Long Power Spectrum

 

The command MAKE/POWER_SPECTRUM produces in the option LONG a power spectrum   from a light curve contained either in a list of BDTs or in a single PET. The maximum length of the input data is 2  bins (it must be a power of two). The power spectrum is produced via Fast Fourier Transform and it is   normalized according to [Leahy et al.1983]. In the BDT case the input tables do have to be necessarily contiguous, and the same bin size is required. Any detected gap in the data (or input table not found) will close the time series accumulation and will force the power spectrum production with a number of bins corresponding to the last power of two reached. All the parameters for the analysis (filenames included) are passed via a parameter file.

Different options are available: it is possible to rebin the input time series (only in the BINNED case), to select a start time for the accepted time series, to specify selected intervals in time for the analysis (windowing) and to select the length of the input time series.

A parameter is available to select the minimum percentage exposure allowed to accept a bin; it is also possible to compute the power spectrum on the percentage exposure only (window function).

The syntax of the command is:

MAKE/POWER\SPECTRUM parfil L

where parfil is the name of the parameter file, created using CREATE/PARFIL LPOWER. The parameter file entries are shown below.

  
Table 7.12: Parameter file entries for MAKE/POWER L

The meaning of the parameters is as follows:

• MODE is the accumulation mode. It can be:
  • BINNED: the input data are in BDT format. Up to 9 different tables can be in input.
  • UNBINNED: the input data are in PET format. Only one table can be in input.
• N_OF_FILES is the number of input tables to be read in for the analysis in the BINNED case. The maximum number allowed is 9. Default is 1. In the UNBINNED case this parameter is unnecessary, because only one table can be in input.
• FILENAME_n is the n-th input table name. Default extension is TBL. There is no default.
• WINDOW_FILENAME is the name of the ASCII file containing the start-end times of the periods accepted for the analysis. The default filename for the window file is dummy.win. If the window file is not found, all times will be accepted. The default extension for window files is WIN.
• START_TIME it is the start time for the time series to be accepted for the analysis. This value is used only if no window file is specified. Default value is 0.0
• LOG2_STRETCH_LENGTH is the log  of the length (in bins) of the time series for the power spectrum evaluation. The maximum value allowed is 19 (524288 bins). The default value is 13 (8192 bins).
• LABEL label for the output. This string will be written to the output table (and optional image) in the descriptor POWER_COMMENT.
• OUTPUT_FILENAME is the filename of the output table containing the ``long'' power spectrum. The default extension is TBL.
• BIN_SIZE is the bin size for the light curve accumulation. It is required only in the UNBINNED case. There is no default.
• REBIN_FACTOR is the rebin factor for the input data, used only in the BINNED case. By means of this parameter it is possible to rebin the data before the analysis. Default value is 1 (no rebinning). The rebinning is done internally and does not affect the data in the input BDTs.
• EXPOSURE_FRACTION is the minimum exposure fraction allowed for a bin to be selected as a ``good'' bin. If this parameter is set to 1.0 (the default) only fully exposed bins will be accepted. If it is set to 0.0 even complete gaps will be accepted as empty bins. All the intermediate values are of course allowed.
• WINDOW_PS_FLAG is the flag for the production of the power spectrum of the window function only. If it is set to 0 (the default), the normal power spectrum of the data will be computed. If it is set to 1, the power spectrum of the window function will be computed: this means that each selected bin will contain the percentage exposure of that bin rather than the detected counts. This option makes sense only if the parameter EXPOSURE_FRACTION is set to a value less than 1.0.
• Format of the input: the files input to this command are:
  • PARAMETER FILE: standard ASCII file. See Table 7.12
  • DATA TABLE(S): they are either BDTs or PETs as produced by the EXSAS projection package. Their format is described in section 7.2.2.
  • WINDOW FILE: it is a standard ASCII file. Each line of the file contains the start and end time of an accepted time interval. No comment lines are allowed.
• Format of the output: the files in output from this command are:
  • POWER SPECTRUM: it is in the format of a MIDAS table with 3 columns: frequency, error on frequency and power. It is possible to look for statistically significant peaks by means of the EXSAS command DETECT/PERIOD. The descriptors of the main EXSAS header are copied from the first input table to the output power spectrum. Some new descriptors are created: the descriptor FFT_NUMBER (I*4), included for consistency with the power spectra in output from the SEGMENTED option, contains the fixed value 1, the descriptor WINDOW_NAME (C*80) contains the window file name, and the descriptor POWER_COMMENT (C*80) contains the LABEL parameter read in from the parameter file.
  • LOGFILE: it is an ASCII file containing a copy of the output to terminal. The useful parameters read in from the parameter file are reported and the data file opening is signalled. The logfile has the standard name ps_long.log