Applying Time Filters the Data

Sometimes, it is necessary to filter on time, in addition to those mentioned above. This is because of soft proton background flaring, which can have count rates of 100 counts/sec or higher.

It should be noted that the amount of flaring that needs to be removed depends in part on the object observed; a faint, extended object will be more affected than a very bright X-ray source.

There are two ways to filter temporally: with an explicit reference to the TIME or RATE parameters in the filtering expression, or by creating a secondary Good Time Interval (GTI) file with the task tabgtigen. Both procedures are described below. For the example data, we will filter by time, though you can just as easily filter by rate.

To explicitly define the TIME or RATE parameters in the GUI or Command Window:

1)

Make a light curve and display it in fv (see §7.3.2). In the fv window, you can zoom in on interesting sections by making a selection box by left-clicking over the graph, beginning with the upper left hand corner of the region.

2)

Mouse over the plot to see the rates and times, and note the times for the ``good'' regions.

The syntax for the time selection is (TIME $<$ 1.54912e8). A more complicated expression which would remove some small flares within an otherwise good interval (e.g., the soft proton flares observed in the light curve plot of Figure 7.3) could be: (TIME $<$ 1.54912e8)&&!(TIME IN [1.54854e8:1.54872e8]). The syntax &&(TIME $<$ 1.54912e8) includes only events with times less than 1.54912e8, and the ``!'' symbol stands for the logical ``not''. So, use &&!(TIME in [1.54854e8:1.54872e8]) to exclude events from the interval 1.54854e8 to 1.54856e8. If combined with the standard filtering expression (see §7.3.2), the full filtering expression would then be:

(PATTERN $<=$ 12)&&(PI in [200:12000])&&#XMMEA_EM
$$ &&(TIME $<$ 1.54912e8)&&!(TIME in [1.54854e8:1.54872e8])

This expression can then be used in either the heraXmmselect GUI or Command Window to filter the event file, as in §7.3.3. If using the GUI, be certain that the updateexposure and filterexposure boxes are checked. Similarly, if using the Command Window, make sure to set those parameters to ``yes'', as in the example above. As ever, give the new file a useful name; here, we will use mos1_filt_time.fits.

To make a secondary GTI file and apply it in the GUI:

1)

Call tabgtigen from the Available Tools panel and enter the light curve, mos1_ltcrv.fits as the input data set. (There is no need to quit heraXmmselect.)

2)

Edit the name of table to hold resulting GTI list; here, we will use gtiset.fits:STDGTI.

3)

In the text box directly below (``Boolean expression controlling the GTI creation''), enter the filtering expression . In this case, it is (TIME $<$ 1.54912e8) &&! (TIME in [1.54854e8:1.54872e8])

4)

Click ``Run'' in the lower left corner.

5)

In heraXmmselect, load the filtered event file by going to File $\rightarrow$ New Table and entering mos1_filt.fits.

6)

In the ``Selection Expression'' box, type GTI(gtiset.fits,TIME).

7)

Click on the ``Filtered Table'' box at the lower left of the heraXmmselect GUI.

8)

Change the filteredset parameter, the output file name, to something useful; here, we will use mos1_filt_time.fits.

9)

Click ``Run''.

To make a secondary GTI file and apply it in the Command Window, type:

tabgtigen table=mos1_ltcrv.fits gtiset=gtiset.fits timecolumn=TIME
$$ expression='(TIME $<$ 1.54912e8)&&!(TIME in [1.54854e8:1.54872e8])'

where

table - input count rate table and extension (see §7.3.2)
expression - filtering expression
gtiset - output file name for selected GTI intervals
timecolumn - time column

And then type:

evselect table=mos1_filt.fits withfilteredset=yes
$$ expression='GTI(gtiset.fits,TIME)' filteredset=mos1_filt_time.fits
$$ filtertype=expression keepfilteroutput=yes
$$ updateexposure=yes filterexposure=yes

where

table - input count rate table and extension (see §7.3.2)
expression - filtering expression
withfilteredset - create a filterered set
filteredset - output file name
filtertype - method of filtering
keepfilteroutput - save the filtered set
updateexposure - update exposure information in event list and in spectrum files
filterexposure - filter exposure extensions of event list with same time
$$ filters as for corresponding CCD