It appears that the effective area has not varied significantly
since launch. Observations of the sources HZ 43, Vega, A 2256,
and N132D at several epochs yield non-varying count rates:
Source
Date of observation
Count rate (counts s )
A 2256*
1990 July 1
0.61 0.01
August 3, 1992
0.62 0.01
HZ 43
1990 July 2
12.3 0.2
1991 December 11
12.0 0.2
1992 June 9
11.91 0.04
Vega
1991 March 20
0.10 0.01
1992 March 21
0.11 0.01
1992 September 18
0.10 0.01
N132D
1990 July 3
4.41 0.13
1991 February 9
4.77 0.02
1992 September 18
4.89 0.06
1993 April 17
4.61 0.04
1994 January 10
4.51 0.04
1994 July 15
4.65 0.05
1995 February 11
4.54 0.04
1995 March 11
4.68 0.04
* The count rate for A 2256 is determined from
the net emission