
A millisecond pulsar (MSP) is a
pulsar
A pulsar (''pulsating star, on the model of quasar'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its Poles of astronomical bodies#Magnetic poles, magnetic poles. This radiation can be obse ...
with a
rotational period less than about 10
millisecond
A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units equal to one thousandth (0.001 or 10−3 or 1/1000) of a second or 1000 microseconds.
A millisecond is to one second, as one second i ...
s. Millisecond pulsars have been detected in
radio
Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...
,
X-ray
An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
, and
gamma ray
A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...
portions of the
electromagnetic spectrum
The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high ...
. The leading hypothesis for the origin of millisecond pulsars is that they are old, rapidly rotating
neutron stars that have been spun up or "recycled" through
accretion of matter from a companion star in a close binary system. For this reason, millisecond pulsars are sometimes called recycled pulsars.
Origins
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
A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
produced by the outer layers of a companion star that has overflowed its
Roche lobe
In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential, ...
. The transfer of
angular momentum
Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...
from this accretion event can increase the rotation rate of the pulsar to hundreds of times per second, as is observed in millisecond pulsars.
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
PSR may refer to:
Organizations
* Pacific School of Religion, Berkeley, California, US
* Palestinian Center for Policy and Survey Research
* Payment Systems Regulator in the United Kingdom
* Physicians for Social Responsibility, US
Political par ...
.
Bülent Kiziltan and
S. E. Thorsett (
UCSC) 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
A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting ...
s. This is consistent with the spin-up hypothesis 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 contains 37 of these, followed by
47 Tucanae with 22 and
M28 and
M15 with 8 pulsars each.
Pulsar rotational speed limits

The first millisecond pulsar,
PSR B1937+21
PSR may refer to:
Organizations
* Pacific School of Religion, Berkeley, California, US
* Palestinian Center for Policy and Survey Research
* Payment Systems Regulator in the United Kingdom
* Physicians for Social Responsibility, US
Political par ...
, was discovered in 1982 by
Backer ''et al''. Spinning roughly 641 times per second, it remains the second fastest-spinning millisecond pulsar of the approximately 200 that have been discovered.
Pulsar
PSR J1748-2446ad, discovered in 2004, is the fastest-spinning pulsar known, as of 2023, spinning 716 times per second.
Current models of neutron star structure and evolution predict that pulsars would break apart if they spun at a rate of c. 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
Gravitational waves are oscillations of the gravitational field that travel through space at the speed of light; they are generated by the relative motion of gravitating masses. They were proposed by Oliver Heaviside in 1893 and then later by ...
faster than the accretion process would accelerate them.
In early 2007 data from the
Rossi X-ray Timing Explorer and
INTEGRAL
In mathematics, an integral is the continuous analog of a Summation, sum, which is used to calculate area, areas, volume, volumes, and their generalizations. Integration, the process of computing an integral, is one of the two fundamental oper ...
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
Sigma ( ; uppercase Σ, lowercase σ, lowercase in word-final position ς; ) is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. In general mathematics, uppercase Σ is used as an operator ...
. 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. 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).
Millisecond pulsars, which can be timed with high precision, have a stability comparable to
atomic-clock-based time standards when averaged over decades.
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
An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first detec ...
, 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 of the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
. One of them,
PSR B1257+12 b, has an even smaller mass, just under twice that of the
Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
, and is still today the smallest-mass object known beyond the Solar System.
Gravitational wave detection using pulsar timing
Gravitational wave
Gravitational waves are oscillations of the gravitational field that Wave propagation, travel through space at the speed of light; they are generated by the relative motion of gravity, gravitating masses. They were proposed by Oliver Heaviside i ...
s are an important prediction from Einstein's
general theory of relativity and result from the bulk motion of matter, fluctuations during the early universe and the dynamics of
space-time
In physics, spacetime, also called the space-time continuum, is a mathematical model that fuses the three-dimensional space, three dimensions of space and the one dimension of time into a single four-dimensional continuum (measurement), continu ...
itself.
Pulsars are rapidly rotating, highly magnetized
neutron stars formed during the
supernova
A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...
explosions of massive stars. They act as highly accurate clocks with a wealth of physical applications ranging from celestial mechanics, neutron star seismology, tests of strong-field gravity and Galactic astronomy.
The proposal to use pulsars as gravitational wave detectors was originally made by Sazhin and Detweiler in the late 1970s. The idea is to treat the solar system barycenter and a distant pulsar as opposite ends of an imaginary arm in space. The pulsar acts as the reference clock at one end of the arm sending out regular signals which are monitored by an observer on the Earth. The effect of a passing gravitational wave would be to perturb the local space-time metric and cause a change in the observed rotational frequency of the pulsar.
Hellings and Downs extended this idea in 1983 to an array of pulsars and found that a stochastic background of gravitational waves would produce a quadrupolar correlation between different pulsar pairs as a function of their angular separations on the sky. This work was limited in sensitivity by the precision and stability of the pulsar clocks in the array. Following the discovery of the first millisecond pulsar in 1982, Foster and
Backer improved the sensitivity to gravitational waves by applying in 1990 the
Hellings-Downs analysis to an array of highly stable millisecond pulsars.
The advent of digital data acquisition systems, new radio telescopes and receiver systems, and the discoveries of many new millisecond pulsars advanced the sensitivity of the
pulsar timing array to gravitational waves in the early stages of the international effort. The five-year data release, analysis, and first NANOGrav limit on the stochastic gravitational wave background were described in 2013 by Demorest et al. It was followed by the nine-year and 11-year data releases in 2015 and 2018, respectively. Each further limited the gravitational wave background and, in the second case, techniques to precisely determine the barycenter of the solar system were refined.
In 2020, the collaboration presented the 12.5-year data release, which included strong evidence for a power-law stochastic process with common strain amplitude and spectral index across all pulsars, but statistically inconclusive data for the critical Hellings-Downs quadrupolar spatial correlation.
In June 2023, NANOGrav published the 15-year data release, which contained the first evidence for a stochastic
gravitational wave background. In particular, it included the first measurement of the Hellings-Downs curve,
the tell-tale sign of the gravitational wave origin of the observations.
References
External links
*
How Millisecond Pulsars Spin So Fast. ''Universe Today''.
Fast-Spinning Star Could Test Gravitational Waves. ''New Scientist''.
. ''Astronomy Now''.
Audio: Cain/Gay - Pulsars''Astronomy Cast'' - Nov 2009.
{{DEFAULTSORT:Millisecond Pulsar
*Millisecond pulsar