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New EXOSAT ME Argon Calibration
Max Planck Institute (MPE)
The new ME Argon calibration is now available as part of the general XSPEC
spectral fitting package. The program VIMAT creates response matrices in XSPEC
format for the EXOSAT GSPC and ME detectors. VIMAT uses the new calibration for
the ME argon detectors, the ME xenon calibration has not been changed. The new
ME calibration is based on seven of the nine Crab observations performed during
the EXOSAT mission. Two observations were rejected due to high solar activity.
The Crab spectra for the argon detectors A-H were fitted to a powerlaw (photon
index -2.1) attenuated by photoelectric absorption (column density 3.1 x
1021 H cm-2) after folding through the response. This
article summarizes the changes which have been made to the old calibration to
create the new response matrices.
The shape of the gain curve has been changed to a polynomial of the form
E = a1 * V + a2 * V2 + a3 * V3 (1)
where V is the output of the front-end electronics in mV and E is the energy in
keV. The relation between V and the channel number C is still V = (C - 0.5) *
1500/128. With the new gain curves, the fits to the Crab spectra improve above
channel ~ 60 and the variations of the coefficients are much smaller than
with the old curve which consisted of exponential terms.
The electronic acceptance factors are now the pure rise time veto acceptances.
These have been determined from Crab observations with rise time veto on and
off. These observations were too short to provide good statistics on the
acceptance factors above channel ~ 35. The acceptance factors have therefore
been averaged for all the detectors above channel 7 and set to constant above
channel 35. The rise time acceptance values are plotted in Figure 1. The
additional factors to account for the sinusoidal residuals around 3 keV have
been removed. A possible origin of these "wiggles" is a variation of the width
of individual energy channels. In the new calibration, the channel boundaries
of each individual channel were allowed to vary. The change of the channel
width obtained from eqn. 1 was limited to +/- 4%. These "channel width ratios"
were determined for each Crab observation and detector separately, but then
averaged for each detector because no time variations were seen.
Two detectors, D and G, showed gas leakage during the mission which resulted in
bad spectral fits towards the end of the mission. For detector D, fits were
improved by a linear decrease in pressure after 1984 day 288 from the nominal
value of 2 atm. to a value of 1.85 atm. at the end of the mission. For
detector G, the pressure value was reduced to 1.97 atm. after 1985 day 294.
A plot of count rates in the channel range 80-120 versus channels 5-40 shows
excess counts in the high channels (Fig. 2). To create Figure 2, all the good
quality spectra from the EXOSAT database were used. Only 3 sources (Crab, Cyg
X-1 and GX301-2) are strong and hard enough to be detected above channel 80.
Their count rate ratios (not shown in Figure 2) lie well above all the others
which show a linear relation. A linear fit gives a ratio of
1.2 x10-3, i.e., 0.1% of the counts are detected above channel 80.
This behavior might be explained by a small region of very high gain in the
detector. To account for it, a second linear gain curve has been added in the
calculation of the response matrix. The average weighting factor for the linear
gain curve is 5 x 10-4 but varies from detector to detector (see
Table 1). An example of the gain curves for the first Crab observation of
detector A is shown in Figure 3. The linear gain coefficient and a weighting
factor were free parameters in the fits to the Crab spectra. The effect of the
high gain curve is very small and only improves the fits to the highest
channels (above 80). At the highest channels a bump in the residuals is still
visible and it is not recommended that one use channels above 100. Also,
channel 4 should not be used due to the unknown position of the low level
Finally, the effective areas had to be adjusted to give, on average, the same
powerlaw normalizations for the Crab spectra as the old calibration. The new
values are given in Table 1. The new boundaries, resolution, and efficiencies
were calculated for each detector and Crab observation and stored in separate
files. These files are used by the program to calculate the detector response
matrix. Between the Crab observation boundaries, resolution and efficiencies
are linearly interpolated.
Fitting Other Spectra
Detector Weighting Effective areas
A 10.0 x 10-4 203.9
B 3.1 x 10-4 193.4
C 0.0 201.1
D 4.2 x 10-4 203.3
E 6.1 x 10-4 195.0
F 0.94 x 10-4 199.7
G 5.5 x 10-4 199.0
H 6.1 x 10-4 204.0
To check the new calibration, spectra from various sources have been
fitted. Spectra which show a strong iron line could be used to check that the
gain is right. Fits to spectra from clusters of galaxies where the line is
expected to be around 6.7 keV and spectra from GX301-2 with a line energy at
6.4 keV show no differences in line energy between the old and new
calibrations. Spectra from detector C give systematically low line energies
with the old calibration. The discrepancy of the line energy increases with
time. The new calibration did not solve this problem. As an example, in Table 2
the iron line energies are compared for an observation of M87 on 1984 day 360
using the old and new calibration. Detector C failed on 1985 day 232. A
comparison of the Crab spectra shows a drop in the count rates in channels 6-10
some time between the second and third Crab observations. This different shape
of the spectra causes too low gain parameters which results in too low line
energies for spectra after the first Crab observation. The effect is not
understood and spectra from detector C should not be used to determine iron
Detector Line energy (keV)
Old cal. New cal.
A 6.68+/- 0.15 6.71+/- 0.15
B 6.66+/- 0.20 6.70+/- 0.20
C 6.48+/- 0.25 6.49+/- 0.25
D 6.93+/- 0.20 6.75+/- 0.20
E 6.91+/- 0.15 6.92+/- 0.15
F 6.77+/- 0.15 6.79+/- 0.15
G 6.76+/- 0.15 6.73+/- 0.15
H 6.65+/- 0.20 6.72+/- 0.20
To summarize, fits to spectra from various sources have shown that the new
calibration gives, on average, the same result up to about channel ~ 65. The
new calibration fits the Crab spectra better at higher channels, and channels
up to ~ 100 can be used in spectra of bright sources.
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