We analyse 15 XMM-Newton observations of the Seyfert galaxy NGC 4051 obtained over 45 days 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 timescales the fractional variability of the UV is F _ { var } \sim 8 \% , and on short ( \sim hours ) timescales F _ { var } \sim 2 \% .
The within-observation excess variance in 4 of the 15 UV observations was found 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 \pm 0.5 .
We compute the UV/X-ray Cross-correlation function for the “ good ” observations and find a correlation of \sim 0.5 at time lag of \sim 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 \sim 3 ks .
Combining the 15 XMM-Newton and the Swift observations we compute the DCF over \pm 40 days but are unable to recover a significant correlation .
The magnitude and direction of the lag estimate from the 4 “ good ” observations indicates a scenario where \sim 25 % of the UV variance is caused by thermal reprocessing of the incident X-ray emission .