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ROSAT PSPC

Version: 1992 Oct 30 	               OGIP Calibration Memo CAL/ROS/92-001a

Summary

This memo is an addendum to OGIP Calibration Memo CAL/ROS/92-001 (Hasinger et al 1992) in which we compare the ROSAT PSPC on-axis point spread function psf with the best signal-to-noise data obtained to date for a point source. We confirm that the parameterization detailed in CAL/ROS/92-001 does satisfactorily describe the observed psf at energies o(>,~)0.19 keV.

1 The Data & Model

As detailed in CAL/ROS/92-001, the psf of the ROSAT X-ray mirror assembly (XMA) + PSPC is a convolution of 5 components, 3 of which (namely the XMA scattering profile, the intrinsic spatial resolution of the PSPC, and Focus and detector penetration effects) have been parameterized for on-axis observations from PANTER ground calibration measurements. In CAL/ROS/92-001 it was shown that the derived analytical functions satisfactorily described 5 moderate signal-to-noise datasets out to a radius ~1-2 arcmin in all but the softest band (B-band; where `ghost imaging' was already known to be a problem). Here we use a very high signal-to-noise (o(>,~)6 x 10⁶ photons) dataset of a (nominal) point source for a detailed comparison of the model psf and data out to a radius ~10 arcmin.

1.1 The Dataset

The data used here is proprietary data (PI Thomas) thus the source is not named here, nor is the observation described in detail. Radial profiles were extracted, centered on the source, using the same method described in CAL/ROS/92-001, with 5 arcsecond incremental radii, in the bandpasses as listed in Table 1 of CAL/ROS/92-001. The lowest 11 PI channels were rejected to exclude problems due to the variable lower limit discriminator for valid events, due to the variable instrument gain which is folded into these data. Any additional sources falling within the specified annuli were masked out of the analysis. No background subtraction was carried out. Background rates were measured from the images and were later folded into the predicted profile template for each band.

1.2 The Model

As the in-flight data are affected by more uncertainties than the ground data (aspect corrections, background subtraction, gain correction, etc.) it was not possible to allow profile fitting with the parameters of the gaussian + exponential + lorentzian components to be free. Instead we calculated the psf for the source in each bandpass.

First, a spectrum was extracted for the source, in a circle of size 200 arcseconds radius (masking out a nearby source). Next, a psf was calculated for each energy channel using the algorithm and parameters given in CAL/ROS/92-001[1]. A predicted psf template was calculated in each band using the source spectrum to determine the photon weighting to be applied to the psf component in each energy channel. Thus, for each band and spectrum a combined psf was produced, including a constant term for the background.

2 Results

These predicted psf templates are overlaid on the appropriate datasets in Figure 1. We stress that the normalization of the model psf in each case was also calculated using the equations in CAL/ROS/92-001, thus no fitting was performed. The normalization of the predicted psf was calculated such that the integral under the predicted template is equal to the integral under the observed psf. Discrepancies between the shape of the two curves thus result in the slight discrepancy at the peak seen in some panels of Figure 1.

It can be seen that even at this high signal-to-noise ratio, the psf model provides a good parameterization of the source profile for all but the B-band. The individual panels give an estimate of the error in the psf, which it was not possible to illustrate with lower signal-to-noise data.

B-band (0.12-0.188 keV; panel a) illustrates the ghost imaging problem present in the softest channels of the PSPC.

C-band (0.188-0.284 keV; panel b) illustrates a slight excess in the data at radii ~1-2 arcmin, and a very small excess in the data at radii ~3-4 arcmin. Previously it was thought that the ghost imaging effect was only important below PI channel 15 (0.15 keV), but recent analysis of other high signal-to-noise data has suggested ghost imaging may be significant up to channel ~30 and thus may significantly affect the C-band (Nousek & Harnden, private communication).

R1-band (0.284-0.5 keV; panel c) illustrates a notable excess in the data at radii ~1.5-3 arcmin.

R2 & R3-bands (0.5-2.48 keV; panels d & e) demonstrate good agreement between the data and model psf.

These small scale discrepancies may reflect some of the differences between the ground calibration and in-flight data:

• the Gaussian component (due to spatial resolution of the detector): the width in the model could be narrower than the data due to residual errors in the attitude corrections;
  
• the exponential component (due to combined focus and penetration effects): the geometry of the beam used in the PANTER calibration facility was different to the parallel beam of the in-flight data. Therefore the detailed shape of the penetration term could be slightly different in-flight;
  
• the Lorentzian component (due to the scattering profile of the XRT mirrors): the average grazing angle is slightly larger in the finite beam than in orbit. Approximately 40% less mirror scattering is expected in-orbit than on the ground. In addition, the mirrors may have suffered dust contamination after the PANTER tests, and may therefore have a greater microroughness in-flight.
  

3 Conclusions

The conclusions given in CAL/ROS/92-001 still hold, and these data give the best estimate to date of the residual error in the psf model.

Work continues to refine and improve this model, and to extend this work to include off-axis angles.

4 Ackowledgements

We thank the PI Thomas (MPE), for allowing his proprietary data to be used in this way, and the many people at MPE involved in the determination and interpretation of the PANTER data. We also thank Dave Davis (GSFC) for his help extracting the data.

References

Hasinger, G., Turner, T. J., George, I. M. & Boese, G., 1992. Legacy, 2, 77 (CAL/ROS/92-001).

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