations of NGC 4593: Evidence for diffuse continuum lags. The Astrophysical Journal, 857 (1). (doi:10.3847/1538-4357/aab4f7).

Record type: Article

Abstract

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 A. 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 ∝ λ 4/3 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 A 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.

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Accretion Disk Reverberation with Hubble Space Telescope Observations of NGC 4593: Evidence for Diffuse Continuum Lags - Accepted Manuscript
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Accepted/In Press date: 5 March 2018
e-pub ahead of print date: 12 April 2018
Published date: April 2018
Keywords: galaxies: active, galaxies: individual (NGC 4593), galaxies: nuclei, galaxies: Seyfert

Identifiers

Local EPrints ID: 420493
URI: https://eprints.soton.ac.uk/id/eprint/420493
ISSN: 0004-637X
PURE UUID: efc9ed6f-60d5-409c-8c47-53b35fb612ae

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Date deposited: 09 May 2018 16:30
Last modified: 13 Mar 2019 18:32

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