Pulsar

A pulsar (portmanteau of pulsating star) 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, much the way a lighthouse can only be seen when the light is pointed in the direction of an observer, and is responsible for the pulsed appearance of emission. Neutron stars are very dense, and have short, regular rotational periods. This produces a very precise interval between pulses that ranges from roughly milliseconds to seconds for an individual pulsars.

The precise periods of pulsars makes them useful tools. Observations of a pulsar in a binary neutron star system were used to confirm the existence of gravitational radiation. The first extrasolar planets were discovered around a pulsar, PSR B1257+12. Certain types of pulsars rival atomic clocks in their accuracy in keeping time.

Contents

History

The first pulsar was observed on November 28, 1967 by Jocelyn Bell Burnell and Antony Hewish.[1][2][3] The observed emission from the pulsar was pulses separated by 1.33 seconds, originated from the same location on the sky, and kept to sidereal time. In looking for explanations for the pulses, the short period of the pulses eliminated most astrophysical sources of radiation, such as stars, and since the pulses followed sidereal time, it could not be man-made radio frequency interference. When observations with another telescope confirmed the emission, it eliminated any sort of instrumental effects. At this point, Burnell notes of herself and Hewish that "we did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem—if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly?"[4] Even so, they nicknamed the signal LGM-1, for "little green men" (a name for intelligent beings of extraterrestrial origin). It was not until a second pulsating source was discovered in a different part of the sky that the "LGM hypothesis" was entirely abandoned.[5] 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 visible light, X-ray, and/or gamma ray wavelengths.[6]

The word "pulsar" is a contraction of "pulsating star",[7] 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."[8]

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 reasonable 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. Bell claims no bitterness upon this point, supporting the decision of the Nobel prize committee.[9]

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.[10]

In 1982, Don Backer led a group which discovered PSR B1937+21, a pulsar with a rotation period of just 1.6 milliseconds.[11] 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 rivaling 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. In June 2006, the 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.

In 1992, Aleksander Wolszczan discovered the first extrasolar planets around PSR B1257+12. 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.

Nomenclature

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.[12]

Formation

The events leading to the formation of a pulsar begin when the core of a massive star is compressed during a supernova, which collapses into a neutron star. The neutron star retains most of its angular momentum, and since it has only a tiny fraction of its progenitor's radius (and therefore its moment of inertia is sharply reduced), it is formed with very high rotation speed. A beam of radiation is emitted along the magnetic axis of the pulsar, which spins along with the rotation of the neutron star. The magnetic axis of the pulsar determines the direction of the electromagnetic beam, with the magnetic axis not necessarily being the same as its rotational axis. This misalignment causes the beam to be seen once for every rotation of the neutron star, which leads to the "pulsed" nature of its appearance. The beam originates from the rotational energy of the neutron star, which generates an electrical field from the movement of the very strong magnetic field, resulting in the acceleration of protons and electrons on the star surface and the creation of an electromagnetic beam emanating from the poles of the magnetic field.[13][14] This rotation slows down over time as electromagnetic power is emitted. When a pulsar's spin period slows down sufficiently, the radio pulsar mechanism is believed to turn off (the so-called "death line"). This turn-off seems to take place after about 10–100 million years, which means of all the neutron stars in the 13.6 billion year age of the universe, around 99% no longer pulsate.[15] The longest known pulsar period is 8.5 seconds.[16]

Though this very general picture of pulsars is mostly accepted, 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."[17]

Categories

Three distinct classes of pulsars are currently known to astronomers, according to the source of the power of the electromagnetic radiation:

The Fermi Space Telescope has uncovered a subclass of rotationally-powered pulsars that emit only gamma rays.[18] There have been only about one hundred gamma-ray pulsars identified out of about 1800 known pulsars.[19][20]

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 rotationally-powered pulsars that have already lost most of their power, and have only become visible again after their binary companions had 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. As this matter lands on the neutron star, it is thought to "bury" the magnetic field of the neutron star (although the details are unclear), leaving millisecond pulsars with magnetic fields 1000-10,000 times weaker than average pulsars. This low magnetic field is less effective at slowing the pulsar's rotation, so millisecond pulsars live for billions of years, making them the oldest known pulsars. Millisecond pulsars are seen in globular clusters, which stopped forming neutron stars billions of years ago.[15]

Of interest to the study of the state of the matter in a neutron stars are the glitches observed in the rotation velocity of the neutron star. This velocity is decreasing slowly but steadily, except by sudden variations. One model put forward to explain these glitches is that they are the result of "starquakes" that adjust the crust of the neutron star. Models where the glitch is due to a decoupling of the possibly superconducting interior of the star have also been advanced. In both cases, the star's moment of inertia changes, but its angular momentum doesn't, resulting in a change in rotation rate.

