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Abstract

We report the results of intensive X-ray, UV, and optical monitoring of the Seyfert 1 galaxy\nNGC 4593 with Swift. There is no intrinsic flux-related spectral change in any variable\ncomponent with small apparent variations being due to contamination by a constant hard\n(reflection) component in the X-rays and the red host galaxy in the UV/optical. Relative to the\nshortest wavelength band, UVW2, the lags of the other UV/optical bands mostly agree with the\npredictions of reprocessing of high energy emission by an accretion disc. The U-band lag is,\nhowever, larger than expected, probably because of reprocessed Balmer continuum emission\nfrom the distant broad line region (BLR). The UVW2 band is well correlated with the X-rays\nbut lags by ∼6× more than expected if the UVW2 results only from reprocessing of X-rays\nby the disc. However, if the light curves are filtered to remove variations on time-scales >5\nd, the lag approaches the expectation from disc reprocessing. MEMECHO analysis shows that\ndirect X-rays can be the driver of most of the UV/optical variations if the response functions\nhave tails up to 10 d, from BLR reprocessing, together with strong peaks at short lag (<1 d)\nfrom disc reprocessing. For the 5 AGN monitored so far, the observed UVW2 to V-band lags\nare <\n∼2 of disc reprocessing expectations and vary little between AGN. However, the X-ray to\nUVW2 lags greatly exceed disc reprocessing expectations and differ between AGN. The two\nmost absorbed AGN have the largest excesses, so absorption and scattering may affect these\nlags, but there is no simple relationship between excess and absorption.

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

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