XMM-Newton observations of a possible light echo in the Seyfert 1 nucleus of NGC 4051

We discuss a 1.5 day long observation of the bright variable Seyfert 1 galaxy NGC 4051 that was obtained with the X-ray and ultraviolet telescopes on XMM-Newton and extended in the X-ray band using the Rossi X-Ray Timing Explorer. The ultraviolet (2900 Angstrom) and X-ray (2-10 keV) light curves varied with peak-to-peak amplitudes of 5% and a factor of similar to4, respectively. A simple interband cross-correlation function shows three broad features: a positive correlation with coefficient at a lag r(M) = +0.63 at a lag (T(opt) - T(X-ray)) tau similar to +0.2 days, an anticorrelation with r(M) = -0.65 at tau similar to +0.7 days, and a broad but slightly weaker feature with r(M) greater than or equal to 0.53 over tau = 1.2-1.4 days. Because of the asymmetric sampling of the optical and X-ray data, only positive lags could be probed. Monte Carlo simulations based on synthetic uncorrelated light curves derived from the power spectrum of the real data give an similar to 85% confidence in features at this level. If the X-ray and UV light curves are indeed correlated, the lag is highly significant (i.e., not consistent with zero), which is suggestive of a scenario in which X-rays are being reprocessed into UV light at a distance from the X-ray source. A simple model in which the reprocessor is an inclined optically thick ring at a radius of 0.14 lt-days from the X-ray source reproduces the UV data well. A comparison of the modulated fractions in the ultraviolet and X-ray bands suggests that 5% of the 2900 Angstrom flux is produced in this way. The reprocessing site is close enough to the X-ray emitter for X-ray heating from a source with the luminosity of NGC 4051 to produce a brightening in the UV band, and the amount of UV flux emitted suggests that the covering fraction of the reprocessor is about 6%. We briefly discuss the possible physical nature of the reprocessing ring structure.