22.2 Pulsars

Can we be sure that objects as strange as neutron stars really exist? The answer is a confident yes. The first observation of a neutron star occurred in 1967, when Jocelyn Bell, a graduate student at Cambridge University, made a surprising discovery. She observed an astronomical object emitting radio radiation in the form of rapid pulses. Each pulse consisted of a 0.01-second (s) burst of radiation, after which there was nothing. Then, 1.34 s later, another pulse would arrive. The time interval between pulses was astonishingly uniform—so accurate, in fact, that the repeated emissions could be used as a very precise clock. Figure 22.2 is a recording of part of the radio radiation from the pulsating object Bell discovered.

Figure 22.2 Pulsars emit periodic bursts of radiation. This recording shows the regular change in the intensity of the radio radiation emitted by the first such object known. It was discovered in 1967. Some of the pulses are marked by arrows.

Many hundreds of these pulsating objects are now known in our Milky Way Galaxy. They are called pulsars. Each has its own characteristic pulse period and duration. The pulse periods of some pulsars are so stable that they are by far the most accurate natural clocks known in the universe—more accurate even than the best atomic clocks on Earth. In some cases the period is predicted to change by only a few seconds in a million years.

Most pulsars emit their pulses in the form of radio radiation, but some have been observed to pulse in the visible, X-ray, and gamma-ray parts of the spectrum as well. Whatever types of radiation are produced, these electromagnetic flashes at different frequencies are all synchronized—that is, they occur at regular, repeated time intervals—as we would expect if they arose from the same object. The period of most pulsars is usually short—ranging from about 0.03 to 0.3 s, corresponding to a flashing rate of between 3 and 30 times per second. The human eye is insensitive to such rapid flashes, making it impossible to observe the flickering of a pulsar with the naked eye or even using a large telescope. Fortunately, instruments can record pulsations of light that the human eye cannot detect.

A few pulsars are clearly associated with supernova remnants, although not all such remnants have a detectable pulsar within them. Figure 22.3(a) shows a pair of optical photographs of the Crab pulsar, at the center of the Crab supernova remnant. (Sec. 21.3) In the left frame, the pulsar is off; in the right frame, it is on. Figure 22.3(b) shows that the Crab also pulses in X-rays. By observing the speed and direction of the Crab's ejected matter, astronomers can work backward to pinpoint the location in space at which the explosion must have occurred and where the supernova core remnant should be located. That is precisely the region of the Crab Nebula from which the pulsating signals arise. The Crab pulsar is evidently all that remains of the once-massive star whose supernova was observed in 1054.

Figure 22.3 The pulsar in the core of the Crab Nebula blinks on and off about 30 times each second. (a) In this pair of closely spaced optical images, the pulsing can be seen clearly. (b) The same phenomenon is also detected in X-rays.

When Bell made her discovery in 1967, she did not know what she was looking at. Indeed, no one at the time knew what a pulsar was. The explanation of pulsars as spinning neutron stars won Bell's thesis advisor, Anthony Hewish, the 1974 Nobel Prize in physics. Hewish reasoned that the only physical mechanism consistent with such precisely timed pulsations is a small, rotating source of radiation. Only rotation can cause the high degree of regularity of the observed pulses, and only a small object can account for the sharpness of each pulse. Radiation emitted from different regions of an object larger than a few tens of kilometers across would arrive at Earth at slightly different times, blurring the pulse profile. The best current model describes a pulsar as a compact, spinning neutron star that periodically flashes radiation toward Earth.

Figure 22.4 outlines the important features of this pulsar model. Two "hot spots" on the surface of a neutron star, or in the magnetosphere just above the surface, continuously emit radiation in a narrow "searchlight" pattern. These spots are most likely localized regions near the neutron star's magnetic poles, where charged particles, accelerated to extremely high energies by the star's rotating magnetic field, emit radiation along the star's magnetic axis. The hot spots radiate more or less steadily, and the resulting beams sweep through space, like a revolving lighthouse beacon, as the neutron star rotates. Indeed, this pulsar model is often known as the lighthouse model. If the neutron star happens to be oriented such that the beam of its pulses sweeps across Earth, we see the pulses. The beams are observed as a series of rapid pulses—each time one of the beams sweeps past Earth, a pulse is seen. The period of the pulses is the star's rotation period.

Figure 22.4 This diagram of the "lighthouse model" of neutron-star emission accounts for many of the observed properties of pulsars. Charged particles, accelerated by the magnetism of the neutron star, flow along the magnetic field lines, producing radiation that beams outward.

All pulsars are neutron stars, but not all neutron stars are pulsars, for two reasons. First, the two ingredients that make the neutron star pulse—rapid rotation and strong magnetic field—both diminish with time, so the pulses gradually weaken and become less frequent. Theory indicates that within a few tens of millions of years, the pulsations all but stop. Second, even a young, bright pulsar is not necessarily visible from Earth. The pulsar beam depicted in Figure 22.4 is relatively narrow—perhaps only a few degrees across. Only if the neutron star happens to be oriented in just the right way do we see pulses. When we can see the pulses from Earth, we call the body a pulsar.

Given our current knowledge of star formation, stellar evolution, and neutron stars, pulsar observations are consistent with the idea that every high-mass star dies in a Type II supernova, leaving a neutron star behind, and that all neutron stars emit beams of radiation, just like the pulsars we actually see.