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The Question

(Submitted April 12, 1997)

At the upper end of the electromagnetic spectrum are Gamma-rays. These "Gamma-rays" have the highest energy content in the electromagnetic spectrum. What is never discussed by is the following: Is there a an upper limit (frequency) to the electromagnetic spectrum? To wit: What is the "highest frequency" Gamma-ray ever detected and is there reason to believe that there are Gamma-rays with even higher levels of energy and if so....does the electromagnetic frequency spectrum have an upper limit....or does it go out to infinity?

The Answer

Thank you for your very good question about the highest energy gamma-rays. Historically, all particles with frequencies greater than about 1019 Hertz (or about 50,000 electron Volts (5x104 eV) where a typical optical photon carries 2-3 eV) are called gamma-rays. Theoretically, there is no hard limit to the energy that a gamma-ray can have. However, there are a number of practical considerations that one needs to take into account involving both astrophysical sources and basic physics.

Before we address this, however, let's tackle the question about the highest energy gamma-rays yet detected. The highest energy measurements of gamma-rays are accomplished using ground-based instrumentation which also measure cosmic rays. Reliable detections of very high energy gamma-ray radiation from individual astrophysical sources, specifically from a couple of active galaxies and from the Crab Nebula, have extended up to about 1027 Hz (5 x 1012 eV). Aside from these individual sources, there is also expected to be a diffuse emission of gamma-rays which accompany the isotropic flux of cosmic rays. This diffuse gamma-ray emission is well measured below around 1024 Hz (109 eV) or so, and is expected to extend up to at least 1030 Hz (1015 eV). There have been reports of measurements of diffuse gamma-ray emission above 1029 Hz, but many other groups have only reported upper limits to emission at these energies. The measurement is exceedingly difficult since cosmic rays can outnumber gamma-rays at these energies by a factor of 10,000 to 1 or more! So you have to sift through a lot of cosmic rays to try to find the gamma-ray signal - a very difficult task.

The truth is we may never actually know to how high an energy nature sees fit to produce gamma-rays. As the gamma-ray is making its way to our telescopes, it has to traverse through space, where there are photons and particles all around us, for example the microwave background. At the highest energies, these photons will scatter down to lower energies before they arrive at Earth. In addition, many sources could produce very high energy gamma-rays which are absorbed and re-processed within the source. As a result, at the most extreme energies, we should see only those gamma-rays produced by relatively nearby sources. In addition, while we expect diffuse gamma-rays up to 1030 Hz, at energies beyond this the basic physics of particle interactions and gamma-ray production is less clear. There could be many surprises.

Nevertheless, from the distribution of gamma-ray energies observed we know we should be able to detect gamma-rays with energies higher than those stated above. There are currently a number of projects being developed that will collect ultra-high energy gamma-rays from cosmic sources, such as OWL and MILAGRO. See

http://owl.gsfc.nasa.gov/

for details on these exciting gamma-ray astronomy projects.

Thanks for you interest,

Padi Boyd and Daryl Macomb,
for the Ask an Astrophysicist Team

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