Ultraviolet and X-ray variability of NGC 4051 over 45 days with XMM-Newton and Swift

We analyse 15 XMM-Newton observations of the Seyfert galaxy NGC 4051 obtained over 45 d to determine the ultraviolet (UV) light curve variability characteristics and search for correlated UV/X-ray emission. The UV light curve shows variability on all time-scales, however with lower fractional rms than the 0.2-10 keV X-rays. On days-weeks time-scales the fractional variability of the UV is F_var ∼ 8 per cent, and on short (∼ hours) time-scales F_var ∼ 2 per cent. The within-observation excess variance in 4 of the 15 UV observations was found to be much higher than the remaining 11. This was caused by large systematic uncertainties in the count rate masking the intrinsic source variance. For the four `good' observations we fit an unbroken power-law model to the UV power spectra with slope -2.6 ± 0.5. We compute the UV/X-ray cross-correlation function for the `good' observations and find a correlation of ∼0.5 at a time lag of ∼3 ks, where the UV lags the X-rays. We also compute for the first time the UV/X-ray cross-spectrum in the range 0-28.5 ks and find a low coherence and an average time lag of ∼3 ks. Combining the 15 XMM-Newton and the Swift observations we compute the discrete correlation function over ±40 d but are unable to recover a significant correlation. The magnitude and direction of the lag estimate from the four `good' observations indicate a scenario where ∼25 per cent of the UV variance is caused by thermal reprocessing of the incident X-ray emission.