Other attributes
Millisecond pulsars are the most rapidly rotating pulsars observed, with rotation periods below 30 milliseconds. Pulsars are highly magnetized, rotating neutron stars that emit a beam of electromagnetic radiation. Astronomers believe they primarily form from binary systems where the initially more massive component becomes a neutron star with accelerated rotation due to the accretion of matter from the secondary star. They are also referred to as recycled pulsars as the accretion of matter recycles the otherwise dead pulsar. Evidence for this theory of formation comes from the combination of rapid spin and relatively weak magnetic fields along with the high incidence of binary systems among observed millisecond pulsars.
Plot showing magnetic field strength vs pulsar period with the lower period/magnetic field millisecond pulsars shown in the bottom left.
Recycled pulsars are theorized to be a later evolutionary phase of other observed types of binary x-ray sources. X-ray binaries are a special type of binary star system emitting x-rays. They consist of a normal star and a later-stage collapsed star such as a white dwarf, neutron star, or black hole. They produce x-rays when the stars are close enough such that material is pulled from the normal star onto the denser, collapsed star. As this material falls onto the collapsed star, it is heated to very high temperatures, producing x-rays.
The first millisecond pulsar was observed in 1982. As of 2016, there were almost 3000 known millisecond pulsars. Roughly five percent are found in globular clusters, spherical structures often only tens of light years in diameter, yet holding up to a million stars. These crowded stellar environments are prone to forming binary star systems, and almost 85% of the pulsars observed in globular clusters are fast-rotating millisecond pulsars. A list of millisecond pulsars not associated with globular clusters is maintained by West Virginia University.
Radio pulsars were discovered in 1967; at the time of the first millisecond pulsar discovery in 1982, there were around 350 known examples of pulsars. These generally had longer periods on the scale of one second, the period of the first observed millisecond pulsar, PSR B1937+21, studied by Backer et al. was only 1.6 ms. A paper by Alpar et al. published shortly after suggested the new short-period pulsar originates from a binary system in which a slowly rotating neutron star is being accelerated by mass transfer from a companion star. For twenty years after its discovery, PSR B1937+21 was the fastest observed pulsar. It has a very small magnetic field strength (~108.5 Gauss) and a rotation period twenty times faster than the next-fastest pulsar, which at the time was the Crab pulsar. The early observed millisecond pulsars appeared to have periods between 1.5 and 6 milliseconds. However, astronomers have since detected pulsars with longer periods, making it harder to separate millisecond pulsars from normal pulsars.
Stars with a mass greater than roughly ten solar masses supernova when they run out of nuclear fuel, leaving behind a small dense neutron star. These stars have masses equal to one or more solar masses with small diameters, down to tens of kilometers. Neutron stars also rotate rapidly, and when charged particles are caught in their magnetic fields, they emit electromagnetic radiation as a beam that sweeps around as it rotates, often described as a lighthouse-like beam. This phenomenon is known as a pulsar, and astronomers detect the signals they produce along with their period (the gap between pulses). Millisecond pulsars are ones that spin particularly fast (hundreds of times per second) with a period on the scale of tens of milliseconds. The most commonly used classification of millisecond pulsars is a period of 30 milliseconds or below.
The leading theory of millisecond pulsar formation involves a binary star system that includes a neutron star. The supernova that forms the neutron star often disrupts the binary system, leaving behind an isolated neutron star and a runaway star. Surviving binary systems typically have high orbital eccentricities due to the extreme forces produced by the supernova. Over the next 107–8 years after the supernova, the neutron star may be observed as a normal radio pulsar spinning down to a period of roughly several seconds. At this point, the energy output of the star diminishes to a point where it no longer produces significant amounts of radio emission.
For surviving neutron star binary systems in which the companion is massive enough to evolve into a red giant and overflow its Roche lobe, the spun-down neutron star (dead pulsar) can restart due to accreting matter and angular momentum at the expense of the orbital angular momentum of the combined system. These systems are visible in the x-ray spectrum as high-energy radiation is produced during the accretion phase with frictional heating from matter falling onto the neutron star. Two classes of x-ray binaries relevant to binary and millisecond pulsars exist—neutron stars with high-mass or low-mass companions. A simplified diagram of this model of millisecond pulsar formation is shown below.
Simplified model of a millisecond pulsar formation.
The low-mass x-ray binary companion evolves and transfers matter to the neutron star over a much longer time scale, producing pulsar periods as short as a few milliseconds. During the accretion process, tidal forces increase the eccentricity of the orbit (closer to a circle). After the spin-up phase, the companion star has shed its outer layer to become a white dwarf orbiting the millisecond pulsar.
High-mass x-ray binary has a companion star massive enough to also supernova producing a second neutron star. If the binary system survives the supernova a double neutron star binary results. With the discovery of the double pulsar J0737-3039, systems have now been observed where the second neutron star is also a pulsar. While double neutron star systems are reasonably well understood, a number of effects reduce the detectability of double pulsar systems. The lifetime of the second pulsar is not being prolonged by accretion and therefore likely to be less than one-tenth of the recycled pulsar. The radio beam of the longer period second pulsar is likely to be much smaller than its spun-up counterpart, making it harder to detect.
There are also a number of pulsars with spin properties suggesting a phase of recycling, yet they do not have an orbiting companion. The existence of these systems in globular clusters are readily explained by the higher probability of stellar interactions; they are also observed in the galactic disk. Some models suggest isolated millisecond pulsars have ablated their companion via their strong relativistic winds, and it is unclear whether the energy required or timescale for this process are feasible.
Millisecond pulsars are excellent clocks, with comparable accuracy to the best atomic clocks, and they can be utilized to measure gravitational waves. If a gravitational wave is present either at the pulsar or at earth, the time dilation it produces can be measured as the pulsar measurements become unsynchronized.
The radio spectra are repeatedly searched for new pulsars in a variety of ways. Major search strategies optimized for binary and millisecond pulsars include the following:
- All-sky searches
- Searches close to the plane of the milky way
- Searches at intermediate and high galactic latitudes
- Targeted searches of globular clusters
- Extragalactic searches