Disrupted recycled pulsar

When two massive stars are born close together from the same cloud of gas, they can form a binary system and orbit each other from birth. If those two stars are at least a few times as massive as our sun, their lives will both end in supernova explosions. The more massive star explodes first, leaving behind a neutron star. If the explosion does not kick the second star away, the binary system survives. The neutron star can now be visible as a radio pulsar, and it slowly loses energy and spins down. Later, the second star can swell up, allowing the neutron star to suck up its matter. The matter falling onto the neutron star spins it up and reduces its magnetic field. This is called “recycling” because it returns the neutron star to a quickly-spinning state. Finally, the second star also explodes in a supernova, producing another neutron star. If this second explosion also fails to disrupt the binary, a double neutron star binary is formed. Otherwise, the spun-up neutron star is left with no companion and becomes a “disrupted recycled pulsar”, spinning between a few and 50 times per second.[21]

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.

The discovery of pulsars allowed astronomers to study an object never observed before, the neutron star. This kind of object is the only place where the behavior of matter at nuclear density can be observed (though not directly). Also, millisecond pulsars have allowed a test of general relativity in conditions of an intense gravitational field.

Pulsar maps have been included on the two Pioneer Plaques as well as the Voyager Golden Record. They show the position of the Sun, relative to 14 pulsars, which are identified by the unique timing of their electromagnetic pulses, so that our position both in space and in time can be calculated by potential extraterrestrial intelligences.[22][23] Because pulsars are emitting very regular pulses of radio waves, its radio transmissions do not require daily corrections. Moreover, pulsar positioning could create a spacecraft navigation system independently, or be an auxiliary device to GPS instruments.[24][25]

Precise clocks

For some millisecond pulsars, the regularity of pulsation is as precise as an atomic clock.[26] This stability allows millisecond pulsars to be used in establishing ephemeris time.[27]

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.[28]

Because of 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 n_e 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.[29]

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.[30] 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.[31]

Probes of space-time

Pulsars orbiting within the curved space-time around Sgr A*, the supermassive black hole at the center of the Milky Way galaxy, could serve as probes of gravity in the strong-field regime.[32] Arrival times of the pulses would be affected by special- and general-relativistic Doppler shifts and by the complicated paths that the radio waves would travel through the strongly curved space-time around the black hole. In order for the effects of general relativity to be measurable with current instruments, pulsars with orbital periods less than about 10 years would need to be discovered;[32] such pulsars would orbit at distances inside 0.01 pc from Sgr A*. Searches are currently underway; at present, five pulsars are known to lie within 100 pc from Sgr A*.[33]

Significant pulsars

Gallery

What is a Pulsar?
Pulsar Detection.
A pulsar blinking.
A star in a binary star system has collapsed to a neutron star.


