# Pulsar

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.

Pulsars 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.[1] 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 are very regular. For some pulsars, the regularity of pulsation is as precise as an atomic clock.[2] 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."[3]

## Discovery

Composite Optical/X-ray image of the Crab Nebula, showing synchrotron emission in the surrounding pulsar wind nebula, powered by injection of magnetic fields and particles from the central pulsar.

The first pulsar was observed in July 1967 by Jocelyn Bell Burnell and Anthony 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.[4] 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 the X-ray and/or gamma ray wavelengths.[5]

The word pulsar is a contraction of "pulsating star", and first appeared in print in 1968:

An entirely novel kind of star came to light on Aug. 6 last year and was referred to, by astronomers, as LGM (Little Green Men). Now it is thought to be a novel type between a white dwarf and a neutron [sic]. The name Pulsar is likely to be given to it. Dr. A. Hewish told me yesterday: "… I am sure that today every radio telescope is looking at the Pulsars."[6]

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 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

The Vela Pulsar and its surrounding pulsar wind nebula.

In 1974, Joseph Hooton Taylor, Jr. and Russell Hulse discovered, for the first time, a 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, a pulsar with a rotation period of just 1.6 milliseconds was discovered, by Shri Kulkarni and Don Backer. 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 rivalling 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.

The first extrasolar planets were found orbiting a MSP, by Aleksander Wolszczan. 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.

## Categories

Three distinct classes of pulsars are currently known to astronomers, according to the source of energy that powers the radiation:

The Fermi Space Telescope has uncovered a subclass of rotation-powered pulsars that emit only gamma ray radiation.[7] There have been only about twelve gamma-ray only pulsars identified out of about 1800 known pulsars.[8][9]

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 rotation-powered pulsars that have already lost most of their energy, and have only become visible again after their binary companions 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.[10]

## 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 to date, note that as pulsars slow, their energy output decreases. Conceivably there are much slower ones, below current levels of detection. On the other hand, the fastest they can spin (e.g. 1.4 ms) 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.
• In June 2006, 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.

## 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.

## 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.[11]

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,

$\mathrm{DM} = \int_0^D n_e(s) ds,$

where D is the distance from the pulsar to the observer and ne 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.[12]

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.[13] 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.[14]

## Significant pulsars

• 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.[15]
• The first binary pulsar, PSR 1913+16, whose orbit is decaying at the exact rate predicted due to the emission of gravitational radiation by general relativity
• The first millisecond pulsar, PSR B1937+21
• The brightest millisecond pulsar, PSR J0437-4715
• The first X-ray pulsar, Cen X-3
• The first accreting millisecond X-ray pulsar, SAX J1808.4-3658
• The first extrasolar planets to be discovered orbit the pulsar PSR B1257+12
• The first double pulsar binary system, PSR J0737−3039
• The magnetar SGR 1806-20 produced the largest burst of energy in the Galaxy ever experimentally recorded on [16]
• 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 a wind of particles leaving the pulsar's magnetosphere and carrying away rotational energy.[17]
• PSR J1748-2446ad, at 716 Hz, the pulsar with the highest rotation speed.
• 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.[18] 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.[19][20]
• PSR J1903+0327, a ~2.15 ms pulsar discovered to be in a highly eccentric binary star system with a sun-like star.[21]
• A pulsar in the CTA 1 supernova remnant 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.[7]

## Notes

1. ^ Young, M.D.; Manchester, R.N.; Johnston, S. "A Radio Pulsar with an 8.5-Second Period that Challenges Emission Models." Nature, Volume 400, 26 August 1999 (pages 848-849).
2. ^ D.N. Matsakis, J.H. Taylor and T.M. Eubanks. "A Statistic for Describing Pulsar and Clock Stabilities." Astronomy and Astrophysics, Volume 326, October 1997 (pages 924-928).
3. ^ Press Release: Old Pulsars Still Have New Tricks to Teach Us. European Space Agency, 26 July 2006.
4. ^ Sturrock, Peter A. The UFO Enigma: A New Review of the Physical Evidence. Warner Books, 1999 (page 154).
5. ^ Courtland, Rachel. "Pulsar Detected by Gamma Waves Only." New Scientist, 17 October 2008.
6. ^ Daily Telegraph, 21/3, 5 March 1968.
7. ^ a b Atkinson, Nancy. "Fermi Telescope Makes First Big Discovery: Gamma Ray Pulsar." Universe Today, 17 October 2008.
8. ^ NASA'S Fermi Telescope Unveils a Dozen New Pulsars http://www.nasa.gov/mission_pages/GLAST/news/dozen_pulsars.html
9. ^ Cosmos Online - New Kind of pulsar discovered (http://www.cosmosmagazine.com/news/2260/new-kind-pulsar-discovered)
10. ^ Lyne, Andrew G.; Graham-Smith, Francis. Pulsar Astronomy. Cambridge University Press, 1998.
11. ^ Ferriere, K. "The Interstellar Environment of Our Galaxy." Reviews of Modern Physics, Volume 73, Issue 4, 2001 (pages 1031–1066).
12. ^ Taylor, J. H.; Cordes, J. M. "Pulsar Distances and the Galactic Distribution of Free Electrons." Astrophysical Journal, Volume 411, 1993 (page 674).
13. ^ Rickett, Barney J. "Radio Propagation Through the Turbulent Interstellar Plasma." Annual Review of Astronomy and Astrophysics, Volume 28, 1990 (page 561).
14. ^ Rickett, Barney J.; Lyne, Andrew G.; Gupta, Yashwant. "Interstellar Fringes from Pulsar B0834+06." Monthly Notices of the Royal Astronomical Society, Volume 287, 1997 (page 739).
15. ^ Hewish, A. et al. "Observation of a Rapidly Pulsating Radio Source." Nature, Volume 217, 1968 (pages 709-713).
16. ^ "Galactic Magnetar Throws Giant Flare." Astronomy Picture of the Day, 21 February 2005.
17. ^ "Part-Time Pulsar Yields New Insight Into Inner Workings of Cosmic Clocks." Particle Physics and Astronomy Research Council, 3 March 2006.
18. ^ Tauris, T. M. et al. "Discovery of PSR J0108-1431: The Closest Known Neutron Star?" Astrophysical Journal, Volume 428, 1994 (page L53).
19. ^ Crowsell, K. "Science: Dim Pulsars May Crowd Our Galaxy." New Scientist, Number 1930, 18 June 2008. (page 16).
20. ^ "Closest Pulsar?" Sky & Telescope, October 1994 (page 14).
21. ^ Champion, David J. et al. "An Eccentric Binary Millisecond Pulsar in the Galactic Plane." Science, 6 June 2008 Volume 320, Number 5881 (pages 1309-1312).