Millisecond pulsar

This diagram shows the steps astronomers say are needed to create a pulsar with a superfast spin. 1. A massive supergiant star and a "normal" Sun-like star orbit each other. 2. The massive star explodes, leaving a pulsar that eventually slows down, turns off, and becomes a cooling neutron star. 3. The Sun-like star eventually expands, spilling material on to the neutron star. This "accretion" speeds up the neutron star's spin. 4. Accretion ends, the neutron star is "recycled" into a millisecond pulsar. But in a densely packed globular cluster (2b)... The lowest mass stars are ejected, the remaining normal stars evolve, and the "recycling" scenario (3-4) takes place, creating many millisecond pulsars.

A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio, X-ray, and gamma ray portions of the electromagnetic spectrum. 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.[1][2] For this reason, millisecond pulsars are sometimes 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 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. Bülent Kiziltan and S. E. Thorsett (UCSC) showed that different millisecond pulsars must form by at least two distinct processes.[3] But the nature of the other process remains a mystery.[4]

Many millisecond pulsars are found in globular clusters. 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.[5] The globular cluster Terzan 5 contains 37 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, have a stability comparable to atomic-clock-based time standards when averaged over decades.[6][7] 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 of the Solar System. One of them, PSR B1257+12 D, has an even smaller mass, comparable to that of the Moon, and is still today the smallest-mass object known beyond the Solar System.[8]

  1. ^ Bhattacharya, D.; Van Den Heuvel, E. P. J. (1991). "Formation and evolution of binary and millisecond radio pulsars". Physics Reports. 203 (1–2): 1. Bibcode:1991PhR...203....1B. doi:10.1016/0370-1573(91)90064-S.
  2. ^ Tauris, T. M.; Van Den Heuvel, E. P. J. (2006). Formation and evolution of compact stellar X-ray sources. Bibcode:2006csxs.book..623T.
  3. ^ Kızıltan, Bülent; Thorsett, S. E. (2009). "Constraints on Pulsar Evolution: The Joint Period-Spin-down Distribution of Millisecond Pulsars". The Astrophysical Journal Letters. 693 (2): L109–L112. arXiv:0902.0604. Bibcode:2009ApJ...693L.109K. doi:10.1088/0004-637X/693/2/L109. S2CID 2156395.
  4. ^ Naeye, Robert (2009). "Surprising Trove of Gamma-Ray Pulsars". Sky & Telescope.
  5. ^ Freire, Paulo. "Pulsars in globular clusters". Arecibo Observatory. Retrieved 2007-01-18.
  6. ^ Matsakis, D. N.; Taylor, J. H.; Eubanks, T. M. (1997). "A Statistic for Describing Pulsar and Clock Stabilities" (PDF). Astronomy and Astrophysics. 326: 924–928. Bibcode:1997A&A...326..924M. Archived from the original (PDF) on 2011-07-25. Retrieved 2010-04-03.
  7. ^ Hartnett, John G.; Luiten, Andre N. (2011-01-07). "Colloquium: Comparison of astrophysical and terrestrial frequency standards". Reviews of Modern Physics. 83 (1): 1–9. arXiv:1004.0115. Bibcode:2011RvMP...83....1H. doi:10.1103/revmodphys.83.1. ISSN 0034-6861. S2CID 118396798.
  8. ^ Rasio, Frederic (2011). "Planet Discovery near Pulsars". Science. doi:10.1126/science.1212489.