X-ray pulsarsoraccretion-powered pulsarsare a class ofastronomicalobjects that areX-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.
Characteristics
editAn X-raypulsaris a type ofbinary star systemconsisting of a typical star (stellar companion) in orbit around a magnetizedneutron star.Themagnetic fieldstrength at the surface of the neutron star is typically about 108Tesla,over a trillion times stronger than the strength of the magnetic field measured at thesurface of the Earth(60μT).
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 twoauroral zoneson Earth, but far hotter. At these hotspots the infalling gas can reach half thespeed of lightbefore it impacts the neutron star surface. So muchgravitational potential energyis released by the infalling gas, that the hotspots, which are estimated to about one square kilometer in area, can be ten thousand times, or more, as luminous as theSun.[1]
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.[1]
Gas supply
editThe 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 (seestellar classification), that emit a radiation drivenstellar 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 asRoche lobeoverflow. The captured material forms a gaseousaccretion discand spirals inwards to ultimately fall onto the neutron star as in the binary systemCen X-3.
For still other types of X-ray pulsars, the companion star is aBe 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.[2][3][4][5]
Spin behaviors
editRadio 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 rotatingmagnetizedneutron 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 and faster or slower and 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.[2]
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 ofrelativistic particlesandmagnetic dipoleradiation, lighting up any nearby nebulae that surround them. In contrast, X-ray pulsars are members ofbinary star systemsand accrete matter from either stellar winds or accretion disks. The accreted matter transfersangular 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.
Observations
editX-ray pulsars are observed usingX-ray telescopesthat are satellites in low Earth orbit although some observations have been made, mostly in the early years ofX-ray astronomy,using detectors carried by balloons or sounding rockets. The first X-ray pulsar to be discovered wasCentaurus X-3,in 1971 with theUhuruX-ray satellite.[1]
Anomalous X-ray pulsars
editMagnetars,isolated and highly-magnetised neutron stars, can be observed as relatively slow x-ray pulsars with periods of a few seconds. These are referred to as anomalous X-ray pulsars, but are unrelated to binary X-ray pulsars.
See also
editReferences
edit- ^abcCharles, Philip A.; Seward, Frederick D. (1995). "7".Exploring the X-ray Universe.Cambridge: Cambridge Univ. Press.ISBN978-0-521-26182-1.
- ^ab Bildsten, L.; Chakrabarty, D.; Chu, J.; Finger, M. H.; Koh, D. T.; Nelson, R. W.; Prince, T. A.; Rubin, B. C.; Scott, D. M.; Vaughan, B.; Wilson, C. A.; Wilson, R. B. (1997). "Observations of Accreting Pulsars".The Astrophysical Journal Supplement Series.113(2): 367–408.arXiv:astro-ph/9707125.Bibcode:1997ApJS..113..367B.doi:10.1086/313060.S2CID706199.
- ^Chandra, Amar Deo; Roy, Jayashree; Agrawal, P C; Choudhury, Manojendu (2020-07-01)."Study of recent outburst in the Be/X-ray binary RX J0209.6−7427 with AstroSat: a new ultraluminous X-ray pulsar in the Magellanic Bridge?".Monthly Notices of the Royal Astronomical Society.495(3): 2664–2672.arXiv:2004.04930.Bibcode:2020MNRAS.495.2664C.doi:10.1093/mnras/staa1041.ISSN0035-8711.
- ^Rowden, Pam (1 June 2020)."Ultra-bright X-ray source awakens near a galaxy not so far away".Royal Astronomical Society.
- ^Carpineti, Alfredo (5 June 2020)."Ultra-Bright Pulsar Awakens Next Door To The Milky Way After 26-Year Slumber".IFLScience.