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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
the High-Energy astronomers 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://lheawww.gsfc.nasa.gov/docs/gamcosray/hecr/owl_new.html
for details on these exciting gamma-ray astronomy projects.
Thanks for you interest,
Padi Boyd and Daryl Macomb,
for the Ask a High-Energy Astronomer Team
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