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The origin of UV-optical variability in AGN and test of disc models: XMM-Newton and ground-based observations of NGC 4395

The origin of UV-optical variability in AGN and test of disc models: XMM-Newton and ground-based observations of NGC 4395
The origin of UV-optical variability in AGN and test of disc models: XMM-Newton and ground-based observations of NGC 4395
The origin of short timescale (weeks/months) variability of AGN, whether due to intrinsic disc variations or reprocessing of X-ray emission by a surrounding accretion disc, has been a puzzle for many years. However recently a number of observational programmes, particularly of NGC 5548 with Swift, have shown that the UV/optical variations lag behind the X-ray variations in a manner strongly supportive of X-ray reprocessing. Somewhat surprisingly, the implied size of the accretion disc is ?3 times greater than expected from a standard, smooth, Shakura-Sunyaev thin disc model. Although the difference may be explained by a clumpy accretion disc, it is not clear whether the difference will occur in all AGN or whether it may change as, eg, a function of black hole mass, accretion rate, or disc temperature. Measurements of interband lags for most AGN require long timescale monitoring, which is hard to arrange. However for low mass (<106 M?) AGN, the combination of XMM-Newton EPIC (X-rays) with the optical monitor in fast readout mode allows an X-ray/UV-optical lag to be measured within a single long observation. Here we summarise previous related observations and report on XMM-Newton observations of NGC 4395 (mass 100 times lower, accretion rate ?20 times lower than for NGC 5548). We find that the UVW1 lags the X-rays by ?470 s. Simultaneous observations at 6 different ground based observatories also allowed the g-band lag (?800 s) to be measured. These observations are in agreement with X-ray reprocessing but initial analysis suggests that, for NGC 4395, they do not differ markedly from the predictions of the standard thin disc model
0004-6337
500-506
Mchardy, I.M.
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Connolly, S.D.
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Peterson, B.M.
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Bieryla, A.
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Chand, H.
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Elvis, M.S.
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Emmanoulopoulos, D.
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Falco, E.
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Gandhi, P.
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Kaspi, S.
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Latham, D.
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Lira, P.
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McCully, C.
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Netzer, H.
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Uemura, M.
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Mchardy, I.M.
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Connolly, S.D.
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Peterson, B.M.
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Bieryla, A.
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Chand, H.
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Elvis, M.S.
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Emmanoulopoulos, D.
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Falco, E.
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Gandhi, P.
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Kaspi, S.
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Latham, D.
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Lira, P.
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McCully, C.
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Netzer, H.
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Uemura, M.
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Mchardy, I.M., Connolly, S.D., Peterson, B.M., Bieryla, A., Chand, H., Elvis, M.S., Emmanoulopoulos, D., Falco, E., Gandhi, P., Kaspi, S., Latham, D., Lira, P., McCully, C., Netzer, H. and Uemura, M. (2016) The origin of UV-optical variability in AGN and test of disc models: XMM-Newton and ground-based observations of NGC 4395. Astronomische Nachrichten, 337 (4-5), 500-506. (doi:10.1002/asna.201612337).

Record type: Article

Abstract

The origin of short timescale (weeks/months) variability of AGN, whether due to intrinsic disc variations or reprocessing of X-ray emission by a surrounding accretion disc, has been a puzzle for many years. However recently a number of observational programmes, particularly of NGC 5548 with Swift, have shown that the UV/optical variations lag behind the X-ray variations in a manner strongly supportive of X-ray reprocessing. Somewhat surprisingly, the implied size of the accretion disc is ?3 times greater than expected from a standard, smooth, Shakura-Sunyaev thin disc model. Although the difference may be explained by a clumpy accretion disc, it is not clear whether the difference will occur in all AGN or whether it may change as, eg, a function of black hole mass, accretion rate, or disc temperature. Measurements of interband lags for most AGN require long timescale monitoring, which is hard to arrange. However for low mass (<106 M?) AGN, the combination of XMM-Newton EPIC (X-rays) with the optical monitor in fast readout mode allows an X-ray/UV-optical lag to be measured within a single long observation. Here we summarise previous related observations and report on XMM-Newton observations of NGC 4395 (mass 100 times lower, accretion rate ?20 times lower than for NGC 5548). We find that the UVW1 lags the X-rays by ?470 s. Simultaneous observations at 6 different ground based observatories also allowed the g-band lag (?800 s) to be measured. These observations are in agreement with X-ray reprocessing but initial analysis suggests that, for NGC 4395, they do not differ markedly from the predictions of the standard thin disc model

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Accepted/In Press date: 15 January 2016
e-pub ahead of print date: 2 May 2016
Published date: May 2016
Organisations: Astronomy Group

Identifiers

Local EPrints ID: 399904
URI: http://eprints.soton.ac.uk/id/eprint/399904
ISSN: 0004-6337
PURE UUID: c42a5b07-81f3-48db-a7d9-bf012312fa6c
ORCID for D. Emmanoulopoulos: ORCID iD orcid.org/0000-0002-3335-2446
ORCID for P. Gandhi: ORCID iD orcid.org/0000-0003-3105-2615

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Date deposited: 02 Sep 2016 11:04
Last modified: 17 Dec 2019 01:42

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