Background Line Energies, Subtraction and Detector Gain
Further work on Cas A has quite clearly demonstrated that the
two lines in the background are caused by the L
and transitions of Bismuth. These lines arise
from the activation of the lead collimator material to Pb21O, which
then decays, with a half-life of 4 days to Bismuth to give the observed L
emission lines. The energies of these lines are 10.83 and 13.01 keV
respectively
(averaged over the 1,
2,
1,
2
branching ratios of 100, 10, 50 and 20).
Experience now shows that, except for
extremely bright sources (e.g. Sco X-1), the channel number of the
13.01 keV line
can be established to within one channel (gain 2.0) by fitting two
gaussian-lines
and a polynomial of order 3 or less to the data. The same is true for the 10.83
keV line although for some brighter sources the uncertainty in the measured
channel number may increase (e.g. for a bright bulge source). The
13.01 keV line
can be reliably used to lock the gain of the GSPC for any particular
observation.
Photomultiplier stimulation data should be used to check only the gain drift
throughout an observation.
It has been found that the
apparent energy of the 4.75 keV feature, when fitted as a line depends by up to
10% on the shape of the underlying continuum. It is now unnecessary to fit this
feature which should be included directly in the response matrix, by
overlapping
the energies of the matrix by 70 eV at 4.78 keV.
The Effective Areas
Analysis of several long exposures on the
Crab in October and November this year produced further refinement of the
effective areas. A series of splined three order polynomials were fitted to the
Crab data in order to force the response to give the correct fit. The increased
exposure time for a burst length window of channels 89-107 of 25800 s has
revealed some small deficiencies in the previously quoted values. The revised
values are given in Table 1 and, are good to a systematic uncertainty
of 1%.
An update to the CCF is in progress and will be distributed as
soon as possible (details in the next EXPRESS).
N.E. White