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. The bottom of image is labeled Time In Fractions of a Pulse Period, the left side of the image is labeled Intensity as a Function of Time. The image is divided vertical into seven columns, for each pulsar detected.  Each column is divided horizontally into radio,optical, x-ray, and gamma-ray. Detected pulsations appear as mountain-shaped humps in various light-curves. Non-detections appear as flat lines.

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.

A cartoon of the outer gap model for pulsar emission. A neutron star emits radio and gamma-rays from different places around the magnetosphere, which has a shape similar to an airplane propellar.

The outer-gap model for pulsar emission. Radio jets emanate from the magnetic poles, while gamma-rays are produced in the "outer gap".