15. Fitting EPIC Timing Data with Xronos

We will do some analysis of PN Timing data of the high mass X-ray binary GX 301-2 (Obs ID 0555200301) with Xronos, which is a powerful timing analysis software package available through the HEASARC. More about Xronos can be found here:

 
http://heasarc.gsfc.nasa.gov/xanadu/xronos/xronos.html

We will assume that the data has been reprocessed with SAS, and a light curve with time bin size of 1 second was made with SAS and named pn_ltcrv. Xronos does not have a GUI, so all tasks will be called from the command line. In the examples below, all the tasks prompt the user for further information. Input that we've been prompted for is shown with the indented lines.

Let's start by pulling up a new window and invoking ftools:

ftools

15.1 Making a Power Spectrum

The task powspec calculates the power spectrum density for a given time series, plots the result, and writes the results to a FITS file. We can use the recommended defaults as a starting point.

powspec
   pn_ltcrv.fits
   - 
   25 
   4096 
   1 
   0 
   [enter] 
   [enter]
   [enter]

The output is shown in Figure 15.1. We can zoom in on a region of interest by resetting the axis minimum and maximum. To quit the plotting software, just type 'exit'.

r x 0 3e-3
exit

Figure 15.1: Top: The power spectrum of GX 301-2. Bottom: Same, zoomed in.

We can also do this with geometric rebinning:

powspec
   pn_ltcrv.fits
   - 
   25 
   4096 
   1 
   -1.01 
   [enter] 
   [enter]
   [enter]
   exit

The output is shown in Figure 15.2.

Figure 15.2: The power spectrum of GX 301-2 with geometric rebinning.

15.2 Finding the Period of a Source

We can also search for the source's period with the task efsearch. The period was determined by Furst et al. (2011, A&A, 535A, 9) to be 685 s, so we will use that as a starting point.

efsearch
   pn_ltcrv.fits
   - 
   INDEF
   685
   8 
   689
   4 
   128 
   [enter]
   [enter]
   [enter]
   exit

The output is shown in Figure 15.2. efsearch automatically indicates the best period and resolution at the top of the plot, with the bin size on the upper right side.

Figure 15.3: The output of efsearch. By default, the X axis represents the difference between the periods attempted and the best period found.

15.3 Making a Folded Light Curve

The task efold will produce a folded light curve.

 
efold
   1
   pn_ltcrv.fits
   -
   INDEF
   689
   10
   857
   1
   [enter]
   [enter]
   [enter]

We can add a line to the plot to make it easier to see.

   line step 
   p

And color, if we are feeling fancy. In the statement below, "3" refers to the color choice and "2" refers to the data on the Y axis. If we wanted to turn off the error bars, we could also enter the command error off.

 
   color 3 on 2
   p
   exit

The plot is shown in Figure 15.4. By default, the folded period is shown in the upper right part of the plot.

Figure 15.4: The folded period of GX 301-2.