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Introduction to LIGO & Gravitational Waves

Continuous Gravitational Waves

Continuous gravitational waves are produced by systems that have a fairly constant and well-defined frequency. Examples of these are binary sytems of stars or black holes orbiting each other (long before merger), or a single star swiftly rotating about its axis with a large mountain or other irregularity on it. These sources are expected to produce comparatively weak gravitational waves since they evolve over longer periods of time and are usually less catastrophic than sources producing inspiral or burst gravitational waves. The sound these gravitational waves would produce is a continuous tone since their frequency is nearly constant.

Fig. 1: A short snippet of an example gravitational-wave strain signal from a continuous gravitational-wave source. Note how the peaks and troughs of the waves come at equal time intervals (hence, constant frequency) and how the height of each peak and trough is the same (constant amplitude). [Image: A. Stuver/LIGO]
Listen to this continuous signal.
Example Continuous Waveform

Fig. 2: Long-term frequency evolution of a continuous gravitational-wave signal. [Image: K. Wette]
Long-term frequency changes of a Continuous Gravitational Wave signal
In practice, the most important LIGO search targets for continuous gravitational waves are neutron stars (ultra-compact stellar remnants) in our own Galaxy. Once we reach sufficient detector sensitivity to find such a signal, we expect we can observe it continuously for many months or years.

Over short times, a continuous gravitational-wave signal from a Galactic neutron star will look almost perfectly constant in both frequency and amplitude, as seen in Fig. 1 for a 0.1 second interval. However, over longer durations, the frequency of the signal will slowly change, for two reasons. The first reason is that, as the neutron star emits gravitational and electromagnetic waves, it loses energy which causes it to rotate more slowly. The second reason is that the detector here on Earth is moving with respect to the neutron star, which changes the frequency of the gravitational waves observed in the detector. The manner in which the signal frequency changes is illustrated in Fig. 2.

The top panel shows the frequency, in blue, changing with a daily cycle due to the rotation of the Earth. The middle panel zooms out to show the frequency, in red, changing on a year's time scale due to the orbit of the Earth around the Sun. The bottom panel zooms out further to show how the frequency, in green, slowly decreases due to the rotation of the neutron star itself slowing down over many years.

Tracking all possible frequency changes is what makes the detection of continuous gravitational waves a computational challenge.

» Next:  Inspiral Gravitational Waves

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