Millisecond pulsar
Encyclopedia
A millisecond pulsar is a pulsar
with a rotational period in the range of about 1-10 millisecond
s. Millisecond pulsars have been detected in the radio, X-ray
, and gamma ray
portions of the electromagnetic spectrum
. The origin of millisecond pulsars is still unknown. The leading theory is that they begin life as longer period pulsars but are spun up or "recycled" through accretion
. For this reason, millisecond pulsars are often called recycled pulsars.
Millisecond pulsars are thought to be related to low-mass X-ray binary systems. It is thought that the X-rays in these systems are emitted by the accretion disk of a neutron star
produced by the outer layers of a companion star that has overflowed its Roche lobe
. The transfer of angular momentum
from this accretion event can theoretically increase the rotation rate of the pulsar to hundreds of times a second, as is observed in millisecond pulsars.
However, there has been recent evidence that the standard evolutionary model fails to explain the evolution of all millisecond pulsars, especially young millisecond pulsars with relatively high magnetic fields, e.g. PSR B1937+21. Kızıltan & Thorsett showed that different millisecond pulsars must form by at least two distinct processes. But the nature of the other process remains a mystery.
Many millisecond pulsars are found in globular cluster
s. This is consistent with the spin-up theory of their formation, as the extremely high stellar density of these clusters implies a much higher likelihood of a pulsar having (or capturing) a giant companion star. Currently there are approximately 130 millisecond pulsars known in globular clusters. The globular cluster Terzan 5
alone contains 33 of these, followed by 47 Tucanae
with 22 and M28
and M15
with 8 pulsars each.
Millisecond pulsars, which can be timed with high precision, are better clocks than the best atomic clocks. This also makes them very sensitive probes of their environments. For example, anything placed in orbit around them causes periodic Doppler shifts
in their pulses' arrival times on Earth, which can then be analyzed to reveal the presence of the companion and, with enough data, provide precise measurements of the orbit and the object's mass. The technique is so sensitive that even objects as small as asteroids can be detected if they happen to orbit a millisecond pulsar. The first confirmed exoplanets
discovered several years before the first detections of exoplanets around “normal” solar-like stars, were found in orbit around a millisecond pulsar, PSR B1257+12
. These planets remained for many years the only Earth-mass objects known outside our solar system. And one of them
, with an even smaller mass, comparable to that of our Moon, is still today the smallest-mass object known beyond the solar system.
Current theories of neutron star structure and evolution predict that pulsars would break apart if they spun at a rate of ~1500 rotations per second or more, and that at a rate of above about 1000 rotations per second they would lose energy by gravitational radiation faster than the accretion process would speed them up.
However, in early 2007 data from the Rossi X-ray Timing Explorer
and INTEGRAL
spacecraft discovered a neutron star XTE J1739-285
rotating at 1122 Hz. The result is not statistically significant, with a significance level of only 3 sigma. Therefore, while it is an interesting candidate for further observations, current results are inconclusive. Still, it is believed that gravitational radiation plays a role in slowing the rate of rotation. Furthermore, one X-ray pulsar
that spins at 599 revolutions per second, IGR J00291+5934, is a prime candidate for helping detect such waves in the future (most such X-ray pulsars only spin at around 300 rotations per second).
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. 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...
with a rotational period in the range of about 1-10 millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s. Millisecond pulsars have been detected in the radio, X-ray
X-ray pulsar
X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity...
, and gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
portions of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. The origin of millisecond pulsars is still unknown. The leading theory is that they begin life as longer period pulsars but are spun up or "recycled" through accretion
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
. For this reason, millisecond pulsars are often called recycled pulsars.
Millisecond pulsars are thought to be related to low-mass X-ray binary systems. It is thought that the X-rays in these systems are emitted by the accretion disk of a neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
produced by the outer layers of a companion star that has overflowed its Roche lobe
Roche lobe
The Roche lobe is the region of space around a star in a binary system within which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material can escape the gravitational pull of the star. If the star is in a binary system then the material...
. The transfer of angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
from this accretion event can theoretically increase the rotation rate of the pulsar to hundreds of times a second, as is observed in millisecond pulsars.
However, there has been recent evidence that the standard evolutionary model fails to explain the evolution of all millisecond pulsars, especially young millisecond pulsars with relatively high magnetic fields, e.g. PSR B1937+21. Kızıltan & Thorsett showed that different millisecond pulsars must form by at least two distinct processes. But the nature of the other process remains a mystery.