See also

Notes

  1. ^ Pranab Ghosh, Rotation and accretion powered pulsars. World Scientific, 2007, p.2.
  2. ^ M. S. Longair, Our evolving universe. CUP Archive, 1996, p.72.
  3. ^ M. S. Longair, High energy astrophysics, Volume 2. Cambridge University Press, 1994, p.99.
  4. ^ S. Jocelyn Bell Burnell (1977). "Little Green Men, White Dwarfs or Pulsars?". Cosmic Search Magazine. http://www.bigear.org/vol1no1/burnell.htm. Retrieved 2008-01-30.  (after-dinner speech with the title of Petit Four given at the Eighth Texas Symposium on Relativistic Astrophysics; first published in Annals of the New York Academy of Science, vol. 302, pages 685-689, Dec., 1977)
  5. ^ Burnell, S. Jocelyn Bell (23 April 2004). "So Few Pulsars, So Few Females". Science 304 (5670): 489. http://www.sciencemag.org/content/304/5670/489.full. 
  6. ^ Courtland, Rachel. "Pulsar Detected by Gamma Waves Only." New Scientist, 17 October 2008.
  7. ^ wiktionary:pulsar
  8. ^ Daily Telegraph, 21/3, 5 March 1968.
  9. ^ Burnell, S. Jocelyn Bell. Little Green Men, White Dwarfs, or Pulsars? Annals of the New York Academy of Science, vol. 302, pages 685–689, Dec., 1977 [1]
  10. ^ "Nobel Prize in Physics 1993". http://nobelprize.org/nobel_prizes/physics/laureates/1993/. Retrieved 2010-01-07. 
  11. ^ D. Backer et al. (1982). "A millisecond pulsar". Nature 300 (5893): 315–318. Bibcode 1982Natur.300..615B. doi:10.1038/300615a0. 
  12. ^ Lyne, Andrew G.; Graham-Smith, Francis. Pulsar Astronomy. Cambridge University Press, 1998.
  13. ^ "Pulsar Beacon Animation". http://www3.amherst.edu/~gsgreenstein/progs/animations/pulsar_beacon/. Retrieved 2010-04-03. 
  14. ^ "Pulsars". http://imagine.gsfc.nasa.gov/docs/science/know_l2/pulsars.html. Retrieved 2010-04-03. 
  15. ^ a b http://www.cv.nrao.edu/course/astr534/Pulsars.html
  16. ^ Young, M. D.; Manchester, R. N.; Johnston, S. (1999). "A Radio Pulsar with an 8.5-Second Period that Challenges Emission Models". Nature 400 (6747): 848–849. Bibcode 1999Natur.400..848Y. doi:10.1038/23650. 
  17. ^ Press Release: Old Pulsars Still Have New Tricks to Teach Us. European Space Agency, 26 July 2006.
  18. ^ a b Atkinson, Nancy. "Fermi Telescope Makes First Big Discovery: Gamma Ray Pulsar." Universe Today, 17 October 2008.
  19. ^ NASA'S Fermi Telescope Unveils a Dozen New Pulsars http://www.nasa.gov/mission_pages/GLAST/news/dozen_pulsars.html
  20. ^ Cosmos Online - New Kind of pulsar discovered (http://www.cosmosmagazine.com/news/2260/new-kind-pulsar-discovered)
  21. ^ Background material on "Disrupted Recycled Pulsar" in press release on the pulsar found by Einstein@Home http://www.aei.mpg.de/pdf/pm_news/2010/PM2010_Einstein_Home_pulsar_engl.pdf
  22. ^ http://astro.ysc.go.jp/pioneer10-plaque.txt
  23. ^ http://voyager.jpl.nasa.gov/spacecraft/goldenrec1.html
  24. ^ Marissa Cevallos, Science News,"HOW TO USE A PULSAR TO FIND STARBUCKS"[2], Discovery News,Wed Nov 24, 2010 10:21 AM ET .
  25. ^ Angelo Tartaglia; Matteo Luca Ruggiero; Emiliano Capolongo (2011). "A null frame for spacetime positioning by means of pulsating sources". Advances in Space Research 47 (4): 645–653. arXiv:1001.1068. Bibcode 2011AdSpR..47..645T. doi:10.1016/j.asr.2010.10.023. 
  26. ^ Matsakis, D. N.; Taylor, J. H.; Eubanks, T. M. (1997). "A Statistic for Describing Pulsar and Clock Stabilities". Astronomy and Astrophysics 326: 924–928. Bibcode 1997A&A...326..924M. http://aa.springer.de/papers/7326003/2300924.pdf. Retrieved 2010-04-03. 
  27. ^ Backer, Don (1984). "The 1.5 Millisecond Pulsar". Annals of the New York Academy of Sciences 422 (Eleventh Texas Symposium on Relativistic Astrophysics): 180–181. Bibcode 1984NYASA.422..180B. doi:10.1111/j.1749-6632.1984.tb23351.x. http://www3.interscience.wiley.com/journal/119527609/abstract. Retrieved 2010-02-14. 
  28. ^ Ferrière, Katia (2001). "The Interstellar Environment of Our Galaxy". Reviews of Modern Physics 73 (4): 1031–1066. arXiv:astro-ph/0106359. Bibcode 2001RvMP...73.1031F. doi:10.1103/RevModPhys.73.1031. 
  29. ^ Taylor, J. H.; Cordes, J. M. (1993). "Pulsar Distances and the Galactic Distribution of Free Electrons". Astrophysical Journal 411: 674. Bibcode 1993ApJ...411..674T. doi:10.1086/172870. 
  30. ^ Rickett, Barney J. (1990). "Radio Propagation Through the Turbulent Interstellar Plasma". Annual Review of Astronomy and Astrophysics 28: 561. Bibcode 1990ARA&A..28..561R. doi:10.1146/annurev.aa.28.090190.003021. 
  31. ^ Rickett, Barney J.; Lyne, Andrew G.; Gupta, Yashwant (1997). "Interstellar Fringes from Pulsar B0834+06". Monthly Notices of the Royal Astronomical Society 287: 739. Bibcode 1997MNRAS.287..739R. 
  32. ^ a b Angelil, R.; Saha, P.; Merritt, D. (2010). "Towards relativistic orbit fitting of Galactic center stars and pulsars". arXiv:1007.0007 [astro-ph.GA]. 
  33. ^ Deneva, J. S.; Cordes, J. M.; Lazio, T. J. W. (2009). "Discovery of Three Pulsars from a Galactic Center Pulsar Population". The Astrophysical Journal Letters 702 (2): L177–182. Bibcode 2009ApJ...702L.177D. doi:10.1088/0004-637X/702/2/L177. 
  34. ^ Hewish, A. et al. "Observation of a Rapidly Pulsating Radio Source." Nature, Volume 217, 1968 (pages 709-713).
  35. ^ "Galactic Magnetar Throws Giant Flare." Astronomy Picture of the Day, 21 February 2005.
  36. ^ "Part-Time Pulsar Yields New Insight Into Inner Workings of Cosmic Clocks." Particle Physics and Astronomy Research Council, 3 March 2006.
  37. ^ Champion, David J. et al. (2008). "An Eccentric Binary Millisecond Pulsar in the Galactic Plane". Science 320 (5881): 1309–1312. Bibcode 2008Sci...320.1309C. doi:10.1126/science.1157580. PMID 18483399. 
  38. ^ Knispel, B.; et al., B; Cordes, JM; Deneva, JS; Anderson, D; Aulbert, C; Bhat, ND; Bock, O et al. (2010). "Pulsar Discovery by Global Volunteer Computing". Science Forthcoming (5997): 1305. Bibcode 2010Sci...329.1305K. doi:10.1126/science.1195253. PMID 20705813. 

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