Gamma-Ray Pulsars
Pulsars are believed to be spinning neutron
stars which emit beams of relativistic
particles at opposite poles. These beams also emit photons
which scientists can detect using ground-based radio telescopes, and with different
types of earth-orbiting X-ray and gamma-ray satellites. In August 1967, the first
pulsating radio source (pulsar)
was discovered by an alert graduate student, Jocelyn Bell, working under the tutelage
of Prof. A. Hewish at the Mullard
Radio Astronomy Observatory, Cambridge, England. Because pulsars are rotating
on an axis and their beams are usually not directly aligned with that axis of
rotation, they appear to us as blinking light houses in space. Just as a ship
at sea sees a light house blink at regular intervals, scientists see pulsars pulse
at regular intervals.
A gamma-ray
pulsar is a rotating neutron star that emits gamma-ray photons. Some of the
gamma-ray pulsars emit at radio wavelengths; while others do not. The Crab and
Vela pulsars were among the first discovered by high-energy astronomers. The
Crab pulsar resulted from a supernova explosion observed by Chinese astronomers
in 1054. It is one of the most well studied sources in high-energy astronomy
and is also a radio pulsar. Detailed radio studies of the Crab in the 1960's
showed that the pulsar's spin is gradually slowing, and that this loss of rotational
energy is the source of the energy which feeds the radio beams.
Gamma-ray pulses from the Crab were discovered
in 1971, when the scientists were first able to put gamma-ray detectors into
space. EGRET observations show
that gamma-ray emissions at energies greater than 10 MeV
dominate the total radiation emitted from young pulsars such as the Crab. Furthermore,
comparing EGRET data on several pulsars to data at other wavelengths, scientists
have learned that as the pulsar beams sweep through our line of sight, we often
see different wavelengths of light at different intensities. The following figure
shows how seven gamma-ray pulsars look at different wavelengths.
Multiwavelength light curves of the seven
pulsars detected with EGRET.
A flat line in the radio, optical or x-ray bands means that no such pulsation
has been detected. GLAST should provide gamma-ray light curves for several
dozen pulsars, which should help us learn more about the physical mechanism
which produces pulsar emission.
Scientists would like to understand why the pulses at different
wavelengths of light appear to have a different relationship for each pulsar
that is observed. The intensity of light that is observed depends on how close
the beam is to being pointed directly at our detectors (either on ground-based
radio telescopes, or in earth-orbiting X-ray and/or gamma-ray satellites). Differences
between pulsars can result from different orientations towards the earth, or
from different angles between the axis of rotation and the magnetic poles. The
figure below illustrates one possible model which seems to explain the origin
of the pulses at different wavelengths.
The outer-gap model for pulsar emission.
Radio jets emanate from the magnetic poles, while gamma-rays are produced in
the "outer gap".
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