Screening Criteria

Bona fide X-ray events occupy a tightly defined region in the plane formed by the gain-corrected - i.e., invariant - rise time (RTI) and the pulse height (PHA). The xselect command gisclean will reject events outside this region and is included in the standard criteria used by ascascreen. In fact, it must be included because the response matrices assume that this selection has been performed.

Applying gisclean to the same GIS2 data from GX301-2 gets rid of an additional 3128 (4 per cent) of the remaining 81990 events not rejected by excluding the high background ring and calibration source. If RTI based rejection is applied before rather than after, 10669 events are removed.

From the satellite, the angle between the Earth's limb and the pointing direction is known as the elevation angle. At low elevation angles the target is viewed through the Earth's outer atmosphere which absorbs X-rays and hence distorts the spectrum. Negative elevation angles in the mkf file identify Earth occultations. We have found that data quality degrades with elevation angles less than 5 degrees. At angles greater than 10 degrees, the data quality no longer depends on elevation angle. This means that users can experiment with setting the minimum acceptable elevation angle at 5 degrees (lax) or 10 degrees (strict), depending on whether they want to boost signal-to-noise at the expense of possibly compromising data quality.

Note that sensitivity to elevation angle for a given observation depends on target position. Targets close to the ecliptic poles are the most susceptible, as the elevation angle can remain modest throughout the observation.

The elevation angle is recorded in the mkf file. In REV0 mkf files, the corresponding column name is ELV_MIN. In REV1 it is ELV.

In our example, setting the minimum elevation angle at 5 degrees removes only one additional event after the two previous criteria have been applied.

Cut-off rigidity (COR) is a local measure of the ability of the geomagnetic field to repel cosmic rays. Specifically, it is the minimum momentum (in units of GeV/c) with which a cosmic particle can penetrate as far as the satellite orbit. Since cosmic rays induce background, low values of COR identify those parts of the orbit which have high background. For the GIS, a value of 6 GeV/c yields plenty of events without seriously compromising quality.

Note that since background depends on COR, the actual spectrum used for background subtraction should have the same distribution of COR. This will automatically be the case if the background spectrum is extracted from the same screened event list as the source spectrum. If, on the other hand, blank-sky background is used, the COR distribution should match.

The cut-off rigidity is recorded in the mkf file. In REV0 mkf files, the corresponding column name is COR_MIN. In REV1 it is COR.

For the GIS2 data from GX301-2, setting COR to be greater than 6 GeV/c removes an additional 3484 events.

The lower discriminator corresponds to the lowest PHA which can be confidently identified above the noise level. If the GIS high voltage is turned off (either by command or by automatic trigger), no events pass the lower discriminator. Such periods should be rejected because they nevertheless appear to software as good time intervals, i.e., they erroneously contribute to the exposure. This is done by selecting on the mkf columns G2_L1 and G3_L1 (for GIS2 and GIS3, respectively) which contain the number of events above the lower discriminator.

The South Atlantic Anomaly (SAA) is a "hole" in the geomagnetic field which allows cosmic rays to penetrate further than usual. The particle background in the SAA is extremely high. In fact, the high voltage of the GIS is usually turned off during passage through the SAA. At such times, no events pass the lower discriminator (see above). SAA passages can be explicitly excluded by selecting on the mkf column SAA: requiring the value be zero excludes SAA passages.

In the case of the GIS2 data from GX301-2, applying the SAA criterion does not exclude any additional events.

SIS Screening Criteria

Ascascreen applies these screening criteria which are described in more detail under the links.

Hot and flickering pixels, which appear as false events, are a manifestation of radiation damage. Although they are unavoidably included in telemetry, they can be straightforwardly removed on the ground. The "sisclean" algorithm rejects those pixels which register an event more often than expected from Poisson statistics. Since they contain no astrophysical information and are not accounted for in the available response matrices, hot and flickering pixels should always be removed.

The number of hot and flickering pixels depends on epoch (the problem has worsened since launch) and on clocking mode (more of a problem in 4-CCD mode than in 1-CCD mode).

In the case of the SIS0 data from GX301-2, sisclean removes 61464 hot and flickering pixel events (45 per cent) from the total of 137415.

