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Calibration Sources for Spectral Analysis

Fred Seward


X-ray Astronomy needs in-flight calibration sources - astronomical objects with known spectra and intensity. Observations of these objects would be used to verify or calibrate the performance of instruments and software. Since the modern astronomer uses software developed by others, and often has neither the time nor the inclination to personally investigate the details, this is important. We, therefore, seek a set of sources which can be used to verify spectral analysis of data from past, present, and future missions. We also wish to compare software environments such as MIDAS, PROS/IRAF, XANADU, and perhaps your own package.

The ideal calibration source would be constant, would have no sharp spectral features, and would be bright enough to produce a high counting rate - but not so bright that the detector response is distorted. Since there are no accurate measurements of line intensities yet, we only consider broad-band calibration here. Accretion powered binaries, cataclysmic variables, and active galactic nuclei are unsatisfactory because of variability. Supernova remnants, clusters, and galaxies with high gas content (if the angular extent is not too great) might be suitable. Supernova remnants with bright non-thermal emission and high-temperature clusters have spectra dominated by a continuum. Isolated white dwarf stars might serve to calibrate at low energy.

We have extracted data from the Einstein and HEASARC archives and used the associated software for spectral analysis. This has been done by someone who is not familiar with all the instrument details. Results illustrate those which might be expected by the average user. This is also a step towards development of a set of calibration sources. The IPC spectral analysis used was that for point sources, which is not strictly correct. The PROS tools for extended sources were not available.

Table 1 lists several sources, the models used to fit them, and results obtained with Einstein/PROS, EXOSAT/XSPEC and Einstein SSS/XSPEC. Errors given are those quoted in the literature or are derived from the 68% confidence contours. The figures associated with this article compare 90% confidence contours obtained from the databases and generated with the data-base software. Inspection of these figures gives a feeling for accuracy.

This article describes work in progress, started in early 1991, and then neglected because of lack of time and also frustration with software still in the development stage. There is obviously more which could be usefully done. A set of data could be analyzed with both PROS and XSPEC. ROSAT observations should be added when the archive is available. I would very much like to hear comments and suggestions before carrying the work further.

The Crab Nebula

All but 5-10% of the Crab X-rays come from the diffuse source. With an extent of 2' or less, it is effectively a point source for calibration purposes. Since it is energized by the pulsar which loses energy at a yearly rate of 0.1%, the X-ray emission is expected to decrease slowly. A change of 10% per century is constant for our purposes.

The Crab Nebula is without doubt the best measured continuum X-ray source. Unfortunately, it is too bright for some detectors (e.g. the Einstein IPC). Figure 1 shows the Einstein MPC and EXOSAT ME results from the Crab Nebula. Contours shown are 68%, 90%, and 99% confidence. Figures 2 and 3 compare results of several observations using only the 90% contour.

We note that:

1. Different observations of the Crab, using the same system, do not give the same answer. Although only two observations were analyzed for each detector, we suspect that systematic errors included in the MPC/PROS program are too large and that those included in the ME/XSPEC program are too small.

2. Different groups might have used different theoretical Crab spectra to calibrate.

3. The Crab spectra in Figure 2 illustrate the optimum results which might now be expected from spectral analysis of these data sets within these archives. Results from other sources will be more uncertain.

Table 1 Best-Fit Spectra for Calibration Sources

Another Crab-like Remnant

The remnant G21.5-0.9 has an X-ray extent of ~1' and is strongly absorbed. It is a synchrotron source but with counting rate a factor of several hundred less than that of the Crab. Figure 2 compares 90% confidence contours of several different observations of this source. The spectral index is not restricted by the IPC. The MPC and ME detectors give about the same results. Better data are needed before this will be useful as a calibration source. It is amusing that the archived ME data yield a different answer than the published result. Presumably the data and analysis software were the same.

Clusters of Galaxies

Here are large diffuse sources which, if there are no AGN within the cluster, will not vary with time. Although the emission is thermal, some clusters have high enough temperature so much of the emission is in the continuum. Both clusters illustrated here are ~20' in X-ray extent, not ideal for absolute intensity calibration, but since most emission is from the bright center, not too bad.

Figure 3 compares data for the clusters, A1796 and A401, which have been observed by many spacecraft. Crosses show results from the literature. These are placed at the 21cm measured NH value since these results do not claim to measure column density. The MPC did not measure NH well, and the IPC did not measure the spectral index well. Einstein data is shown for A401 to illustrate the intersection of these two results.


Low energy photons from the above sources are all absorbed by the ISM and are not useful for calibrations below 0.5 keV. A number of white dwarf stars emitting soft X-rays are nearby. Since these are thought not to vary, they might be good for low-energy calibrations. HZ43 is one of the brightest and Table 1 contains some (rather incomplete) information concerning this source.

Figure 1 The Crab Nebula

Figure 2 SNRs

Figure 3 Clusters

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