Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission beams.
Cycle of pulsed gamma rays from the Vela pulsar.
are highly magnetized, rotating
neutron stars that emit a beam of
electromagnetic radiation. The observed periods of their pulses range from 1.4
milliseconds to 8.5 seconds. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name. Because neutron stars are very dense objects, the rotation period and thus the interval between observed pulses is very regular. For some pulsars, the regularity of pulsation is as precise as an
atomic clock. A few pulsars are known to have planets orbiting them, as in the case of
PSR B1257+12. Werner Becker of the
Max Planck Institute for Extraterrestrial Physics said in 2006, "The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work."
Discovery
The first pulsar was observed in July 1967 by
Jocelyn Bell Burnell and
Antony Hewish. Initially baffled as to the seemingly unnatural regularity of its emissions, they dubbed their discovery
LGM-1, for "
little green men" (a name for intelligent
beings of extraterrestrial origin). The hypothesis that pulsars were beacons from extraterrestrial civilizations was never serious, but some discussed the far-reaching implications if it turned out to be true. Their pulsar was later dubbed
CP 1919, and is now known by a number of designators including
PSR 1919+21,
PSR B1919+21 and
PSR J1921+2153.
Although CP 1919 emits in
radio wavelengths, pulsars have, subsequently, been found to emit in visible light,
X-ray, and/or
gamma ray wavelengths.
The word "pulsar" is a contraction of "pulsating star", and first appeared in print in 1968:
The suggestion that pulsars were rotating neutron stars was put forth independently by
Thomas Gold and
Franco Pacini in 1968, and was soon proven beyond reasonable doubt by the discovery of a pulsar with a very short (33-
millisecond) pulse period in the
Crab nebula.
In 1974, Antony Hewish became the first astronomer to be awarded the
Nobel Prize in physics. Considerable controversy is associated with the fact that Professor Hewish was awarded the prize while Bell, who made the initial discovery while she was his Ph.D student, was not.
Subsequent history
In 1974,
Joseph Hooton Taylor, Jr. and
Russell Hulse discovered the first time pulsar in a
binary system,
PSR B1913+16. This pulsar orbits another neutron star with an orbital period of just eight hours.
Einstein's theory of
general relativity predicts that this system should emit strong
gravitational radiation, causing the orbit to continually contract as it loses
orbital energy. Observations of the pulsar soon confirmed this prediction, providing the first ever evidence of the existence of gravitational waves. As of 2004, observations of this pulsar continue to agree with general relativity. In 1993, the Nobel Prize in Physics was awarded to Taylor and Hulse for the discovery of this pulsar.
In 1982,
Don Backer led a group which discovered
PSR B1937+21, a pulsar with a rotation period of just 1.6 milliseconds. Observations soon revealed that its magnetic field was much weaker than ordinary pulsars, while further discoveries cemented the idea that a new class of object, the "
millisecond pulsars" (MSPs) had been found. MSPs are believed to be the end product of
X-ray binaries. Owing to their extraordinarily rapid and stable rotation, MSPs can be used by
astronomers as clocks rivaling the stability of the best
atomic clocks on Earth. Factors affecting the arrival time of pulses at the Earth by more than a few hundred
nanoseconds can be easily detected and used to make precise measurements. Physical parameters accessible through pulsar timing include the 3D position of the pulsar, its
proper motion, the
electron content of the
interstellar medium along the propagation path, the orbital parameters of any binary companion, the pulsar rotation period and its evolution with time. (These are computed from the raw timing data by
Tempo, a computer program specialized for this task.) After these factors have been taken into account, deviations between the observed arrival times and predictions made using these parameters can be found and attributed to one of three possibilities: intrinsic variations in the spin period of the pulsar, errors in the realization of
Terrestrial Time against which arrival times were measured, or the presence of background gravitational waves. Scientists are currently attempting to resolve these possibilities by comparing the deviations seen amongst several different pulsars, forming what is known as a
Pulsar Timing Array. With luck, these efforts may lead to a
time scale a factor of ten or better than currently available, and the first ever direct detection of gravitational waves.
In June 2006, the astronomer
John Middleditch and his team at
LANL announced the first prediction of
pulsar glitches with observational data from the
Rossi X-ray Timing Explorer. They used observations of the pulsar
PSR J0537-6910.
