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X-ray/UV/optical variability of NGC 4593 with Swift : reprocessing of X-rays by an extended reprocessor.

McHardy, I. M. and Connolly, S. D. and Horne, K. and Cackett, E. M. and Gelbord, J. and Peterson, B. M. and Pahari, M. and Gehrels, N. and Goad, M. and Lira, P. and Arevalo, P. and Baldi, R. D. and Brandt, N. and Breedt, E. and Chand, H. and Dewangan, G. and Done, C. and Elvis, M. and Emmanoulopoulos, D. and Fausnaugh, M. M. and Kaspi, S. and Kochanek, C. S. and Korista, K. and Papadakis, I. E. and Rao, A. R. and Uttley, P. and Vestergaard, M. and Ward, M. J. (2018) 'X-ray/UV/optical variability of NGC 4593 with Swift : reprocessing of X-rays by an extended reprocessor.', Monthly notices of the Royal Astronomical Society., 480 (3). pp. 2881-2897.


We report the results of intensive X-ray, UV, and optical monitoring of the Seyfert 1 galaxy NGC 4593 with Swift. There is no intrinsic flux-related spectral change in any variable component with small apparent variations being due to contamination by a constant hard (reflection) component in the X-rays and the red host galaxy in the UV/optical. Relative to the shortest wavelength band, UVW2, the lags of the other UV/optical bands mostly agree with the predictions of reprocessing of high energy emission by an accretion disc. The U-band lag is, however, larger than expected, probably because of reprocessed Balmer continuum emission from the distant broad line region (BLR). The UVW2 band is well correlated with the X-rays but lags by ∼6× more than expected if the UVW2 results only from reprocessing of X-rays by the disc. However, if the light curves are filtered to remove variations on time-scales >5 d, the lag approaches the expectation from disc reprocessing. MEMECHO analysis shows that direct X-rays can be the driver of most of the UV/optical variations if the response functions have tails up to 10 d, from BLR reprocessing, together with strong peaks at short lag (<1 d) from disc reprocessing. For the 5 AGN monitored so far, the observed UVW2 to V-band lags are ∼<2 of disc reprocessing expectations and vary little between AGN. However, the X-ray to UVW2 lags greatly exceed disc reprocessing expectations and differ between AGN. The two most absorbed AGN have the largest excesses, so absorption and scattering may affect these lags, but there is no simple relationship between excess and absorption.

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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:23 July 2018
Date deposited:08 November 2018
Date of first online publication:02 August 2018
Date first made open access:08 November 2018

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