Determine Source Extent

 

The extent of a source can be small, comparable to or large compared to the point spread function of the experiment. In the first two cases the maximum likelihood technique is recommended to be applied. The command

DETECT/MAXLIK extent edit

has to be submitted for the PSPC in pointing mode. To get the proper defaults, for other detectors it is recommended to use (e.g. for HRI) the commands

CREATE/PARFIL EXSP extent ROSAT,HRI
DETECT/MAXLIK extent edit

A parameter file extent.par will be created and offered to be edited (cf. Table 5.16 for PSPC, pointing).

  
Table 5.16: Parameter file for DETECT/MAXLIK for ROSAT PSPC (fit source extent)

A complete description of all parameters from Table 5.16 can be found in section 5.2.8. Here only some parameters of interest are described:

• CUT_RADIUS is the cut-out-radius (in units of the FWHM of the source at the calibrated amplitude channel 30) for source photons centered at the source position. A large value of about 10 should be chosen if an extended source is analysed or if only a few events fall into the extract circle.
• OFF_AX_MIN should be set to 0.
• OFF_AX_MAX should be set to 57 (not active in the Survey case).
• ML_THR is the threshold of maximum likelihood. If the existence likelihood falls above ML_THR then the source will be written to the table with the name specified under the parameter ML_LIST_ACCEP, otherwise if the source is not rejected, it will be written to the table with the name specified under the parameter ML_LIST_ALL.
• START_POSITION is the mode of input of the source positions and has to be set to user_list.
• FIT_POSITION determines whether the source positions specified in tables MERGED_LIST or USER_LIST should be kept fixed or free. fixed means the source positions will not be varied during the likelihood fit, free means they are allowed to be varied.
• USER_LIST is the name of the user defined source table.
• ML_LIST_ACCEP is the name of the source table, which contains the source in case it is accepted in the maximum likelihood detection with an existence likelihood equal to or above the likelihood threshold ML_THR.
• ML_LIST_ALL is the name of the source table, which contains the source in case it is accepted in the maximum likelihood detection with any positive existence likelihood.

The Gaussian sigma of the extent and the error in extent (in units of image pixels)is written to the columns :EXT and :EXTERR of the tables ML_LIST_ACCEP or/and ML_LIST_ALL. The likelihood of extent is written to the table column :EXT_ML. Only in case of a large extent likelihood :EXT_ML> 50. the source should be considered as extended. This is because of the limited model used for the point spread function, the Gaussian model.

In case of a bright source a ring integrated profile can be created   and tested with ring profiles from more complicated models coming from calibration data. Such a calibration table EXSAS_CAL:psf is existing for the on-axis point spread function. It gives the ring integrated (not yet normalized) surface brightness for energies of 0.1 to 1.7 keV in steps of 0.1 keV and for radii of 1 to 3600 arcsec in steps of 1 arcsec.

Ring integrated profiles from observational (photon event file) data can be created with the command PLOT/RING_PROFILE, see section 5.2.21.

The calibrated on-axis ring profiles can be overplotted to the observed ring profile in the command.   In the off-axis case the calibrated ring profiles can be overplotted if they are corrected for the total number of counts in the observed ring profile. In the following example calibrated ring profiles for an off-axis angle of 10 arcmin are overplotted for energies of 0.8 and 1.5 keV onto a ring profile created from a source at an off-axis angle of about 10 arcmin. So a direct comparison is not possible. One should also compute the amplitude weighted calibrated ring profile to test with the data. The amplitude histogram can be created by the BIN/AMPL 10 command and one has to create the ring profile weighted with the amplitudes.

PLOT/RING testdata1:events 851.,-1541.,1000. ? ? ? 100
SET/GRAP stype=1 ltype=0 pmode=1
PLOT/TABLE radevents :RING_RADIUS :INTENSITY
SET/GRAP stype=0 ltype=1
OVERPLOT/ERROR radevents :RING_RADIUS :INTENSITY :ERROR
OVERPLOT/ERROR radevents :RING_RADIUS :INTENSITY :RING_STEP 5
STATISTICS/TABLE radevents :INTENSITY
COPY/TABLE EXSAS_CAL:psf_pspc_10 psf
SET/GRAP ltype=1
COMPUTE/TABLE psf :INTENSITY = :RCTS0D8*{OUTPUTR(3)}*{OUTPUTI(2)}
OVERPLOT/TABLE psf :ANGLE :INTENSITY
SET/GRAP ltype=2
COMPUTE/TABLE psf :INTENSITY = :RCTS1D5*{OUTPUTR(3)}*{OUTPUTI(2)}
OVERPLOT/TABLE psf :ANGLE :INTENSITY

In case of image data the command DRAW/RADIAL_PROFILE   can be used. One first has to convert the radius from image pixels to arcsec and the normalized counts to counts per arcmin before one can overplot the calibrated profiles.