Using the Output of Detect Tools

Edit the List of Sources

Once the source list is overlaid on the image, you may see some sources that you want to exclude from the analysis. Currently, ds9 does not number the sources in the field of view. However, double-clicking on a source region brings up the region info box, which lists the region "ID". In Figure 2 , the source on the left side of the field of view was chosen -- the ID is 14. This number corresponds to the row the region occupies in the FITS file.

NOTE: this technique only works with the first region file loaded into ds9. After that, the imager starts counting up. For example, if you load region_a.fits that has 10 sources, then region_b.fits that has 6, the regions of the second file will be counted from 11 onward. One may, of course, always examine the region file visually to determine which rows to delete.

To create a new region file that doesn't have row 14 in it, dmcopy is used:

unix% dmcopy "s3_src.fits[#row=1:13,15:17]" s3_1gone_src.fits

unix% dmlist s3_1gone_src.fits blocks
 
--------------------------------------------------------------------------------
Dataset: s3_1gone_src.fits
--------------------------------------------------------------------------------
 
     Block Name                          Type         Dimensions
--------------------------------------------------------------------------------
Block    1: PRIMARY                        Null        
Block    2: SRCLIST                        Table        31 cols x 16       rows

The dmlist command shows that there are only 16 rows in the file now. This new file may be displayed in ds9  to ensure that the correct source was removed.

This filter was fairly simple. It is possible to make more complicated filters with the DM syntax. If you wanted to exclude rows (sources) 8, 12, and 14 from the original file:

unix% dmcopy "s3_src.fits[#row=1:7,9:11,13,15:17]" s3_3gone_src.fits

Figure 4  shows the new source list, indicating where the deleted objects used to be.

Sort on a Particular Column

The tool dmsort makes it possible to sort on any column in the FITS file. To order the rows by NET_COUNTS:

unix% dmlist "s3_src.fits[cols net_counts]" data
 
--------------------------------------------------------------------------------
Data for Table Block SRCLIST
--------------------------------------------------------------------------------
 
ROW    NET_COUNTS  
 
     1                 60.0
     2              544.750
     3             48.81250
(cut)
    15                 47.0
    16       112.9583358765
    17              80.8750

unix% dmsort s3_src.fits s3_sorted_src.fits keys=net_counts copyall=yes

unix% dmlist "s3_sorted_src.fits[cols net_counts]" data
 
--------------------------------------------------------------------------------
Data for Table Block SRCLIST
--------------------------------------------------------------------------------
 
ROW    NET_COUNTS  
 
     1        22.7142848969
     2        25.1428565979
     3        33.1428565979
(cut)
    15       112.9583358765
    16       190.2857208252
    17              544.750

The default behavior of dmsort is to organize the results in ascending order. To sort NET_COUNTS in descending order, add a dash ("-") before the column name:

unix% dmsort s3_src.fits s3_descend_src.fits keys=-net_counts copyall=yes

unix% dmlist "s3_descend_src.fits[cols net_counts]" data
 
--------------------------------------------------------------------------------
Data for Table Block SRCLIST
--------------------------------------------------------------------------------
 
ROW    NET_COUNTS  
 
     1              544.750
     2       190.2857208252
     3       112.9583358765
(cut)
    15        33.1428565979
    16        25.1428565979
    17        22.7142848969

It is also possible to have several sort priorities (i.e. sort by NET_COUNTS, then NET_COUNTS_ERR); see ahelp dmsort for more information on how to sort files.

Filtering the Source List

If you are interested in only using a certain range of data in your analysis, use the DM filtering syntax to create a new source list with only those items.

To keep the sources that have 40 or more net counts:

unix% dmcopy "s3_src.fits[net_counts=40:]" s3_40cts_src.fits
unix% dmlist "s3_40cts_src.fits[cols net_counts]" data
 
--------------------------------------------------------------------------------
Data for Table Block SRCLIST
--------------------------------------------------------------------------------
 
ROW    NET_COUNTS  

     1                 60.0
     2              544.750
     3             48.81250
     4             79.43750
     5             104.6250
     6             44.93750
     7       190.2857208252
     8        54.1428565979
     9                 47.0
    10       112.9583358765
    11              80.8750

or those with NET_COUNTS between 25 and 80:

unix% dmcopy "s3_src.fits[net_counts=25:80]" s3_25-80cts_src.fits
unix% dmlist "s3_25-80cts_src.fits[cols net_counts]" data
 
--------------------------------------------------------------------------------
Data for Table Block SRCLIST
--------------------------------------------------------------------------------
 
ROW    NET_COUNTS  
 
     1                 60.0
     2             48.81250
     3             79.43750
     4             44.93750
     5        39.4285697937
     6        25.1428565979
     7        33.1428565979
     8        54.1428565979
     9        37.2857131958
    10        38.4772720337
    11                 47.0

See ahelp dmfiltering for other variations on the filtering syntax.

