New Constraints on the Continuum Emission Mechanism of AGN:
Intensive Monitoring of NGC 7469 in the X-Ray and Ultraviolet
K. Nandra,
J. Clavel,
R.A. Edelson,
I.M. George,
M.A. Malkan,
R.F. Mushotzky,
B.M. Peterson, and
T.J. Turner 1998, ApJ, 505, 594 - 606.
We have undertaken near-continuous monitoring of the Seyfert 1 galaxy NGC 7469 in the X-ray with XTE over a ~30 d baseline. The source shows strong variability with a root-mean-square (rms) amplitude of ~16 per cent, and peak-to-peak variations of a factor of order 2. Simultaneous data over this period were obtained in the ultraviolet (UV) using IUE, making this the most intensive X-ray UV/X-ray variability campaign performed for any active galaxy. Comparison of the continuum light curves reveals very similar amplitudes of variability, but different variability characteristics, with the X-rays showing much more rapid variations. The data are not strongly correlated at zero lag. The largest absolute value of the correlation coefficient occurs for an anticorrelation between the two bands, with the X-ray variations leading the UV by ~4 d. The largest positive correlation is for the ultraviolet to lead the X-rays by ~4 d. Neither option appears to be compatible with any simple interband transfer function. The peak positive correlation at ~4 d occurs because the more prominent peaks in the UV light curve appear to lead those in the X-rays by this amount. However, the minima of the light curves are near-simultaneous. These observations provide new constraints on theoretical models of the central regions of active galactic nuclei. Models in which the observed UV emission is produced solely by re-radiation of absorber X-rays are ruled out by our data, as are those in which the X-rays are produced solely by Compton upscattering of the observed UV component by a constant distribution of particles. New or more complex models must be sought to explain the data. We require at least two variability mechanisms, which have no simple relationship. We briefly explore means by which these observations could be reconciled with theoretical models.