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

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 ∼