OGIP Calibration Note CAL/ROS/94-005

ROSAT PSPC: Informal Study Note

Region Selection based on Detector Coordinates: A Cautionary note on Extracted Spectra

ROSAT PSPC: Informal Study Note

Region Selection based on Detector Coordinates: A Cautionary note on Extracted Spectra


Code 668
NASA/GSFC,
Greenbelt, MD20771

Version: 1995 Jan 31

SUMMARY

This memo is intended to remind Guest Investigators (GOs) of the importance of the effects related to ``Ghost Imaging'' in extracted spectra extracted under certain criteria.

LOG OF SIGNIFICANT CHANGES

Release Sections Changed Brief Notes
Date
1994 Apr 13 First public version
1995 Jan 31 All Made compatible with LaTeX2HTML software

1  INTRODUCTION

The fact that the ROSAT PSPC can not always properly position X-ray events with very low energy has been well documented. This is the ``Ghost Imaging'' effect discussed at length in Nousek and Lesser 1993 (ROSAT newsletter 8). We have had several guest investigator questions concerning spectra extracted under certain criteria, and this memo is intended to remind investigators of the importance of a spectral effect related to ``Ghost Imaging''.

2  DETECTOR COORDINATE SELECTION

The PSPC uses induced charge on the cathode wires to obtain photon positions. For very low pulse-heights sometimes only one or two cathodes register a signal above the lower level discriminator. In these cases the position determination degenerates to a line or point (respectively). This is known as the ``Ghost Imaging'' effect and is detailed in Snowden, McCammon, Burows & Mendenhall (1994, Ap. J. 424, 714) and Nousek & Lesser (1993). Figure 1 shows a high signal-to-noise image in the detector coordinate system in the ~ channel 12 -17 range, illustrating the checkered image effect. Examples of ghost imaging can be seen by examination of the detector maps in channel ranges 8-19 & 11-19 (available from the legacy ftp area on /caldb/data/rosat/pspc/cpf/).

The cathode strips are ~ 5 arcmin (3.5 mm) in width, this produces the size- scale of the checkered pattern. During a typical observation, the source is wobbled across a path several arcminutes in length over a timescale of typically 400 seconds. If a selection is made on detector coordinates such that spectra are preferentially extracted on or off the wire locations, then the wrong soft count rate will be inferred and a skewed spectrum may be extracted. As the cathode wire spacing is ~ 5 arcminutes, this becomes a pronounced problem if the extraction cell is small (radius less than ~ 2.5 arcmin) . Figure 2 shows a ratio of vela pulsar spectra taken in a circular cell of radius 40 arcsec, the numerator is the on-wire spectrum, the denominator is the off-wire spectrum. The spectrum taken at the wire intersection is very much harder than the spectrum taken between the wires. In Figure 1, the effect is a factor ~ 4 difference in counts in channel 12 (the high energy ratio is not exactly 1 because of the slight difference in time spent at the on- and off-wire locations). It is obvious that these two spectra will give very different spectral fit results. Investigators may notice a similar effect if they use a small extraction cell in (one of the rare) unwobbled or very brief observations (such as some early calibration observations where the exposure time was comparable to the wobble cycle time of ~ 400 seconds), when soft enhancements and deficits are not averaged out.

In summary, investigators should be careful when using a small extraction cell based on detector coordinate selections, or in brief or unwobbled observations. Spectra extracted under these criteria can have a soft count rate (below channel 20) which is significantly enhanced or suppressed.

FIGURES

Figure

Figure 1: A high signal-to-noise image in the detector coordinate system in the ~ channel 12-17 range, illustrating the checkered image effect.

Figure

Figure 2: The ratio of Vela pulsar spectra taken in a circular cell of radius 40 arcsec, the numerator is the on-wire spectrum, the denominator is the off-wire spectrum. It can be seen that the spectrum taken at the wire intersection is very much harder than the spectrum taken between the wires.

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