Many millisecond pulsars are found in globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
s. This is consistent with the spin-up theory of their formation, as the extremely high stellar density of these clusters implies a much higher likelihood of a pulsar having (or capturing) a giant companion star. Currently there are approximately 130 millisecond pulsars known in globular clusters. The globular cluster Terzan 5
Terzan 5
Terzan 5 is a heavily obscured globular cluster belonging to the bulge of the Milky Way galaxy. It was one of six globulars discovered by French astronomer Agop Terzan in 1968 and was initially labeled Terzan 11. The cluster was cataloged by the Two-Micron Sky Survey as IRC–20385...
alone contains 33 of these, followed by 47 Tucanae
47 Tucanae
47 Tucanae or just 47 Tuc is a globular cluster located in the constellation Tucana. It is about 16,700 light years away from Earth, and 120 light years across. It can be seen with the naked eye, with a visual magnitude of 4.0...
with 22 and M28
Messier 28
Messier 28 is a globular cluster in the constellation Sagittarius. It was discovered by Charles Messier in 1764. In the sky it is very close to the 3rd magnitude star Kaus Borealis....
and M15
Messier 15
Messier 15 or M15 is a globular cluster in the constellation Pegasus. It was discovered by Jean-Dominique Maraldi in 1746 and included in Charles Messier's catalogue of comet-like objects in 1764...
with 8 pulsars each.
Millisecond pulsars, which can be timed with high precision, are better clocks than the best atomic clocks. This also makes them very sensitive probes of their environments. For example, anything placed in orbit around them causes periodic Doppler shifts
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
in their pulses' arrival times on Earth, which can then be analyzed to reveal the presence of the companion and, with enough data, provide precise measurements of the orbit and the object's mass. The technique is so sensitive that even objects as small as asteroids can be detected if they happen to orbit a millisecond pulsar. The first confirmed exoplanets
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
discovered several years before the first detections of exoplanets around “normal” solar-like stars, were found in orbit around a millisecond pulsar, PSR B1257+12
PSR B1257+12
PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located roughly 2000 light-years from the Sun. In 2007, it was confirmed that three extrasolar planets orbit the pulsar.- Pulsar :...
. These planets remained for many years the only Earth-mass objects known outside our solar system. And one of them
PSR B1257+12 D
PSR B1257+12 D is a possible extrasolar dwarf planet approximately 980 light-years away in the constellation of Virgo. It is suspected that a dwarf planet is orbiting PSR B1257+12 at an average orbital distance of 2.6 AU with an orbital period of approximately 3.5 years.Originally, in 1996, a...
, with an even smaller mass, comparable to that of our Moon, is still today the smallest-mass object known beyond the solar system.
Pulsar rotational speed limits
The first millisecond pulsar, PSR B1937+21, was discovered in 1982 by Backer et al. Spinning roughly 641 times a second, it remains the second swiftest-spinning millisecond pulsar of the approximately 180 that have been discovered. Pulsar PSR J1748-2446ad, discovered in 2005, is, as of 2010, the swiftest spinning pulsar currently known, spinning 716 times a second.Current theories of neutron star structure and evolution predict that pulsars would break apart if they spun at a rate of ~1500 rotations per second or more, and that at a rate of above about 1000 rotations per second they would lose energy by gravitational radiation faster than the accretion process would speed them up.
However, in early 2007 data from the Rossi X-ray Timing Explorer
Rossi X-ray Timing Explorer
The Rossi X-ray Timing Explorer is a satellite that observes the time structure of astronomical X-ray sources. The RXTE has three instruments—the Proportional Counter Array, the High-Energy X-ray Timing Experiment , and one instrument called the All Sky Monitor...
and INTEGRAL
INTEGRAL
The European Space Agency's INTErnational Gamma-Ray Astrophysics Laboratory is an operational Earth satellite, launched in 2002 for detecting some of the most energetic radiation that comes from space. It is the most sensitive gamma ray observatory ever launched.INTEGRAL is an ESA mission in...
spacecraft discovered a neutron star XTE J1739-285
XTE J1739-285
|- style="vertical-align: top;"| Distance | 39.000 LyXTE J1739-285 is a neutron star, in the constellation Ophiuchus, situated approximately 39,000 light-years from Earth...
rotating at 1122 Hz. The result is not statistically significant, with a significance level of only 3 sigma. Therefore, while it is an interesting candidate for further observations, current results are inconclusive. Still, it is believed that gravitational radiation plays a role in slowing the rate of rotation. Furthermore, one X-ray pulsar
X-ray pulsar
X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity...
that spins at 599 revolutions per second, IGR J00291+5934, is a prime candidate for helping detect such waves in the future (most such X-ray pulsars only spin at around 300 rotations per second).
External links
- "Pinning Down a Pulsar’s Age". Science News.
- "How Millisecond Pulsars Spin So Fast". Universe Today.
- "Fast-Spinning Star Could Test Gravitational Waves". New Scientist.
- "Astronomical whirling dervishes hide their age well". Astronomy Now.
- Audio: Cain/Gay - Pulsars Astronomy Cast - Nov 2009.