Converting Coordinates to Sexagesimal

Often it is desirable to have the list of sources in sexagesimal format, i.e. (RA,Dec) = (hms,dms). There are two ways that this may be done - in ds9 or with S-Lang.

• In ds9: load the event file and display the source list on top of it. From the "Region" menu, set "File Format -> CIAO" and "File Coordinate System -> WCS and Equatorial J2000". Finally, "Save Regions" as s3_src_ds9.reg.

  The output (an ASCII region file) still contains the shape definitions, but the coordinates are now given as sexagesimal:

  unix% more s3_src_ds9.reg
  # Region file format: CIAO version 1.0
  ellipse(14:11:27.380,+52:11:31.70,0.0195851',0.0173948',45)
  ellipse(14:11:23.422,+52:13:32.14,0.0183431',0.0158548',53.264)
  ellipse(14:11:20.701,+52:12:07.98,0.0197903',0.0184924',15.4458)
  ellipse(14:11:20.530,+52:12:10.08,0.0191862',0.0170336',155.484)
  ellipse(14:11:20.387,+52:12:11.51,0.0203776',0.0173993',75.4574)
  ellipse(14:11:13.633,+52:13:41.46,0.0169192',0.0153512',12.7196)
  ellipse(14:11:30.846,+52:14:24.20,0.0390769',0.0242176',31.4398)
  ellipse(14:11:29.124,+52:13:33.01,0.0321857',0.0254928',125.887)
  ellipse(14:11:20.686,+52:14:11.97,0.03602',0.0308016',32.1849)
  ellipse(14:11:19.464,+52:14:00.94,0.0286652',0.0194748',33.3924)
  ellipse(14:10:59.522,+52:11:53.84,0.0395356',0.0245289',151.558)
  ellipse(14:10:57.363,+52:11:31.25,0.0386767',0.0297596',123.502)
  ellipse(14:11:14.499,+52:16:11.60,0.0546014',0.0455196',5.8862)
  ellipse(14:11:29.153,+52:16:43.48,0.0937733',0.0753438',149.301)
  ellipse(14:11:08.981,+52:16:45.68,0.0790725',0.0635626',174.887)
  ellipse(14:11:04.245,+52:17:56.15,0.0995391',0.089148',15.4741)
  ellipse(14:11:24.716,+52:19:23.96,0.131417',0.107319',167.549)
  

  The benefit of this method is that the file may be redisplayed in ds9, as it retains the CIAO region format.

• With S-Lang: use a small slang script, deg2sexag.sl, to convert the FITS source list. Download the script and load it into ChIPS with the evalfile command:

  unix% chips
  
  Welcome to ChIPS, version CIAO 3.4
  Copyright (C) 1999-2003, Smithsonian Astrophysical Observatory
  
  chips> evalfile("deg2sexag.sl")
  1
  chips> sources=readfile("s3_src.fits") 
  chips> (ra,dec)=deg2sexag(sources.RA,sources.DEC)
  chips> writeascii(stdout,sources.COMPONENT,ra,dec,sources.X,sources.Y)
  1       14:11:27.38     +52:11:31.69    3961.14 4035.14
  2       14:11:23.42     +52:13:32.14    4035.16 4279.93
  3       14:11:20.70     +52:12:07.97    4085.98 4108.85
  4       14:11:20.53     +52:12:10.08    4089.18 4113.13
  5       14:11:20.39     +52:12:11.51    4091.84 4116.04
  6       14:11:13.63     +52:13:41.46    4217.98 4298.88
  7       14:11:30.85     +52:14:24.19    3896.6  4385.79
  8       14:11:29.12     +52:13:33.00    3928.69 4281.72
  9       14:11:20.69     +52:14:11.96    4086.27 4360.86
  10      14:11:19.46     +52:14:00.94    4109.08 4338.45
  11      14:10:59.52     +52:11:53.84    4481.75 4080.36
  12      14:10:57.36     +52:11:31.25    4522.16 4034.49
  13      14:11:14.50     +52:16:11.60    4201.7  4604.05
  14      14:11:29.15     +52:16:43.48    3928.33 4668.88
  15      14:11:08.98     +52:16:45.68    4304.62 4673.37
  16      14:11:04.25     +52:17:56.15    4392.81 4816.67
  17      14:11:24.72     +52:19:23.95    4011.19 4995.01
  
  chips> writeascii("s3_src_sexag.reg",sources.COMPONENT,ra,dec)
  
  chips> exit
  

  The output (s3_src_sexag.reg) is an ASCII source file which contains the positions in the new format:

  unix% more s3_src_sexag.reg 
  1       14:11:27.38     +52:11:31.69
  2       14:11:23.42     +52:13:32.14
  3       14:11:20.70     +52:12:07.97
  (cut)
  15      14:11:08.98     +52:16:45.68
  16      14:11:04.25     +52:17:56.15
  17      14:11:24.72     +52:19:23.95
  

  This output cannot be loaded into ds9 as it no longer contains the shape definitions. It would, however, be more straightforward to incorporate this output into a latex table than the ds9 output.

