Abstract
When a galaxy acts as a gravitational lens it behaves more like a raindrop deflecting sunlight than like a true lens. The images formed by a galaxy exist as directions of incidence of rays on the observer and the best a galaxy can do is to generate complex surfaces called caustics1–4. Here we point out that the caustic surfaces of galactic lenses are powerful telescopes, with point-source intensifications varying with frequency from 105 for radio waves to 108 for X rays. The diffraction limit to the angular resolution varies from 10–11 to 10–17 arcs from radio waves to X rays and exceeds the resolution of man-made telescopes by many orders of magnitude. The signature of a point source crossing the caustic is a burst of radiation modulated in time by a characteristic diffraction pattern. The microcaustic surfaces generated by stars in the lensing galaxy are also powerful telescopes that yield large intensifications of small sources crossing the microcaustic. Decaying cosmic strings could be an important source of cosmic rays with energies above 1010 GeV.
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McBreen, B., Metcalfe, L. A galactic gravitational lens as the ultimate astronomical telescope. Nature 330, 348–350 (1987). https://doi.org/10.1038/330348a0
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DOI: https://doi.org/10.1038/330348a0
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