National Aeronautics and Space Administration

A near-continuous monitoring campaign of the Seyfert 1 galaxy NGC 7469 was undertaken in the X-ray with RXTE, investigating time scales from hours to one month. Simultaneous monitoring was performed in the UV with IUE, making this the most intensive X-ray/UV variability study performed for an active galaxy. The UV and X-ray light curves show amplitude variations of similar magnitude, but are not strongly correlated at zero lag time. While the maxima in the UV light curve appear to precede those in the X-ray by ~4 d, the minima of the light curves are near-simultaneous. This implies that models of the continuum emission of AGN which invoke simple UV re-radiation of X-ray photons, as well as models in which the X-ray are produced solely by Compton upscattering of UV radiation, must be revised. Two variability mechanisms which have no simple relationship are required to explain the observed UV/X-ray variability.

Fig. 1: X-ray light curve (RXTE PCA, 2-10 keV) binned at 512 s time resolution (red) together with the IUE 1315 Angstrom light curve in the UV (blue, Wanders, 1997). Both light curves have been normalized to their mean value and are plotted on the same time axis. We note the remarkable similarity in the amplitudes of variability on long time scales, but clear evidence for more rapid X-ray variations which are not observed in the UV. The light curves are not well correlated at zero lag, but the two prominent troughs appear to line up rather well in the time domain. In contrast, the apparent peaks in the UV light curve appear to precede the X-ray peaks.

Figure and caption courtesy of Paul Nandra.

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