gtvcut Help File
gtvcut is used to view the Data Sub-Space (DSS) keywords in a given extension, where the EVENTS extension is assumed by default. The DSS keywords scheme was introduced for recording data selection criteria in a systematic manner.
For Fermi data, DSS keywords store information in the FITS headers about the selection criteria that have been applied to the data using the gtselect tool (see the gtselect help). The DSS keywords are used by the exposure-related tools (gtexpmap, gtrspgen) and by gtlike for calculating various quantities, such as the predicted number of detected events given the source model. These keywords must be the same for all of the filtered event files considered in a given analysis. gtlike will check to ensure that all of the DSS keywords are the same in all of the relevant files.
A filtering condition is expressed with a set of three or four keywords, DSTYP1, DSUNI1, DSVAL1, and DSREF1.
For example, if we select events in a given time interval (TIME>start_time&&TIME<end_time), the filtering condition are stored as
If you select events in a given energy range (ENERGY>start_energy&&ENERGY<end_energy), the filtering conditions are stored as
Parameters are passed following the FTOOLs model (i.e., they can be passed interactively by: answering a prompt; as a list in a command line; or by editing the parameter file). To be prompted for gttsmap parameter values, enter (at the command line): > gtvcut
Note: Not all parameters are prompted; some are "hidden". In order to change one of the "hidden" parameters, specify its value in the command line. For example, if you want to
An example of how to run the tool is shown below:
In this example the gtselect tool was applied first, selecting a time range of the events in the original event file between 252374400 (MET sec) and 252460800 (MET sec), an energy range between 30 MeV and 200000 MeV, and a region of 20 degrees from a central point at coordinates RA=193.98 DEC=-5.82.
The example above can also be run from the command line as follows: