Abstract

We present the results of a monitoring campaign of the low luminosity Seyfert 1 NGC 4593, at X-ray, ultraviolet, optical and near IR frequencies. The rapid and large amplitude fluctuations detected set interesting upper limits to the sizes of the various emitting regions. We investigate the properties of the nuclear energy distribution, its pattern of variability and the relationship between the emission processes at different frequencies. We also study the variations of the broad emission lines in relation to those of the continuum. The implications of our data are discussed. It is found that they are consistent with a very hot accretion disk illuminated by the hard X-ray source.

Observations

Repeated observations of the nucleus of NGC 4593 have been performed from the near IR to X-rays. Though the temporal density of our observations is poor, they have the advantage of a wide wavelength coverage. Besides, NGC 4593 turned out to be highly variable at all frequencies.

Results and implications

The strong and rapid variations detected in all wavebands imply that the continuum source is unusually compact: ≤ 1.1 lt-hr in the hard X-rays, ≤ 3 lt-hrs in the soft X-rays, 70 lt-hrs at 1450 Å, and 37 lt-days at 2.5 microns. The latter corresponds roughly to the dust evaporation radius (∼ 33 lt-d, Barvainis, 1987) in such a low luminosity source. Such an IR behaviour is reminiscent to that of Fairall 9, a more luminous Seyfert 1, (Clavel et al., 1989).