Determine Upper Limits for Counts

 

If a source is not expected to be prominent (or known) in the data, then an upper limit of the counts can be determined with the command (PSPC pointing only)  

COMPUTE/UPPER_LIMITS ulpar edit

  or (HRI)

CREATE/PARFIL EXUL ulpar ROSAT,HRI
COMPUTE/UPPER_LIMITS ulpar edit

or (WFC)

CREATE/PARFIL EXUL ulpar ROSAT,WFC
COMPUTE/UPPER_LIMITS ulpar edit

or (PSPC, survey)

CREATE/PARFIL EXUL ulpar ROSAT,SURVEY
COMPUTE/UPPER_LIMITS ulpar edit

A parameter file ulpar.par is created and offered to be edited (cf. Table 5.13).

  
Table 5.13: Parameter file for COMPUTE/UPPER for ROSAT PSPC, pointing

The source positions have to be specified in equatorial coordinates right ascension (degrees) and declination (degrees) in the table user.tbl in the two columns with the labels :RA_DEG and :DEC_DEG for equinox 2000.0. Such a table can be created with the command WRITE/RADEC_TABLE, see chapter 5.2.29. The parameter file ulpar.par will be created with default parameters for the ROSAT PSPC pointing case.

A description of most of the parameters of the parameter file is given in Table 5.13. There are a few extra parameters which will be described here:

• UL_THR is the threshold of maximum likelihood. If the existence likelihood of the source falls above UL_THR then this source will be written to the table with the name specified with the parameter UL_LIST_ACCEP, otherwise upper limits in the counts for the confidence CONF_UPPER_LIMIT will be calculated and written to the table UL_LIST_ALL in the column :UL_CTS.
• FIT_EXTENT this parameter determines whether the source extent specified in table USER_LIST should be kept free or fixed. If the mode fixed is chosen, a column :EXT must be provided with the table USER_LIST. Presently only the setting :EXT=0. is allowed.
• CONF_UPPER_LIMIT is the confidence of the upper limit flux in percent.   Confidence levels of 68.3%, 95.4%, 99.7%, and 99.99%   correspond to 1, 2, 3 and 4 Gaussian sigma.  

The user must in any case install the parameters INPUT_DATASET, AMPLITUDE_LOW, AMPLITUDE_HIGH, USER_LIST, BG_IMAGE_1. For the parameters AMPLITUDE_LOW and AMPLITUDE_HIGH the values must agree with the values of the descriptors AMP_SEL (RAW_SEL for HRI) of the background image:

AMPLITUDE_LOW(1)  = AMP_SEL(1)
AMPLITUDE_HIGH(1) = AMP_SEL(2)

The background image BG_IMAGE_1 can be created with the command CREATE/BG_IMAGE (or CREATE/SMOOTHED_BG) in case it is not yet available. The source counts are written to the column :CTS in the source table UL_LIST_ACCEP in case the likelihood falls above the threshold UL_THR and the upper limits of the counts are written to the table UL_LIST_ALL in the column :UL_CTS in case the likelihood falls below the threshold UL_THR. The counts :CTS and upper limits of the counts :UL_CTS are not corrected for the efficiency of the mirror of the X-ray telescope (vignetting). The column :VIG_COR in the tables solst.tbl and ullst.tbl contains the vignetting correction factor.
 
CTS VIG_COR

are then the vignetting corrected counts and

UL_CTS VIG_COR

are the vignetting corrected upper limit counts.