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Applying Standard Filters the Data

Whether using the GUI or the command line, the needed filtering ``expressions'' for the MOS and PN are, respectively:

(PATTERN $<=$ 12)&&(PI in [200:12000])&&#XMMEA_EM


(PATTERN $<=$ 12)&&(PI in [200:15000])&&#XMMEA_EP

If the PN data is timed, then the PATTERN parameter should be set to 4:

(PATTERN $<=$ 4)&&(PI in [200:15000])&&#XMMEA_EP.

The first two expressions will select good events with PATTERN in the 0 to 12 range, and the last will select events with PATTERN between 0 and 4. The PATTERN value is similar the GRADE selection for ASCA data, and is related to the number and pattern of the CCD pixels triggered for a given event.The PATTERN assignments are: single pixel events: PATTERN == 0, double pixel events: PATTERN in [1:4], triple and quadruple events: PATTERN in [5:12].

The second keyword in the expressions, PI, selects the preferred pulse height of the event; for the MOS, this should be between 200 and 12000 eV. For the PN, this should be between 200 and 15000 eV. This should clean up the image significantly with most of the rest of the obvious contamination due to low pulse height events. Setting the lower PI channel limit somewhat higher (e.g., to 300 eV) will eliminate much of the rest.

Finally, the #XMMEA_EM (#XMMEA_EP for the PN) filter provides a canned screening set of FLAG values for the event. (The FLAG value provides a bit encoding of various event conditions, e.g., near hot pixels or outside of the field of view.) Setting FLAG == 0 in the selection expression provides the most conservative screening criteria and should always be used when serious spectral analysis is to be done.

It is a good idea to keep the output filtered event files and use them in your analyses, as opposed to re-filtering the original file with every task. This will save much time and computer memory. As an example, the pipelined Lockman Hole data for the MOS1 is 44Mb; the quality-filtered list is 29Mb, and when the temporal filter is applied, the final ``clean'' event list is only 7Mb!

To filter the data using the heraXmmselect GUI:

Enter the filtering criteria in the ``Selection Expression'' area at the top of the heraXmmselect GUI:
(PATTERN $<=$ 12)&&(PI in [200:12000])&&#XMMEA_EM
Click on the ``Filtered Table'' box at the lower left of the heraXmmselect GUI.
In the General tab, change the filteredset parameter, the output file name, to something useful, e.g., mos1_filt.fits
Click ``Run''.

To filter the data using evselect in the Command Window, type:

evselect table=mos1.fits withfilteredset=yes
$   $ expression='(PATTERN $<=$ 12)&&(PI in [200:12000])&&#XMMEA_EM'
$   $ filteredset=mos1_filt.fits filtertype=expression keepfilteroutput=yes
$   $ updateexposure=yes filterexposure=yes


table - input event table
filtertype - method of filtering
expression - filtering expression
withfilteredset - create a filtered set
filteredset - output file name
keepfilteroutput - save the filtered output
updateexposure - for use with temporal filtering
filterexposure - for use with temporal filtering

next up previous contents
Next: Applying Time Filters the Up: Examine and Filter the Previous: Create and Display a   Contents
Lynne Valencic 2011-07-26