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# Using Timing Data to Understand Periodic Sources

There are many objects in the sky whose appearances change with time. Some change over time because they are evolving, such as supernovae, which undergo a tremendous explosion and then fade over time (albeit a very long, long time!!). Others change periodically, with a pattern of brightness and dimness that repeats again and again. Examples include binary stars, and pulsars and neutron stars. The period of such a system is the time it takes to return to the same level of brightness or dimness.

We learn about systems that vary periodically with time by studying data collected from the same source over time (many times the period of the pattern). Data is displayed in the form of a lightcurve. A lightcurve has time as its X-axis (units of seconds, hours, or days, for example) and some measure of brightness, intensity or counts received as the Y-axis.

In this guided activity, you will learn how to use Hera's suite of timing analysis tools to find the orbital period of a system where a neutron star is in orbit with a supergiant B star. But before starting, try these exercises to help familiarize you with periodic sources you may see in everyday life.

### Exploration of Periodic Sources in Everyday Life

 A lighthouse is a good everyday example of a source of light that varies in a regular pattern with time. As the top of a lighthouse rotates around, the beam of light sweeps through a complete 360 degrees. To a person (or boat) standing still and observing the lighthouse, the light from the lighthouse gets brighter as the beam sweeps over the observer and then fades and disappears as the beam passes to the other side.

Now that you have thought about periodic sources in everyday life, let's next think about why the intensity of X-rays might vary from an x-ray binary star system.

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