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# Cen X-3

Cen X-3 is the third X-ray source to be discovered in the constellation Centaurus. It is a binary system first observed in the early 1970s by the Uhuru satellite, and found to have interesting and peculiar repeating patterns in its light curve. See if you can use the timing analysis tools from the previous exercises to analyze the lightcurve of Cen X-3 and uncover the nature of this system.

The data set cenx3.lc that is provided with the Hera module is taken from the All Sky Monitor of the RXTE satellite. You can follow the steps used to find the period for the source GX301-2, but with the data set cenx3.lc selected to find the best period for Cen X-3.

To find the best period, run File Plot to get a general sense of the data. It will be helpful to look at only a small section of the data at a time (the period is fairly short). Try four or five hundred rows, and try looking at different samples. You can then refine your plot using Plot Light Curve to get a first guesstimate of the period.

The period of this source is known very accurately (down to 0.001 days). In order to find the period, it will be necessary to use a very small value for the resolution of period search and search over a large value of number of periods to search. Try a resolution of period search value of no more than 0.001 days, and search over many periods (a thousand is not unreasonable). If you do not get a strong single peak (high chi-squared value), it is possible that your search did not include the actual period. Try changing the initial guess slightly, and use a fairly large value for the Number of Periods to Search parameter.

Once you have gotten a best period using Search with Fold, try generating a folded light curve with the period using Epoch Fold to verify that you have the right period. If you see only a weak period (the folded curve looks like the New York skyline instead of a strong peak and trough), you may not have the right period. Try going back and running Search With Fold with slightly different starting parameters.

### Exercise V3

What is the best period for this source? Print out a copy of the output for Search with Fold showing a best period with a strong peak. Convert the outputted best period from seconds to days.

After you have sucessfully found the period for Cen X-3, you can generate a good folded light curve using Epoch Fold. You can vary the value for Phasebins/Period (the size of the bins, as a fraction of the entered period on which the data is folded), which may give a clearer plot. Remember, to connect the points, type in "line step" and "p" at the PLT> prompt in the efold Output window (see Plotting Basics for more ways to manipulate the plotted output). The Y-axis is a measure of the normalized intensity of the source (the higher the value, the brighter the source).

### Exercise V4

Print out a folded light curve for this source and answer the following questions: