Pulsars

Pulsars

(Section Not Complete)
A pulsar is a rapidly spinning neutron star that has a mechanism to beam light, much like a lighthouse. This mechanism is only partially understood, but is connected with very strong magnetic fields spinning with the star. If the beam of the pulsar sweeps over the Earth as the neutron star rotates, the light from the pulsar appears to pulse on and off. Several hundred pulsars are now known, with periods ranging from seconds down to sub-milliseconds.

The Crab Pulsar
The first pulsar discovered was found by Anthony Hewish and Jocelyn Bell at the Cambridge radio astronomy observatory in 1967. The most famous pulsar was discovered shortly after that. It lies in the Crab Nebula (M1), which is about 7000 light years away in the constellation Taurus. The Crab Pulsar rotates about 30 times a second, emitting a double pulse in each rotation in the RF through gamma-ray spectrum. The adjacent image catches the Crab Pulsar (in the yellow region in the center) pulsing in X-rays. In visible light, the Crab Pulsar appears to be a magnitude 16 star near the center of the nebula (see below), but stroboscopic techniques show it to be pulsing.
The Crab Nebula (adjacent image) is a strong source of electromagnetic radiation across the spectrum, and most of the power for this emission is being derived from the spinning neutron star and its strong fields lying at the center of the nebula. Although the Crab Pulsar emits visible light (and X-rays and gamma-rays), most pulsars are detectable only by their RF radiation. The following image illustrates the broad-band emission of the Crab Nebula.

Two pulsars, the Crab and the Vela Pulsars, are strong sources of gamma rays. The following figure is an all-sky map in galactic coordinates of gamma ray intensity in the 1-3 MeV (million electron-Volt) range, as obtained by the COMPTEL instrument on the orbiting Compton Gamma Ray Observatory (Ref). Several strong sources in the galactic plane are marked, including two pulsars, the galactic center (which may contain a supermassive black hole), and the black hole binary system Cygnus X-1. Sources above and below the plane of the galaxy are mostly extragalactic gamma sources like quasars.

Powering the Crab Nebula
The tiny Crab Pulsar, which is not much more than 10 kilometers in diameter, powers the enormous energy output of the Crab Nebula, which is 10 light years in diameter. To set things in perspective in terms of relative sizes, this is as if a 1 kilometer wide volume of space were radiating strongly at various wavelengths and most of the power were being supplied by a single hydrogen atom at the center of that volume!
Recently, Hubble Space Telescope observations illustrated in the figure below and the animation adjacent right have shown that the Crab Nebula undergoes substantial changes in time as energy is being fed into it by the pulsar (Ref). For example, energy ripples outward at speeds near half that of light through the nebula in the vicinity of the pulsar.

Here is a movie (550 kB MPEG) of the ripples moving outward in the Crab Nebula from central pulsar partially illustrated in the above animation. Here is an animation (292 kB streaming) illustrating what may be responsible for these observations (Ref). The movie imagines that you are close to the Crab Pulsar and slowly pull away, allowing you to see more and more of what is going on to power the inner part of the nebula and to account for the detailed activity seen in the preceding animation (more info).

The Sound of Pulsars
Since pulsars pulse in the RF part of the spectrum, it is possible to "listen" to them by connecting the amplified signal from the radio telescope to a radio speaker (Ref). Here are sound files for three pulsars of very different period.

PSR 0329+54 has a pulse period of about 0.715 seconds.
PSR 0833-45 (Vela Pulsar) has a period of 89.3 milliseconds.
PSR 1937+21 is a very fast pulsar, with a period of 1.56 milliseconds (640 revolutions per second, which is faster than a kitchen blender).
For the first two you can hear the pulses clearly. For the last one the period is so short that the pulses blend into a continuous whine (like a 15 kilometer wide kitchen blender spinning at 640 revolutions per second!).
The names for these pulsars are derived from their position on the celestial sphere. The first part of the number gives the approximate right ascension in hours and minutes. The second part of the number gives the declination (with a plus or negative sign). For example, the pulsar labeled PSR 0329+54 is located on the celestial sphere at right ascension 3 hours and 29 minutes and declination +54 degrees. Here is a map from the Princeton Pulsar Group of pulsar positions given in right ascension and declination (the links from this map to some databases with further information do not always work at present).

Change in Spin Rate
As a pulsar radiates away its energy, its spin rate decreases slowly. This change is very slow, but can be measured very precisely. The rate of change in the rotational period for a radio pulsar is very important for several reasons. One is that it can be used to estimate the magnetic field associated with the neutron star. The resulting measurements indicate that pulsars have enormous magnetic fields, typically in the range of 10^8 - 10^12 gauss (there is some evidence that the fields in neutron stars may be as large as 10^14 gauss). In addition, the change in the spin rate for pulsars has been used to detect indirectly the radiation of gravitational waves (a key prediction of the General Theory of Relativity), and to infer the possible existence of planets orbiting a neutron star (see "New Views of Neutron Stars", L. Boldsten and T. Strohmayer, Physics Today, Feb. 1999, p. 40).
The adjacent image (Source) shows the Crab Nebula, which is the remains of a supernova whose light reached the Earth in 1054 A.D. The red color in this image indicates regions where electrons are combining with protons to form neutral hydrogen. The green color indicates regions in which electrons are being accelerated in strong magnetic fields in the inner part of the expanding nebula. The pronounced filament structure is not well understood. At the center of the nebula, not visible in this image, is the neutron star rotating 30 times a second.
Guest Star in Sung dynasty chronicles (1054 AD).

Image from cliff dwelling at Chaco Canyon that may be the guest star of 1054 A. D.

Princeton Pulsar Group
binary pulsar

Typical Magnetic Field Strengths
Object Strength (Gauss)

Earth's magnetic field 0.6

Simple iron bar magnet 100

Strongest sustained
laboratory fields 4 x 10⁵

Strongest man-made fields (millisecond duration) 10⁷

Maximum field for ordinary stars 10⁶

Typical fields for radio pulsars 10¹²

Magnetars 10¹⁴ - 10¹⁵

The following image illustrates the location of the strongest magnetar known (Ref). Magnetars are super-magnetized spinning neutron stars. The magnetar SGR 1900+14 has the strongest magnetic field known in the galaxy: it is approximately 1,000,000,000,000,000 times larger than the magnetic field of the Earth. If a magnet that strong were placed halfway to the Moon, it could pull metal pens out of your pocket on Earth. This magnetar has been observed to emit powerful flashes of gamma rays (Ref).

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