  For ease in using this frequently, it may be added to the /home/username/.varmmrc resource file. See ahelp configure for information on using resource files.

Creating Individual Spectra

It is possible to create a spectrum from any of the regions in the source list without having to divide up the file. To extract a PI spectrum of the second source  in the file and bin it by a factor of 5:

unix% punlearn dmextract
unix% dmextract "s3_evt2.fits[sky=region(s3_src.fits[#row=2])][bin pi=::5]" \
      spectrum_2.fits

The contents of the parameter file may be checked using plist dmextract. The resultant spectrum can be plotted in ChIPS:

unix% chips

Welcome to ChIPS, version CIAO 3.4
Copyright (C) 1999-2003, Smithsonian Astrophysical Observatory

chips> curve "spectrum_2.fits[cols channel,counts]"
chips> curve simpleline; symbol none
chips> limits x 0 100
chips> xlabel "channel"; xlabel size 1.5
chips> ylabel "counts"; ylabel size 1.5
chips> exit

Figure 6  shows the plot. This data may also be fit in Sherpa, once the proper response files have been created; see the Step-by-Step Guide to Creating ACIS Spectra for Pointlike Sources and Independent Background Responses threads for more information.

Creating Multiple Spectra

To create an individual spectrum for each of the sources, use the lgrid(1:n:steps) stack syntax, where n is the number of sources in the file.

For s3_src.fits, which has 17 sources in it:

unix% dmextract "s3_evt2.fits[sky=region(s3_src.fits[component=lgrid(1:17:1)])][bin pi=::5]" \
      multi_spectra_phaII.fits

The output file is a type II PHA file that has one spectrum per row:

unix% dmlist multi_spectra_phaII.fits blocks
 
--------------------------------------------------------------------------------
Dataset: multi_spectra_phaII.fits
--------------------------------------------------------------------------------
 
     Block Name                          Type         Dimensions
--------------------------------------------------------------------------------
Block    1: PRIMARY                        Null        
Block    2: SPECTRUM                       Table         7 cols x 17       rows
Block    3: GTI7                           Table         2 cols x 1        rows

For more information on working with this type of file, see the Displaying the Spectrum section of the Examining Grating Spectra and Regions: PHA2 files thread (the information in that section applies to imaging files as well).

Creating a Composite Spectrum

It is also possible to extract a composite spectrum of all the (faint) sources in a field for a single spectral analysis. This may be useful in the case where the sources might be of similar characteristics, but there are not enough counts in any given source to do individual spectra.

Here we have deleted the sources associated with 3C295 (IDs 3-5):

unix% dmcopy "s3_src.fits[#row=1:2,6:]" s3_nocore_src.fits

Now we can use dmextract to accept all the regions together and create a spectrum:

unix% dmextract "s3_evt2.fits[sky=region(s3_nocore_src.fits)][bin pi=::5]" \
      spectrum_faint.fits

Again, the resultant spectrum can be plotted in ChIPS:

unix% chips

Welcome to ChIPS, version CIAO 3.4
Copyright (C) 1999-2003, Smithsonian Astrophysical Observatory

chips> curve "spectrum_faint.fits[cols channel,counts]"
chips> curve simpleline; symbol none
chips> limits x 0 100
chips> xlabel "channel"; xlabel size 1.5
chips> ylabel "counts"; ylabel size 1.5
chips> exit

Figure 7  shows the plot. As mentioned before, this data may also be fit in Sherpa, once the proper response files have been created; see the Step-by-Step Guide to Creating ACIS Spectra for Pointlike Sources and Independent Background Responses threads for more information.

Removing Unwanted Point Sources

Often it is desirable to remove point sources from an image for both cosmetic and scientific reasons. For example, if you are analyzing an extended source, an unfortunately located point source may affect the count rates in the object.

Suppose that we want to remove all the point sources from our field of view, except for the three in the core (sources 3-5). First, create a source list that excludes these three regions:

unix% dmcopy "s3_src.fits[#row=1:2,6:17]" s3_holes_src.fits

Now filter the event file with those regions:

unix% dmcopy "s3_evt2.fits[exclude sky=region(s3_holes_src.fits)]" s3_holes_evt2.fits

unix% ds9 s3_holes_evt2.fits &

The resulting image, Figure 8 , has holes where the sources used to be. The Obtain and Fit a Radial Profile thread illustrates another case in which you might use this syntax (see the Removing Contaminating Point Sources section).

A more sophisticated means of removing point sources is to use dmfilth. This tool removes the point source and then fills in the hole where the source was. See ahelp dmfilth for more information and the Identify and Remove Point Sources section of the Create an Image of Diffuse Emission thread for an example.