A pulsar is a neutron star that emits beams of radiation that sweep through Earth’s line of sight. Like a black hole, it is an endpoint to stellar evolution. The “pulses” of high-energy radiation we see from a pulsar are due to a misalignment of the neutron star’s rotation axis and its magnetic axis. Pulsars seem to pulse from our perspective because the rotation of the neutron star causes the beam of radiation generated within the magnetic field to sweep in and out of our line of sight with a regular period, somewhat like the beam of light from a lighthouse. The stream of light is, in reality, continuous, but to a distant observer, it seems to wink on and off at regular intervals.
A diagram of a pulsar showing its rotation axis,
its magnetic axis, and its magnetic field.
Observations of Pulsars
Neutron stars have very intense magnetic fields, about a trillion times stronger than Earth’s own field. However, the axis of the magnetic field is not aligned with the neutron star’s rotation axis. The combination of this strong magnetic field and the rapid rotation of the neutron star produces extremely powerful electric fields, with electric potential in excess of 1 trillion volts. To put this power into perspective: A single cubic meter of the magnetic field in the Crab pulsar contains more energy than humans have been able to generate, to date.
Electrons are accelerated to high velocities by these strong electric fields. These high-energy electrons produce radiation (light) in two ways. In the first, they act as a coherent plasma, and the electrons work together to produce radio emission by a process whose details are still being researched; and secondly, the electrons interact individually with photons or the magnetic field to produce high-energy emissions in optical, X-ray and gamma-ray wavelengths. The exact locations where the radiation is produced are uncertain, and they may be different for different types of radiation, but they must occur somewhere above the magnetic poles. External viewers see pulses of radiation whenever this region above the the magnetic pole is visible. Because of the rotation of the pulsar, the pulses thus appear much as a distant observer sees a the light from a lighthouse, which seems to blink as its beam rotates. The pulses come at the same rate as the rotation of the neutron star, and, thus, appear periodic.
Pulsars are the original gamma-ray astronomy point sources. A few years after the discovery of pulsars by radio astronomers, the Crab and Vela pulsars were detected at gamma-ray energies. Pulsars accelerate particles to tremendous energies in their magnetospheres. These particles are ultimately responsible for the gamma-ray emission seen from pulsars.
By the end of 2010 there were about 1800 pulsars known through radio detections, but only about 70 had been detected in the gamma-rays. Gamma-ray telescopes preferentially detect young, nearby pulsars. These pulsars tend to have large magnetic fields and to be spinning rapidly. It is the loss of the pulsar’s spin energy which eventually appears as radiation across the electromagnetic spectrum, including gamma-rays. Both observations and models indicate that pulsars eventually lose the ability to emit gamma-rays as the pulsar slowly takes longer and longer to rotate.
|The folded light curves of some of the known gamma-ray pulsars compared to other energies. Explaining the differences is an important part of pulsar studies.|
Gamma-ray astronomers measure both the pulsar’s light curve and how the gamma-ray energy is distributed within the light curve. In this way, detailed models of how this light is created can be compared to observations to better understand pulsars. In addition, comparison of the gamma-ray energy to energy emitted at other wavelengths gives scientists other important clues to how the radiation is created. Hopefully, future gamma-ray instruments will greatly increase the number of detected sources and allow astronomers greater insight into these fascinating objects.
|The pulsed emission from Geminga, formerly an unidentified gamma-ray source. In the bottom set of images, the region of the sky containing Geminga is shown as a function of pulsar phase.|
Although all pulsars are neutron stars, not all neutron stars are pulsars, and not all pulsars shine in the same way. X-ray pulsars in particular illustrate several ways in which pulsar emission can originate:
- Magnetospheric Emission: Like gamma-ray pulsars, X-ray pulsars can be produced when high-energy electrons interact in the magnetic field regions above the neutron star’s magnetic poles. Pulsars seen this way, whether in the radio, optical, X-ray, or gamma-ray, are often referred to as “spin-powered pulsars,” because the ultimate source of energy comes from the neutron star’s rotation. The eventual loss of rotational energy results in a slowing of the pulsar spin period.
- Cooling Neutron Stars: When a neutron star is first formed in a supernova, its surface is extremely hot (more than 1 million degrees). Over time, the surface cools. While the surface is still hot enough, it can be seen with X-ray telescopes. If some parts of the neutron star are hotter than others, such as the magnetic poles, then pulses of thermal X-rays from the neutron star surface can be seen as the hot spots pass through our line of sight. Some pulsars, including Geminga (see above), show both thermal and magnetospheric pulses.
- Accretion: If a neutron star is in a binary system with a normal star, the powerful gravitational field of the neutron star can pull material from the surface of the normal star. As this material spirals around the neutron star, it is funneled by the magnetic field toward the neutron star magnetic poles. In the process, the material is heated until it becomes hot enough to radiate X-rays. As the neutron star spins, these hot regions pass through the line of sight from Earth and X-ray telescopes see these as X-ray pulsars. Because the gravitational pull on the material is the basic source of energy for this emission, these are often called “accretion-powered pulsars.”