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Student Hera: HERA Timing Tools - Epoch Fold

Tools covered in this section
* List Data
*Plot Light Curve
*Search via Fold
*Epoch Fold

Epoch Fold

Now that you have found the orbital period of GX301-2 using the Search with Fold tool, you can plot a folded light curve of the data. The tool Epoch Fold allows you to do this. Remember, since the folded light curve overlaps many periods' worth of data, the signal (from the source) will be much greater than that of the raw data.

Click on the data set to select it, then run Epoch Fold. Fill in your best period in the parameter box (make sure the format for the period is the same units you are using for the period, probably days, by filling in the appropriate parameter number, 1 or 2). You will also need to fill in values for Phasebins/Period, Newbins/Interval, and Number of Intervals/Frame.

The parameter Phasebins/Period determines the time binning of the raw data (equal to the number of bins in each period). Start with a value around 20 and later you can test the effects of varying this parameter.

The Newbins/Interval parameter affects the size of the error bars on the outputted plot. This can affect how easy it is to see patterns. A large value (1000 or more) for Newbins/Interval will yield a plot with smaller error bars.

As with Search with Fold, the Intervals/Frame parameter determines how much of the raw data will be folded to get the plot of the new light curve. It is wise to use a large value, so that as much of the original data set as possible is utilized (and multiple plots are not generated). A value of at least 1000 will yield good results. The output will be one plot showing the folded light curve with the parameters you have specified. The X-axis shows two complete periods. The two periods are exact copies of each other. Both are shown because there may be interesting features at the edges or transitions of a plot of just one period that could be missed if just one period were plotted.

It may be useful to connect the data points on your folded light curve. To do that, run Epoch Fold and wait for the POW window with your plot to come up. Then, in the text window that appears when you run the program, type in "line step," and then the enter button. This tells the plotting program to connect the points. Then type in "p" and hit the enter button and a new plot will be generated in the POW window. For more detailed information and help with basic plotting commands, go to PLT plotting basics.

Exercise T7

Using your best period (from Search with Fold, vary the parameters of Epoch Fold until you are satisfied with the appearance of your folded light curve for this source. Connect the points of the graph and print out a copy of your folded light curve.

Describe the shape of GX301-2's folded light curve. Remember that one period of the light curve is half of the outputted plot. the folded light curve can look deceptively periodic, since two periods are plotted (and they are exactly alike!). You know that you have got the right one because the folded curve is smooth, without a lot of random peaks, not just because it repeats itself. This represents the best "true" lightcurve for this source with this data, where the light from the source adds constructively while the noise adds destructively. Does the lightcurve look sinusoidal (with a strong peak and little random fluxuations)? Do you see evidence of an eclipse (when the light is completely blocked from the satellite)? What might the shape of a folded light curve reveal about this source?

Now test the effects of varying the Period parameter on which the data are folded. Keep all the other parameters the same - you are just studying the effects on the folded light curve of giving the wrong period. Answer the following questions:

Exercise T8

  • At what point does the folded lightcurve no longer appear to be periodic? It will look more like a city skyline, with many peaks, rather than a smooth curve with a distinct peak and trough. Print out a copy of the folded light curve for this period.

  • At what point does the shape of this source look significantly different (though perhaps still periodic)?

  • What does the folded light curve look like for your "best by eye guesstimate" for the period?

  • Can you explain why it is important to use a tool such as Search with Fold as well as Epoch Fold when trying to pin down the period of a source? Can guessing by eye alone be misleading?


A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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