ME CALIBRATION UPDATE
Although the present calibration of the ME detector is adequate for the majority
of sources, some problems have been noticed when carrying out spectral fits to
bright sources. In particular:
1. An excess of counts above channel ~ 60 in, the argon chamber.
2. Systematic trends in the residuals around 3 keV.
3. Poorer fits to the spectra of bright sources obtained during the last year of
4. Differences in the relative effective areas of the argon and xenon chambers
when simultaneously fitting argon and xenon data to sources with quite different
These problems reflect three basic uncertainties in the calibration of the ME. 1)
The value of the effective area of the argon and xenon areas corrected for
collimator transmission effects, 2) the thickness of the windows to the argon
chamber for each detector and 3) the non-linearity of the gain of the argon
detectors. In order to overcome these problems the ME is being recalibrated using
a new three stage approach in which each of the above uncertain parameters are
determined by matching the predicted count rate and spectrum to those observed
from the Crab. The measurements used for this recalibration are the 9 Crab Nebula
observations made at various intervals through the mission, the original ground
calibration and the in-flight radioactive calibration data. Final details of the
implementation of this new calibration will be provided in the near future when a
calibration update tape is issued. The following gives an overview of the
methodology that is being used.
1. Determine the geometric area of the detectors
Each of the eight xenon chambers is located behind an argon chamber and so the
xenon sensitivity is modified by the absorption properties of the argon chamber
and the associated detector windows. The overall count rate in any particular
xenon detector is determined by the following:
a. the transmission of the collimators.
b. the detector geometric area.
c. the thickness of the front window.
d. the depth of the argon chamber.
e. the pressure in the argon chamber.
f. the thickness of the window seperating the argon and xenon chambers.
g. the depth of the xenon chamber.
h. the pressure in the xenon chamber.
i. the loss of area associated with the xenon guard wires (1-2% the total
j. sampling losses from the onboard computer (0BC).
The overall counting rate from the Crab in each xenon chamber
is not strongly dependent on the gain or resolution functions because at low
energies the overlying material (i.e. the argon chamber and the intermediate
window) limits the count rate, while at high energies the steeply falling Crab
spectrum means that few counts are observed above ~40 keV. In the above list the
largest uncertainty is associated with the total effective area and the
efficiency of the collimator. The low energy response is principly determined by
the depth of the argon chamber, and the thickness of the intervening window and
are both well known. Uncertainties in the other properties that effect the low
energy response such as the thickness of the windows, the kapton foil, the column
density to the Crab and the pressure of the argon chamber do not significantly
effect the total count rate. The loss of area associated with the guard wires is
small and well known from ground calibrations. The OBC sampling losses are well
calibrated. A power law photon spectrum with an index of -2.10 and a
normalization of 9.7 (the generally accepted values for the Crab) was folded
through the response of each detector and the geometric area/collimator
transmission adjusted until the predicted count rate agreed with that
2. Determine the Front Window Thicknesses
One of the critical parameters for the argon detectors is the thickness of the
window. Small variations in thickness can lead to large changes in the spectral
response at low energies. Again the overall counting rate is relatively
insensitive to uncertainties in the gain. The counting rate in the argon chamber
a. The column density to the Crab.
b. The thickness of the kaptan foil.
c. The front window thickness.
d. The area/collimator transparency product.
e. The depth of the argon chamber.
f. The loss of area associated with the argon anticoincidence guard wires.
g. The pressure in the argon chambers.
h. The losses associated with the risetime vetoing.
i. OBC sampling losses.
The geometric area/collimator transparency product was assumed to be the value
obtained from matching the observed and predicted xenon counting rates. The value
used for the column density to the Crab was the value of 3 x 1021 H atoms cm-2
measured by EXOSAT by fitting various absorption models to the count rates
obtained in the low energy telescope using different filters. This value is in
good agreement with previous measurements. The risetime and guard losses were
measured by observing the Crab with each turned on and off. Pre-launch
measurements of the kaptan foil thickness and gas pressure were assumed. All the
other parameters are relatively well known from ground or inflight calibration.
The front window thickness could then be optimized by varying the window
thickness of each detector until the predicted and observed counts from the Crab
were equal. Non-uniform window thicknesses did not significantly improve the
3. Determine the Gains and Energy Resolutions.
Once the geometic areas and window thicknesses are known the gain and energy
resolution functions can be determined by iteratively comparing an observed
spectrum with that predicted using trial resolution and gain functions until
agreement is reached. In the current calibration data the gains and resolutions
are given analytically.
In the ground calibrations it was noticed that there was a small dependence in
the gain with position on the face of each detector. The dependence was such that
the gain was highest over the anode wires and lowest between them. In the case of
the argon chambers gain changes as large as 10% were noticed, while for xenon the
effect was about a factor of two smaller. The effect is likely caused by
space-charge effects. Since the gain measurements were not made at energies above
8 keV it is possible that the effects are larger at higher energies. The new
calibration data includes the effects of this gain modulation by allowing for a
more complex resolution function than hithertoo used. Such a gain modulation
seems consistent with a decrease in resolution seen at high energies and the
observation of genuine x-ray counts in high channel numbers in the argon
Possible origins for the systematic "wiggle" in the residuals at around 3 keV
have been considered. Using the present calibration data these tend to have a
characteristic shape with an excess of observed counts at ~3 keV and a deficit
at slightly higher energies. It is likely that this effect is caused by
variations in the widths of individual energy channels. In the new calibration
data this is accounted for by providing individual channel boundaries, rather
than analytic functions for the gain curves. Channel boundaries are obtained by
fits to each of the Crab observations and the time dependence is modelled by
assuming a linear change between observations.
The new calibration scheme will involve 1) slightly revised areas and window
thicknesses, 2) Individual energy channel boundaries as a function of time, 3)
Revised resolution functions and 4) new electronic acceptance values (in fact
unmodified guard acceptance values). Currently we are comparing the results of
fits to a range of bright sources using the current calibration data and a
preliminary set of revised data. Another cross check being made is a comparison
of GSPC and ME results on bright sources.
A. N. Parmar