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An increasing number of active galactic nuclei are being found by EXOSAT to exhibit rapid X- ray variability, on a timescale less than a typical observation duration. However, study of the X-ray variations in the CMA have been hampered by the filter wheel rotation policy used to date, which involved obtaining an equal number of counts in the Lexan, Al-P and Boron filters in order to obtain maximum spectral information. As a result, most of the observation duration was usually spent in the filter with the lowest throughput! A typical example is shown in Figure 1, where a Seyfert galaxy exhibited marked variations in the ME waveband, but the soft X-ray light curve was seriously disrupted by the filter changes.

Therefore, the default filter strategy (and the one recommended by the Observatory!) will be changed as follows:

a. The observation will be started in Lexan.

b. If the count rate is high enough ( and spectral information is desired), an Al-P exposure will be performed until approx. 30 counts from the source are obtained.

c. If the count rate is sufficiently high, a Boron exposure to be performed until approx. 50 counts obtained.

HOWEVER the Al-P and Boron exposures would only be performed if more than 80% of the observation would still take place in Lexan. (The amount of time needed for the Al-P and Boron exposures can be predicted by the Observatory Controller from the initial Lexan count rate in real time).

(Of course if the PI is interested principally in variability we recommend spending the entire observation in Lexan).

This procedure is now the default strategy for AGN and will be adhered to unless the PI specifically requests otherwise for his/ her observation.

P. Barr

fig1 effect of filter change on a lightcurve of a 
variable AGN

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