therefore consisted of observations approximately every 7 to 10 days throughout the first year, followed by observations approximately every 2 weeks thereafter. In addition, during the first year, there were two more intensive monitoring periods: a two-week period of twice-daily observations and a four-week period of of daily observations. Each observation lasted for typically $1$ksec. In 1996 December, there was also a period of 3 days during which NGC 4051 was observed for a total of 70ksec.

The PCA consists of 5 proportional counter units (PCUs) but typically only 3 PCUs (numbers 0, 1 and 2) were operational and all count rates refer to the total counts from those 3 PCUs. Where other than 3 PCUs were in operation, the count rate has been normalized to 3 PCUs. The PCUs have 3 layers. Here we only include data from the upper layer as this layer provides the highest signal-to-noise ratio for photons in the energy range 2–20 keV where the flux from NGC 4051 is strongest. We used standard “good time interval” (GTI) criteria to select data with the lowest background. We reject data obtained when the Earth elevation angle was less than 10$^{\circ}$, when the pointing offset from the 