time of 73.4 ms.

Observation Data Files (ODFs) were obtained from the on-line [*XMM-Newton*]{} Science Archive and later reduced with the [*XMM-Newton*]{} Science Analysis Software (SAS, v. 7.0.0, 6.1.0, 5.4.1). Source light curves, with 5 s bins, were extracted for a circular region centered on the source ($\sim$20$^\prime$$^\prime$). Background light curves were obtained from a nearby rectangular source-free region and subtracted from the source light curves. These rectangular background regions were larger than the source regions and were accordingly scaled down. Due to strong flaring, the last few kiloseconds of data are excluded from most observations. The count rates for the target sources are orders of magnitude greater than the background count rate in the detection cell, so a rise in the background is unimportant. We removed these last few kiloseconds of data from the data stream just to be very cautious. We note that in the observation of 3C 273 with the claimed detection, including the periods with flaring does not change our results.

Some of the observations in our sample are affected by pile-up. Pile-up occurs when more than one X-ray photon arrives in a pixel before the pixel is read out by the CCD, making it difficult 