Ultraviolet and X-ray variability of active galactic nuclei with Swift

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Buisson, DJK 
Lohfink, AM 
Alston, WN 
Fabian, AC 

We analyse a sample of 21 active galactic nuclei (AGN) using data from the Swift satellite to study the variability properties of the population in the X-ray, UV and optical band. We find that the variable part of the UV-optical emission has a spectrum consistent with a powerlaw, with an average index of −2.21±0.13, as would be expected from central illumination of a thin disc (index of -7/3). We also calculate the slope of a powerlaw from UV to X-ray variable emission, αOX,Var; the average for this sample is αOX,Var=−1.06±0.04. The anticorrelation of αOX with the UV luminosity, LUV, previously found in the average emission is also present in the variable part: αOX,Var=(−0.177±0.083)log(Lν,Var(2500\AA))+(3.88±2.33). Correlated variability between the emission in X-rays and UV is detected significantly for 9 of the 21 sources. All these cases are consistent with the UV lagging the X-rays, as would be seen if the correlated UV variations were produced by the reprocessing of X-ray emission. The observed UV lags are tentatively longer than expected for a standard thin disc.

accretion, accretion discs, black hole physics, galaxies: active, galaxies: nuclei, ultraviolet: galaxies, X-rays: galaxies
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Monthly Notices of the Royal Astronomical Society
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Oxford University Press (OUP)
Science and Technology Facilities Council (ST/N000927/1)
European Research Council (340442)
European Commission (312789)
ACF, AML and DJKB acknowledge support from the ERC Advanced Grant FEEDBACK 340442. WNA acknowledges support from the European Union Seventh Framework Programme (FP7/2013-2017) under grant agreement n.312789, StrongGravity. DB acknowledges an STFC studentship. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.