Grade is a one-dimensional description of the shape of the charge cloud created by an event. Certain grades have better spectral resolution than others (grade 0 has the best resolution) or are more likely to be bona fide X-ray events (grades 1, 5 & 7 contain mostly non X-ray events). Experience with lab and in-flight data has shown that the best combination of resolution and signal-to-noise is provided by using grades 0, 2, 3 & 4, while discarding the others. In fact, most of the available response matrices have been created for this combination which is selected in ascascreen by default. However, there are two important cases where users might want a different selection. First, Fast mode data have different grade assignments: only grade 0 events should be selected; they should be analyzed spectrally with "g02" matrices. Second, since analyzing a light curve does not require a response, users can increase the signal-to-noise of an SIS light curve by including grade 6 events.

In addition, please note that:

• On-board Bright mode data contain only grades 0-4 (up to late November 1993) or grades 0-6 (after November 1993).

• Goddard-converted Bright mode data contain only grades 0-4.

Selecting grades 0, 2, 3 & 4 for the SIS0 data from GX301-2 removes an additional 17239 events (23 per cent) from the 75951 left after the removal of the hot and flickering pixels.

From the satellite, the angle between the Earth's limb and the pointing direction is known as the elevation angle. At low elevation angles the target is viewed through the Earth's outer atmosphere which can absorb X-rays and hence distort the spectrum. Moreover, in the case of the SIS, which is also sensitive to optical and UV radiation, data quality is further impaired when the portion of the outer atmosphere in the FOV is illuminated by the Sun. This necessitates two elevation angle criteria: one for when the Earth's limb is dark, and a second, stricter one for when the limb is bright. Negative elevation angles in the mkf file identify Earth occultations.

For the dark limb we have found that data quality is significantly reduced with elevation angles less than 5 degrees. At angles greater than 10 degrees, the effect no longer depends on elevation angle. This means that users can experiment with setting the minimum acceptable elevation angle at 5 degrees (lax) or 10 degrees (strict), depending on whether they want to boost signal-to-noise at the expense of possibly compromising data quality.

For the bright limb the elevation angle can be set in the range 15-40 degrees for SIS0, with 20 degrees being a sensible default. For SIS1, the recommended range is 15-20 degrees.

Note that sensitivity to elevation angle for a given observation depends on target position. Targets close to the ecliptic poles are the most susceptible, as the elevation angle can remain modest throughout the observation.

The elevation angle, regardless of whether dark or bright, is recorded in the mkf file. In REV0 mkf files, the corresponding column name is ELV_MIN. In REV1 it is ELV. The Bright Earth elevation angle is BR_EARTH in the mkf file.

In our example, setting the minimum (dark) elevation angle at 5 degrees and the bright earth angle at 20 degrees removes an additional 8170 events (14 per cent) after the two previous criteria have been applied.

Cut-off rigidity (COR) is a local measure of the ability of the geomagnetic field to repel cosmic rays. Specifically, it is the minimum momentum (in units of GeV/c) with which a cosmic particle can penetrate as far as the satellite orbit. Since cosmic rays induce background, low values of COR identify those parts of the orbit which have high background. For the SIS, a value of 6 GeV/c yields plenty of events without seriously compromising quality. Note that since background depends on COR, the actual spectrum used for background subtraction should have the same distribution of COR. This will automatically be the case if the background spectrum is extracted from the same screened event list as the source spectrum. If, on the other hand, blank-sky background is used, the COR distribution should match.

The cut-off rigidity is recorded in the mkf file. In REV0 mkf files, the corresponding column name is COR_MIN. In REV1 it is COR.

For the SIS0 data from GX301-2, setting COR to be greater than 6 GeV/c removes an additional 2946 events.

To avoid telemetry saturation and pile-up the SIS is not used to observe very bright objects. This means that when high count rates do occur in the SIS, it is often the result of such non astrophysical occurrences as passing through the SAA or observing the bright limb of the Earth's atmosphere. The mkf column Sn_PIXLm records the number of pixels per readout (REV0) or per second (REV1) which exceed the event threshold in SISn/chipm. By imposing an upper limit on SISn/chipm, these non X-ray peaks can be rejected. In practice, however, the same default settings are used for all eight chips and depend only on the readout time (i.e., on clocking mode).

Setting S0_PIXL1 to be less than 400 events per readout (i.e., 400 events per 4 seconds in 1-CCD mode or 100 events per second) results in an additional 1983 events being removed.

The South Atlantic Anomaly (SAA) is a "hole" in the geomagnetic field which allows cosmic rays to penetrate further than usual. The particle background in the SAA is extremely high, though not so high that the SIS is turned off (unlike the GIS). SAA passages can be explicitly excluded by selecting on the mkf column SAA: requiring that the value be zero excludes SAA passages.

In the case of the SIS0 data from GX301-2, applying the SAA criterion after the lower discriminator criterion excludes an additional 627 events.

