Local and Map Source Detection on WFC data

 

In this section an example for source detection on WFC data     is given. In comparison to the previous sections only the local and the map detections are done (as a maximum likelihood detection is not yet possible on WFC data in EXSAS). There are a few preparatory steps to be done before source detection can be started.

1. An image must be created. Images appropriate for source detection must have a size of 512 512 pixels and the image pixel size must be 7 sky pixels (42 arcsec) for ROSAT WFC. There is an automatic command to bin an image fulfilling these requirements and it is highly recommended to use this command:

   Midas 002> create/source_detect_image  testdata2:OPA

    

   An image image1.bdf is created from the photon event table OPA.tbl in the testdata area testdata2.

2. A mask will be created to screen out the outer edge of the detector. This is done by a shift of an existing mask image (testdata2:OPADEX.bdf, a circular mask with a radius equal to the field of view of the WFC). The translation will be done by guess with the command ADJUST/MASK and the result has to be checked by eye.

   Mean xoffset, and yoffset are determined by guess. In this example rough values have been determined in advance (rotation angle zero, since the mask is circular).

   Midas 003> adjust/mask image1 testdata2:OPADEX 0. 0. 25. ? load

    

   An adjusted mask called mask.bdf is created. It is strongly recommended to control at this point the correct overlay of the mask on the image (always use the default load display option as is done in the example before).

Next the local detection is started. First a parameter file dlpar.par has to be created, the name of the mask image is written to the parameter MASK, for the minimum ML a value of 10. is written to the parameter MIN_ML, and then the local detection is started.

Midas 004> create/parfil EXDL dlpar ROSAT,WFC
Midas 005> write/parfil dlpar MASK mask.bdf
Midas 006> write/parfil dlpar MIN\_ML 10.
Midas 007> detect/local dlpar

  Next a background image bacmp1.bdf is created by a bicubic spline fit. A WFC-filter specific FWHM calibration table has to be specified. In this case fwros1.tbl for WFC filter S1 is used.

Midas 008> create/parfil EXDB dbpar ROSAT,WFC
Midas 009> write/parfil dbpar MASK mask.bdf
Midas 010> write/parfil dbpar EXPOSURE_FLAG T
Midas 011> write/parfil dbpar EXPOSURE_IMAGE_1 testdata2:OPAMEX.bdf
Midas 012> write/parfil dbpar MIN_ML 10.
Midas 013> write/parfil dbpar FWHM_CAL_FILE EXSAS_CAL:fwros1.tbl
Midas 014> create/bg_image

  Finally the map detection is started. First a parameter file dmpar.par is created and some parameters in the parameter file are set.

Midas 015> create/parfil EXDM dmpar ROSAT,WFC
Midas 016> write/parfil  dmpar MASK mask.bdf
Midas 017> write/parfil  dmpar MIN_ML 10.
Midas 018> write/parfil  dmpar CUT_RADIUS_FWHM 2.0
Midas 019> detect/map    dmpar

  A table mslst1.tbl containing all sources accepted by the map detection method and having a likelihood 10. is created. We read the table, which contains 4 sources.

Midas 020> read/tab mslst1

  Table : mslst1

 Sequence INDEX  X_POS     Y_POS     COUNT     BACKG     SIGNA WINDO SOURD
 -------- ------ --------- --------- --------- --------- ----- ----- ---------
        1      1    302.50    313.59     17.10      0.74    11     1     10.67
        2      2    294.02    320.07    990.18      0.74  3162     1     10.67
        3      3    319.00    331.00     16.87      0.76    10     1     23.98
        4      4    381.50     81.50     29.64      0.75    10     2    245.17
 -------- ------ --------- --------- --------- --------- ----- ----- ---------
 Sequence RIBD      PRIO  COF EXTFL     CLUFL     DUMMY     X_SKY     Y_SKY
 -------- --------- ----- --- --------- --------- --------- --------- ---------
        1    130.11    32   0      0.00      0.00      0.00    321.50    399.13
        2    130.11    32   0      0.00      0.00      0.00    262.14    444.49
        3    106.07    32   0      0.00      0.00      0.00    437.00    521.00
        4      5.66    32   0      0.00      0.00      0.00    874.50  -1225.50
 -------- --------- ----- --- --------- --------- --------- --------- ---------

The parameters of the source tables are described in Table 5.29. We convert the sky pixel coordinates to equatorial, ecliptic and galactic coordinates for equinox 2000 with the command

Midas 021> transform/coord mslst1 mslst1_2000 2000.0

and we read the table mslst1_2000

  Table : mslst1_2000

 Sequence INDEX  X_SKY            Y_SKY            RA_DEG    DEC_DEG
 -------- ------ ---------------- ---------------- --------- ---------
        1      1           321.50           399.13 157.88110  53.57219
        2      2           262.14           444.49 158.04901  53.49774
        3      3           437.00           521.00 157.56291  53.36620
        4      4           874.50         -1225.50 156.16061  56.25626
 -------- ------ ---------------- ---------------- --------- ---------
 Sequence RA_HMS       DEC_DMS      LAMDA     BETA      LII       BII
 -------- ------------ ------------ --------- --------- --------- ---------
        1 10H31M31.46S +53D34M19.8S 136.32744  40.48684 157.49055  53.12143
        2 10H32M11.76S +53D29M51.8S 136.48964  40.47010 157.50827  53.24548
        3 10H30M15.09S +53D21M58.3S 136.24036  40.21499 157.94778  53.06737
        4 10H24M38.54S +56D15M22.5S 133.42871  42.34622 154.65752  50.85432
 -------- ------------ ------------ --------- --------- --------- ---------
 Sequence OFFAX      FWHM       DIST
 -------- ---------- ---------- ----------------
        1      51.25       9.94           510.23
        2      51.60       9.95           510.24
        3      68.00      10.37          1162.63
        4     150.55      18.07          6987.28
 -------- ---------- ---------- ----------------

Four sources have been found by the map detection technique. The source with sequence number 4 is very close to the edge of the field of view and has to be rejected. A maximum likelihood detection   technique is not yet available in this release of EXSAS for the WFC data.

  
Figure 5.8: ROSAT WFC image (left upper figure). Cheesed image (right upper figure) created by the command CREATE/BG_IMAGE. This is the image with the removed source positions, found by the command DETECT/LOCAL in circles with a radius of 2 the FWHM of the point spread function. Binned cheesed image before bicubic spline fit has been applied (left lower figure). Background image (right lower figure) created by the command CREATE/BG_IMAGE. It is a bicubic spline-fit to the binned cheesed image.