In 1992,
Aleksander Wolszczan discovered the first
extrasolar planets around
PSR B1257+12. This discovery presented important evidence concerning the widespread existence of planets outside the
solar system, although it is very unlikely that any
life form could survive in the environment of intense radiation near a pulsar.
Theory
There is general agreement that what we observe as a pulse is what happens when a beam of radiation points in our direction, once for every rotation of the neutron star. The origin of the beam is related to the misalignment of the rotation axis and the axis of the
magnetic field of the star. The beam is emitted from the poles of the neutron star's magnetic field, which may be offset from the rotational poles by a wide angle. The source of the power of the beam is the
rotational energy of the neutron star. This rotation slows down over time as
electromagnetic power is emitted.
Millisecond pulsars are thought to have been spun up to high rotational speed by matter falling in that had been pulled off from a companion star.
Of interest to the study of the state of the matter in a neutron
stars are the
glitches observed in the rotation velocity
of the neutron star. This velocity is decreasing slowly but steadily, except by sudden variations. One model put forward to explain these glitches is that they are the result of "starquakes" that adjust the crust of the neutron star. Models where the glitch is due to a decoupling of the possibly
superconducting interior of the star have also been advanced. In both cases, the star's
moment of inertia changes, but its
angular momentum doesn't, resulting in a change in rotation rate.
In 2003, observations of the
Crab nebula's pulsar electromagnetic signal revealed "sub-pulses" within the main signal with durations of only nanoseconds. It is thought that these nanosecond pulses are emitted by regions on the pulsar's surface 60 cm in diameter or smaller, making them the smallest structures outside the solar system to be measured.
Categories
Three distinct classes of pulsars are currently known to
astronomers, according to the source of the power of the electromagnetic radiation:
The
Fermi Space Telescope has uncovered a subclass of rotationally-powered pulsars that emit only
gamma rays.
[Atkinson, Nancy. "." Universe Today, 17 October 2008.] There have been only about twelve gamma-ray pulsars identified out of about 1800 known pulsars.
Although all three classes of objects are neutron stars, their observable behavior and the underlying physics are quite different. There are, however, connections. For example,
X-ray pulsars are probably old rotationally-powered pulsars that have already lost most of their power, and have only become visible again after their
binary companions had expanded and began transferring matter on to the neutron star. The process of accretion can in turn transfer enough
angular momentum to the neutron star to "recycle" it as a rotation-powered
millisecond pulsar.
Naming
Initially pulsars were named with letters of the discovering observatory followed by their
right ascension (e.g. CP 1919). As more pulsars were discovered, the letter code became unwieldy and so the convention was then superseded by the letters PSR (Pulsating Source of Radio) followed by the pulsar's right ascension and degrees of
declination (e.g. PSR 0531+21) and sometimes declination to a tenth of a degree (e.g. PSR 1913+167). Pulsars that are very close together sometimes have letters appended (e.g. PSR 0021-72C and PSR 0021-72D).
The modern convention is to prefix the older numbers with a B (e.g. PSR B1919+21) with the B meaning the coordinates are for the 1950.0 epoch. All new pulsars have a J indicating 2000.0 coordinates and also have declination including minutes (e.g. PSR J1921+2153). Pulsars that were discovered before 1993 tend to retain their B names rather than use their J names (e.g. PSR J1921+2153 is more commonly known as PSR B1919+21). Recently discovered pulsars only have a J name (e.g.
PSR J0437-4715). All pulsars have a J name that provides more precise coordinates of its location in the sky.
Miscellaneous facts
- The magnetic axis of pulsars determines the direction of its jets (the lighthouse spewing out the north and south poles of the magnetic axis of rotation), and their magnetic axis is not necessarily the same as their spin axis - just as Earth's magnetic north pole is not the same as its true (spin) north pole. That's why pulsars don't just "sit there" and beam at the same point in their own celestial sphere (if their outer spin axis coincided with their magnetic spin axis). If this happened, they would not pulse... there would just be detectable sources of radiation (when their jets pointed straight at us), or not, but no pulsing.