To derive the true PHA in each pixel, the "dark frame" is subtracted from the total PHA. Dark frame varies across each chip and depends on the radiation environment of the instrument. Because of these dependencies, the on-board computer calculates a 16 x 16 map of the dark frame for each chip and updates the map periodically. However, when the satellite clears the SAA, radioactive decays persist for a few seconds, raising the dark frame. This elevation of the dark frame occurs so rapidly that the dark frame map is not accurately calculated until about 4 readout times have elapsed, i.e., 16, 32 and 64 seconds for 1-CCD, 2-CCD and 4-CCD mode, respectively. The mkf column T_SAA gives the time in seconds after a passage through the SAA. Note that before the first SAA passage of an observation, T_SAA is negative. This means that for a 1-CCD mode observation, for example, the appropriate setting is that T_SAA be less than zero and greater than 16. Users should also be aware that mkf files have 32-s bins.

Since an inaccurate dark frame affects light curves less than spectra, users extracting light curves might want to omit the T_SAA criterion.

WARNING. The current version of ascascreen does not apply the correct value of T_SAA. Instead of converting 4 readout times into the appropriate number of seconds, ascascreen always sets T_SAA to be greater than 4 seconds, regardless of clocking mode. This is not a serious bug because applying the T_SAA criterion, even correctly, does not screen many events. Since ascascreen does not prompt the user to set T_SAA, the way around the problem is to edit the _mkf.sel file. Specifically, you should first run ascascreen as usual, but type "ascascreen -q" instead of "ascascreen". This will cause ascascreen to stop before running xselect. Second, type "ls" to identify two files which ascascreen has just produced: the _mkf.sel file, which contains the screening criteria, and the .xco file, which contains the xselect commands. Third, use an editor to overwrite the correct values of T_SAA in the _mkf.sel file (16, 32 or 64 seconds for 1-CCD, 2-CCD and 4-CCD modes, respectively). Finally, run xselect: after choosing a session name, type @ followed (without a space) by the name of the .xco file. By following these steps, you're effectively running ascascreen as usual, but with an interruption in the middle to correct the bug.

Applying this criterion to the SIS0 data from GX301-2 does not exclude any additional events.

To derive the true PHA in each pixel, the "dark frame" is subtracted from the total PHA. Dark frame varies across each chip and depends on the radiation environment of the instrument. Because of these dependencies, the on-board computer calculates a 16 x 16 map of the dark frame for each chip and updates the map periodically. However, when the satellite crosses the Day-Night (or Night-Day) Terminator, the optical illumination, and hence the dark frame, changes rapidly - so rapidly, in fact, that the dark frame map is not accurately calculated until about 4 readout times have elapsed, i.e., 16, 32 and 64 seconds for 1-CCD, 2-CCD and 4-CCD mode, respectively. The mkf column T_DY_NT gives the time in seconds after a passage through the Terminator (whether it's Day-Night or Night-Day). Note that before the first Terminator passage of an observation, T_DY_NT is negative. This means that for a 1-CCD mode observation, for example, the appropriate setting is that T_DY_NT be less than zero and greater than 16. Users should also be aware that mkf files have 32-s bins.

WARNING. The current version of ascascreen does not apply the correct value of T_DY_NT. Instead of converting 4 readout times into the appropriate number of seconds, ascascreen always sets T_DY_NT to be greater than 4 seconds, regardless of clocking mode. This is not a serious bug because applying the T_DY_NT criterion, even correctly, does not screen many events. Since ascascreen does not prompt the user to set T_DY_NT, the way around the problem is to edit the _mkf.sel file. Specifically, you should first run ascascreen as usual, but type "ascascreen -q" instead of "ascascreen". This will cause ascascreen to stop before running xselect. Second, type "ls" to identify two files which ascascreen has just produced: the _mkf.sel file, which contains the screening criteria, and the .xco file, which contains the xselect commands. Third, use an editor to overwrite the correct values of T_DY_NT in the _mkf.sel file (16, 32 or 64 seconds for 1-CCD, 2-CCD and 4-CCD modes, respectively). Finally, run xselect: after choosing a session name, type @ followed (without a space) by the name of the .xco file. By following these steps, you're effectively running ascascreen as usual, but with an interruption in the middle to correct the bug.

Since an inaccurate dark frame affects light curves less than spectra, users extracting light curves might want to omit the T_DY_NT criterion.

Applying this criterion to the SIS0 data from GX301-2 does not exclude any additional events.