Search for fast ultraviolet variability in NGC 5548 and non-linearities in IUE's SWP camera.

Abstract

, Large observation campaigns of Seyfert galaxies give the opportunity to check the linearity of IUE short wavelength camera at low flux number and moderate background. In particular, many pairs of quasi simultaneous observations of NGC 5548 have been obtained with IUE. We selected pairs of spectra which include one well exposed and one poorly exposed spectrum. The ratio of the two broad band continuum fluxes measured in these paired spectra is variable and well correlated to the integrated flux of the underexposed observation indicating that this apparent variability is due to instrumental effects rather than source variability. The response of the SWP system (camera + processing) decreases by up to 25% at low flux numbers. Taking these non linearities into account, NGC 5548 did not show variability in the ultraviolet on time scale of a few hours like in the soft X-ray.