Interstellar Holography

Some radio sources, called "pulsars" show strong interference effects: the intensity of the signal at each radio-frequency oscillates on time-scales of seconds to minutes. These oscillations come about because the radio-signals are scattered by lumpiness in the interstellar medium - especially by lumpiness in the ionised interstellar medium - so the received signal is the sum of many waves which have taken different paths from the pulsar to the telescope. This scattering occurs for all radio sources, but most types of radio source are large enough that the oscillations become smoothed out and cannot be detected. An example of interference between two waves is shown below.

The intensity of the sum of two waves of different amplitudes and frequencies. Try changing their values using the slider bars. If you click on the "+" icons, you can see the numerical values of the amplitude and frequency.

When we record these oscillations for many different radio-frequencies we obtain the "dynamic spectrum", which contains a lot of information on the various scattered waves which are reaching the telescope. The image below shows an example of a dynamic spectrum in the artificially-simple case where there is only a single scattering lump near the line-of-sight.

The intensity shown in grey-scale as a function of radio-frequency (vertical axis) and time (horizontal axis). Try changing the angle of the scattering lump, relative to the position of the pulsar.

In the case shown above we see a single ripple running across the image. This ripple is the interference between the single scattered wave and the wave coming directly from the pulsar, and the properties of the ripple tell us which direction the scattered wave has come from, relative to the location of the pulsar.

Examples with larger numbers of scattered waves can be seen in the animated demonstration below. When there are many scattered waves the interference pattern appears very complicated (sometimes yielding results that are similar to real pulsar dynamic spectra). But the principle remains the same - the properties and locations of all the various scattering lumps, relative to the line-of-sight to the pulsar, determine the observed pattern - it's just that now we need a computer to help us to interpret the observations.

Hit the Play button, at top left, to see how this spectrum evolves with time.

The dynamic spectrum of a pulsar is, in fact, a type of hologram, so this sort of approach to studying the structure of the interstellar medium can be termed "interstellar holography". The key to this technique is being able to determine the individual scattered waves (E1, E2, E3 …) which make up the pulsar signal: E = E1 + E2 + E3 + …     Breaking down the signal in this way is possible because each scattered wave produces a distinct ripple. But it is complicated by the fact that the thing we measure - the dynamic spectrum - is not the signal itself (E, the electric field), but the signal-squared E2 (the intensity). One way around this problem is to record the dynamic cyclic spectrum of the pulsar.

Once we know the make-up of the pulsar signal in terms of scattered waves, we can proceed to make an image of the interstellar material which has caused the scattering. Early results showed that sometimes the scattering material is almost completely anisotropic - meaning that scattering takes place in one direction but not the other, leading to an image which resembles a line drawn on the sky. That property can be seen with the relatively poor-quality images one obtains in a single observation. By taking observations on many different days we can build-up a picture with much more detail.

We're developing holographic techniques for making detailed images of the interstellar medium using scattered radio waves. These images will help us to resolve some fundamental puzzles about the nature of the material which scatters radio-waves in interstellar space - see the page on interstellar scintillation. A better understanding of radio-wave scattering will also allow us to gain a better understanding of the radio sources themselves, which will assist the various international teams aiming to detect low-frequency gravitational waves using pulsar timing.

This work is in collaboration with Paul Demorest (NRAO), Willem van Straten (Swinburne) and Aris Karastergiou (Oxford).


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