X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are
X-raysources displaying strict periodic variations in X-ray intensity. The X-ray periods range from as little as a fraction of a second to as much as several minutes. An X-ray pulsar consists of a magnetized neutron starin orbit with a normal stellar companion and are a type of binary star system. The magnetic fieldstrength at the surface of the neutron star is typically about 1012 gauss, over a trillion times stronger than the strength of the magnetic fieldmeasured at the surface of the Earth (0.6 gauss). Gas is accreted from the stellar companion and is channeled by the neutron star's magnetic field on to the magnetic poles producing two or more localized X-ray hot spots similar to the two auroral zones on the Earth but far hotter. At these hotspots the infalling gas can reach half the speed of lightbefore it impacts the neutron star surface. So much gravitational potential energyis released by the infalling gas, that the hotspots, which are estimated to about one square kilometer in area, can be up to ten thousand times or more luminous than the Sun. Temperatures of millions of degrees are produced so the hotspots emit mostly X-rays. As the neutron star rotates, pulses of X-rays are observed as the hotspots move in and out of view if the magnetic axis is tilted with respect to the spin axis.
The gas that supplies the X-ray pulsar can reach the neutron star by a variety of ways that depend on the size and shape of the neutron star's orbital path and the nature of the companion star. Some companion stars of X-ray pulsars are very massive young stars, usually OB supergiants (see
stellar classification), that emit a radiation driven stellar windfrom their surface. The neutron star is immersed in the wind and continuously captures gas that flows nearby. Vela X-1is an example of this kind of system. In other systems, the neutron star orbits so closely to its companion that its strong gravitational force can pull material from the companion's atmosphere into an orbit around itself, a mass transfer process known as Roche lobeoverflow. The captured material forms a gaseous accretion discand spirals inwards to ultimately fall onto the neutron star as in the binary system Cen X-3. For still other types of X-ray pulsars, the companion star is a Be starthat rotates very rapidly and apparently sheds a disk of gas around its equator. The orbits of the neutron star with these companions are usually large and very elliptical in shape. When the neutron star passes nearby or through the Be circumstellar disk, it will capture material and temporarily become an X-ray pulsar. The circumstellar disk around the Be star expands and contracts for unknown reasons, so these are transient X-ray pulsars that are observed only intermittently, often with months to years between episodes of observable X-ray pulsation. Radio pulsars (rotation-powered pulsars) and X-ray pulsars exhibit very different spin behaviors and have different mechanisms producing their characteristic pulses although it is accepted that both kinds of pulsar are manifestations of a rotating magnetized neutron star. The rotation cycle of the neutron star in both cases is identified with the pulse period. The major differences are that radio pulsars have periods on the order of milliseconds to seconds, and all radio pulsars are losing angular momentum and slowing down. In contrast, the X-ray pulsars exhibit a variety of spin behaviors. Some X-ray pulsars are observed to be continuously spinning faster or slower (with occasional reversals in these trends) while others show either little change in pulse period or display erratic spin-down and spin-up behavior. The explanation of this difference can be found in the physical nature of the two pulsar classes. Over 99% of radio pulsars are single objects that radiate away their rotational energy in the form of relativistic particles and magnetic dipoleradiation, lighting up any nearby nebulae that surround them. In contrast, X-ray pulsars are members of binary star systems and accrete matter from either stellar winds or accretion disks. The accreted matter transfers angular momentumto (or from) the neutron star causing the spin rate to increase or decrease at rates that are often hundreds of times faster than the typical spin down rate in radio pulsars. Exactly why the X-ray pulsars show such varied spin behavior is still not clearly understood.
X-ray pulsars are observed using that are satellites in low Earth orbit although some observations have been made, mostly in the early years of
X-ray astronomy, using detectors carried by balloons or sounding rockets.
The first X-ray pulsar to be discovered was
Centaurus X-3in 1971 with the UhuruX-ray satellite.
1. [http://adsabs.harvard.edu/abs/1997ApJS..113..367B Observations of Accreting Pulsars] , Bildsten, L., et al., 1997, Astrophysical Journal Supplement Series, 113, p. 367
Anomalous X-ray pulsar
List of X-ray pulsars
* [http://www.batse.msfc.nasa.gov/batse/pulsar/ BATSE Pulsar Studies]
Wikimedia Foundation. 2010.
Look at other dictionaries:
X-ray pulsar — X ray pulsar, a pulsar that is the source of powerful X ray emissions: »One of the most remarkable developments of X ray astronomy was the…discovery of an X ray pulsar in the Crab Nebula (Richard B. Hoover) … Useful english dictionary
X-ray pulsar based navigation — and Timing (XNAV) is a navigation technique whereby the periodic X ray signals emitted from pulsars are used to determine the location of a spacecraft in deep space. A spacecraft using XNAV compares received X ray signals with a database of known … Wikipedia
Anomalous X-ray pulsar — Anomalous X ray Pulsars (AXPs) are now widely believed to be magnetars mdash;young, isolated, highly magnetized neutron stars. These energtic X ray pulsars are characterized by slow rotation periods of 5 ndash;12 seconds and large magnetic fields … Wikipedia
Pulsar x anormal — Pour les articles homonymes, voir AXP. En astronomie, le terme de pulsar X anormal (ou AXP, sigle de l expression anglaise Anomalous X ray Pulsar) désigne un pulsar X (c est à dire un pulsar émettant principalement dans le domaine des rayons X),… … Wikipédia en Français
Pulsar — Pulsars are highly magnetized rotating neutron stars that emit a beam of electromagnetic radiation in the form of radio waves. Their observed periods range from 1.4 ms to 8.5 s. [ M.D. Young, R.N. Manchester and S. Johnston. A radio pulsar with… … Wikipedia
Pulsar du Crabe — PSR B0531+21 Pulsar du Crabe (PSR B0531+21) … Wikipédia en Français
Pulsar X anormal — Pour les articles homonymes, voir AXP. En astronomie, le terme de pulsar X anormal (ou AXP, sigle de l expression anglaise Anomalous X ray Pulsar) désigne un pulsar X (c est à dire un pulsar émettant principalement dans le domaine des rayons X),… … Wikipédia en Français
Pulsar gamma — En astronomie, le terme de pulsar gamma désigne un pulsar présentant une émission modulée périodique dans le domaine des rayons gamma. Ce terme ne désigne donc pas des pulsars sièges d émissions irrégulières de rayons gamma comme les sursauteurs… … Wikipédia en Français
Pulsar de Vela — El Pulsar de Vela y su nebulosa circundante. Constelación Vela … Wikipedia Español
Ray Bradbury — Born August 22, 1920 (1920 08 22) (age 91) Nationality American … Wikipedia