National Aeronautics and Space Administration

powspec

In this example, we plot the power spectrum of a time series from EXOSAT ME of the transient pulsar, EXO 2030+375.

Executing the powspec task (Screen output is in a fixed width font, while commentary is italicized):

powspec 1.0 (xronos5.18)
 
Ser. 1 filename +options (or @file of filenames +options)[file1] me_a52385.lc.Z
 Series 1 file   1:me_a52385.lc.Z

We enter a single filename here, however a list of files in the series may be given as a filelist beginning with an `@` symbol if there are multiple files.

 
 Selected FITS extensions: 1 - RATE TABLE;
 
 Source ............ EXO2030+375         Start Time (d) ....  6208 05:05:35.725
 FITS Extension ....  1 - `RATE      `   Stop Time (d) .....  6208 06:41:19.250
 No. of Rows .......         5737        Bin Time (s) ......    1.000
 Right Ascension ... 307.588622760       Internal time sys.. Converted to TJD
 Declination ....... 37.382217390        Experiment ........ EXOSAT   ME
 
 Corrections applied: Vignetting - Yes; Deadtime - Yes; Bkgd - Yes; Clock - No
             values: .983574270       .756429610       1.00000000
 
 Selected Columns:  1- Y-axis;  2- Y-error;
 
 File contains binned data.
 
Name of the window file ('-' for default window)[-] 

Entering a - indicates that the default exposure windows are to be applied ($XRDEFAULTS/default_win.wi). A custom windows file may be generated with the xronwin task.

 
 Expected Start ...  6208.21221903629  (days)       5: 5:35:725  (h:m:s:ms)
 Expected Stop ....  6208.27869502456  (days)       6:41:19:250  (h:m:s:ms)
 
 **** Warning: Newbin Time must be an integer multiple of Minimum Newbin Time
 Minimum Newbin Time    1.0000000      (s)
 for Maximum Newbin No..             5744
 
 Default Newbin Time is:   1.0000000    (s) (to have 1 Intv. of    8192 Newbins)
 Type INDEF to accept the default value
 
Newbin Time or negative rebinning[4.6692607009327] 1

You may not enter a newbin value less than the Minimum Newbin Time, which corresponds to the largest bin size found in the input file. In this example, we choose a newbin time of 1, thus the original binning is preserved.

 
 Newbin Time ......     1.0000000      (s)
 Maximum Newbin No.              5744
 
 Default Newbins per Interval are:        8192
 (giving       1 Interval of         8192 Newbins)
 Type INDEF to accept the default value
 
Number of Newbins/Interval[10] INDEF

Entering INDEF includes the whole timespan of the input in the resulting plot. In order to subdivide the time series into multiple intervals, resulting in multiple plots, give a smaller number than the indicated INDEF value.

 Maximum of       1 Intvs. with         8192 Newbins of       1.00000     (s)
Default intervals per frame are:         1
Type INDEF to accept the default value
Number of Intervals/Frame[1] 

If multiple intervals exist, frames may be calculated from the average of some of them and then plotted. One frame corresponds to one plot. In this example, we have only one interval, and consequently one frame and one plot.

 Results from up to       1 Intvs. will be averaged in a Frame
Rebin results? (>1 const rebin, <-1 geom. rebin, 0 none)[0] 

The first time through we will use no binning, setting the value to 0. The unbinned plot appears first at the end of the example. Then we will use a geometric rebinning, entering a value of -1.01, the results of which are represented in the second plot below. A rebin value less than -1 corresponds to binning as a geometrical series with a step equal to the modulus. In this example, the step is 0.01.

Name of output file[default]  

In this example, we enter a space before pressing Return to indicate that an output FITS file is not desired, however, entering default for the output file, will result in a binned FITS light curve file to be written with the extension .fps.

Do you want to plot your results?[yes] 

In general, we want to plot the power spectrum, however, entering no will prevent powspec from plotting the output. This is useful when only the output FITS file is desired.

Enter PGPLOT device[/XW] 

The Xwindows device, /xw is recommended as the results may be viewed immediately and the plot may be manipulated interactively with PLT. In some cases, however, other file-based devices such as /ps and /gif may be desired.

 
    4096 analysis results per interval
 
 Intv    1   Start  6208  5: 5:35
     Ser.1     Avg  401.0        Chisq 0.1627E+06   Var 0.1594E+05 Newbs.   5713
               Min  152.8          Max  826.5    expVar  559.6      Bins   5713
Power spectrum ready !

By default, the following plot is produced (no rebinning):

Now, assume we run powspec again using the rebin value of -1.01, and then use the following PLT commands:

PLT> color 2 on 2

Change group 2 (the data representing Y) to color 2 (red).

PLT> cpd exo2030_375ps.gif/gif

Change the plot device to a gif.

PLT> p

Plot to the gif.

PLT> quit

The resulting gif file looks like this: