Create a simulated observation

 

For testing e.g. of user-customized procedures it is sometimes useful to have a test data set with exactly known parameters. Ideally such a test data set would consist of an observation with a single point source superposed on a well-behaved background. A simulated observation of this kind can be created with the command

SIMULATE/SOURCE [det] [source] [backgr] [ampl] [offset] [wobble]

 

• det : ROSAT Detector (PSPC or HRI). Defaulted to PSPC.
• source : unvignetted source counts. Defaulted to 10000.
• backgr : mean background level in units of counts/sky_pixel, defaulted to 0.00005 counts/sky_pixel.
• ampl : amplitude channels of source and background, defaulted to 100,30 for the PSPC and 8,2 for the HRI.
• offset : X- and Y-offset of the simulated source from the nominal pointing direction in units of sky pixel. Defaulted to 0,0.
• wobble : Three parameters describing the virtual spacecraft wobble: wobble amplitude in sky pixels, wobble period in seconds, and angle of wobble direction with respect to north in degrees. Defaulted to 0,0,0 (no spacecraft wobble).

The output consists of a simulated events table (events.tbl), attitude table (attitude.tbl) and an image (image1.bdf) in the proper source detection binning. Attention: pre-existing files with these names will be overwritten !

The simulated source SIM X-1 will be wobbled, vignetted, and smeared according to the Gaussian model of the PSF as used in the EXSAS source detection. An analytic model of the PSF for the simulated source is not yet available. The dead-time correction is set to 1.0. Since vignetting is taken into account, the number of simulated source events actually written to the events table is equal or smaller than that given in parameter source. For calculation of the vignetting, the PSPC window support structure is not taken into account.

The simulated background is uniform over the field of view and not affected by vignetting.

Both source and background are monoenergetic. The respective channels are defined in parameter ampl.

The observation time of the simulated data set is 3600 seconds, without any time gap.

Since the random number generator is initialized from the system time for each run, successive runs with the same parameters will give statistically independent results. Poissonian statistics (in good approximation) is used for the creation of the simulated source.

The simulation parameters are written to descriptors (SIM_xxx) of the events table.

Examples:

SIMULATE/SOURCE
SIMULATE/SOURCE HRI 4000 0.00002 10,2 800,400 600,402,45

Note that the output of this command is not suited to compare it with actual observations to assess possible source extents, for two reasons: the used Gaussian PSF is only an approximation lacking the PSF wings typical for actual observations, and attitude uncertainty is not considered for the simulations.