Lunar Occulations of Radio Sources

The intensity variation of a star being occulted by the moon can be described by Fresnel diffraction convolved with the intensity structure of the star (most often approximated by a Gaussian). The plot shown below is a Fresnel diffraction curve before convolution. The x-axis is time and the y-axis is intensity.


The shape of the diffraction curve depends primarily on the angular size of the stellar disk and the frequency of observation. So, lunar occultations have been successfully used to measure stellar angular diameters.

In the same sense, radio astronomers in the 1960ís realized that the same technique could be applied to measure the arcsecond angular structure of radio sources. More importantly, the radio structure could be measured with radio telescopes which have tens of meters diameters and beams with angular sizes of tens of arcminutes.  For example, one of the 26 meter radio telescopes at PARI has a 30 arcminute FWHM beam. So, the telescope cannot realisitically resolve radio sources better than about 1/2 beamwidth. Yet, applying the lunar occultation technique, we can probe the arcsecond structure of radio sources. 

The Lunar Occultation Program at PARI will take advantage of the ability of multiple simultaneous frequency measurements, probing the arcsecond structure of radio sources at those frequencies. We can explore the nature and source of the energetic radio emissions.

Predictions of lunar occultations of radio sources are made using the software called Lunar Occultation Workbench (LOW) written by Eric Limburg.  Radio Sources with 1420 MHz and 4.8 GHz fluxes greater than 1 Jy are included in the special version of LOW made for our project. The LOW software provides an excellent summary of all aspects of an occultation.

The Fresnel diffraction pattern shown above is an ideal case.  Now for some reality.  On 9 May 2002 we measured the occultation reappearance of the radio source 0106+0119 which has a flux of 4 Jy at 1420 MHz.  The raw data is shown in the plot below.  The data was taken by measuring only the intensity in a 50 KHz bandpass across the 1420 MHz peak intensity of the source.

The x-axis is time increasing from left to right and the y-axis is intensity with highest at the top. The lowest intensity occured at the moment the radio source started to reappear.  As the rest of the radio source reappears, we see a coarse diffraction pattern indicating a very broad radio source.  Had the radio source been more pointlike, the diffraction pattern would have looked more like the ideal case shown at the beginning of this web page.

Now that we can make the measurements, the next step is to reconstruct the radio source structure.  Thatís where I am now.  The method of reconstruction used for optical lunar occultations works well for the small subarcsecond structure of stellar disks. However, radio sources are often nebuluos in nature, so I will first use a method that reconstructs the nebular structure that was described by Scheuer (1965, MNRAS, 129, 199). Scheuer defines a restoring function which is the second derivative of the fresnel diffraction pattern convolved with a gaussian. The restoring function is convolved with the observed data to produce the radio source structure.

More efficient data-taking software is being developed - software that samples the 1420 MHz peak and a point in the bandpass free of 1420 MHz emission.  This will provide a measure of the moonís intensity as an occultation occurs.

Modified 31 Dec 2002. mwc

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