- Although 8.5 seconds is the slowest observed pulsar period to date, note that as pulsars slow, their power output decreases. Conceivably there are much slower ones, below current levels of detection. On the other hand, the fastest that they can spin (e.g. 1.4 msec) seems to be dependent on the speed at which a pulsar can rotate without neutronium breaking up. In summary, young pulsars are fast and energetic; old ones are slow and weak, with the exception of millisecond pulsars, which are old but have been "recycled" to very short periods.
- It is currently not known if original star mass (pre supernova) or current neutron star mass is related to pulse period.
Applications
The study of pulsars has resulted in many applications in physics and astronomy. Striking examples include the confirmation of the existence of
gravitational radiation as predicted by
general relativity and the first detection of an extrasolar planetary system.
The discovery of pulsars allowed astronomers to study an object never observed before, the
neutron star. This kind of object is the only place where the behavior of matter at
nuclear density can be observed (though not directly). Also, millisecond pulsars have allowed a test of
general relativity in conditions of an intense gravitational field.
As probes of the interstellar medium
The radiation from pulsars passes through the
interstellar medium (ISM) before reaching Earth. Free
electrons in the warm (8000 K), ionized component of the ISM and
H II regions affect the radiation in two primary ways. The resulting changes to the pulsar's radiation provide an important probe of the ISM itself.
[Ferriere, K. "." Reviews of Modern Physics, Volume 73, Issue 4, 2001 (pages 1031–1066).]Due to the
dispersive nature of the interstellar
plasma, lower-frequency radio waves travel through the medium slower than higher-frequency radio waves. The resulting delay in the arrival of pulses at a range of frequencies is directly measurable as the
dispersion measure of the pulsar. The dispersion measure is the total
column density of free electrons between the observer and the pulsar,
where
is the distance from the pulsar to the observer and
is the electron density of the ISM. The dispersion measure is used to construct models of the free electron distribution in the
Milky Way Galaxy.
[Taylor, J. H.; Cordes, J. M. "." Astrophysical Journal, Volume 411, 1993 (page 674).]Additionally,
turbulence in the interstellar gas causes density inhomogeneities in the ISM which cause
scattering of the radio waves from the pulsar. The resulting
scintillation of the radio waves—the same effect as the twinkling of a star in
visible light due to density variations in the Earth's atmosphere—can be used to reconstruct information about the small scale variations in the ISM.
[Rickett, Barney J. "." Annual Review of Astronomy and Astrophysics, Volume 28, 1990 (page 561).] Due to the high velocity (up to several hundred km/s) of many pulsars, a single pulsar scans the ISM rapidly, which results in changing scintillation patterns over timescales of a few minutes.
Significant pulsars
Gamma-ray pulsars detected by the Fermi Gamma-ray Space Telescope.
- The first radio pulsar CP 1919 (now known as PSR 1919+21), with a pulse period of 1.337 seconds and a pulse width of 0.04 second, was discovered in 1967. A drawing of this pulsar's radio waves was used as the cover of British rock band Joy Division's debut album, Unknown Pleasures.
- The magnetar SGR 1806-20 produced the largest burst of power in the Galaxy ever experimentally recorded on 27 December 2004
- PSR B1931+24 "... appears as a normal pulsar for about a week and then 'switches off' for about one month before emitting pulses again. [..] this pulsar slows down more rapidly when the pulsar is on than when it is off. [.. the] braking mechanism must be related to the radio emission and the processes creating it and the additional slow-down can be explained by the pulsar wind leaving the pulsar's magnetosphere and carrying away rotational energy.
- PSR J0108-1431, the closest known pulsar to the Earth. It lies in the direction of the constellation Cetus, at a distance of about 85 parsecs (280 light years). Nevertheless, it was not discovered until 1993 due to its extremely low luminosity. It was discovered by the Danish astronomer Thomas Tauris. in collaboration with a team of Australian and European astronomers using the Parkes 64-meter radio telescope. The pulsar is 1000 times weaker than an average radio pulsar and thus this pulsar may represent the tip of an iceberg of a population of more than half a million such dim pulsars crowding our Milky Way.
- A pulsar in the CTA 1 supernova remnant (4U 0000+72, in Cassiopeia) initially emitted radiation in the X-ray bands. Strangely, when it was observed at a later time X-ray radiation was not detected. Instead, the Fermi Gamma-ray Space Telescope detected the pulsar was emitting gamma ray radiation, the first of its kind.
See also
