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

Abstract

We report the results of intensive X-ray, UV and optical monitoring of the\nSeyfert 1 galaxy NGC 4593 with Swift. There is no intrinsic flux-related\nspectral change in the the variable components in any band with small apparent\nvariations due only to contamination by a second constant component, possibly a\n(hard) reflection component in the X-rays and the (red) host galaxy in the\nUV/optical bands. Relative to the shortest wavelength band, UVW2, the lags of\nthe other UV and optical bands are mostly in agreement with the predictions of\nreprocessing of high energy emission from an accretion disc. The U-band lag is,\nhowever, far larger than expected, almost certainly because of reprocessed\nBalmer continuum emission from the more distant broad line region gas. The UVW2\nband is well correlated with the X-rays but lags by ~6x more than expected if\nthe UVW2 results from reprocessing of X-rays on the accretion disc. However, if\nthe lightcurves are filtered to remove variations on timescales >5d, the lag\napproaches the expectation from disc reprocessing. MEMEcho analysis shows that\ndirect X-rays can be the driver of most of the variations in the UV/optical\nbands as long as the response functions for those bands all have long tails (up\nto 10d) in addition to a strong peak (from disc reprocessing) at short lag\n(<1d). We interpret the tails as due to reprocessing from the surrounding gas.\nComparison of X-ray to UVW2 and UVW2 to V-band lags for 4 AGN, including NGC\n4593, shows that all have UVW2 to V-band lags which exceed the expectations\nfrom disc resprocessing by factor < 2. However the X-ray to UVW2 lags are,\nmostly, in greater excess from the expectations from disc reprocessing and\ndiffer between AGN. The largest excess is in NGC 4151. Absorption and\nscattering may be affecting X-ray to UV lags.\n