View source for Binary pulsar

{{Short description|Two pulsars orbiting each other}}
[[File:J0737-3039 still1 large.jpg|thumb|Artist's impression of a binary pulsar]]
A '''binary pulsar''' is a [[pulsar]] with a [[binary star|binary companion]], often a [[white dwarf]] or [[neutron star]]. (In at least one case, the double pulsar [[PSR J0737-3039]], the companion neutron star is another pulsar as well.)  Binary pulsars are one of the few objects which allow physicists to test [[general relativity]] because of the strong gravitational fields in their vicinities. Although the binary companion to the pulsar is usually difficult or impossible to observe directly, its presence can be deduced from the timing of the pulses from the pulsar itself, which can be measured with extraordinary accuracy by [[radio telescope]]s.

== History ==

The binary pulsar [[PSR B1913+16]] (or the "Hulse-Taylor binary pulsar") was first discovered in 1974 at [[Arecibo Observatory|Arecibo]] by [[Joseph Hooton Taylor, Jr.]] and [[Russell Hulse]], for which they won the 1993 [[Nobel Prize in Physics]]. While Hulse was observing the newly discovered pulsar PSR B1913+16, he noticed that the rate at which it pulsed varied regularly. It was concluded  that the pulsar was orbiting another star very closely at a high velocity, and that the pulse period was varying due to the [[Doppler effect]]: As the pulsar was moving towards Earth, the pulses would be more frequent; and  conversely, as it moved away from Earth fewer would be detected in a given time period. One can think of the pulses like the ticks of a clock; changes in the ticking are indications of changes in the pulsars speed toward and away from Earth. Hulse and Taylor also determined that the stars were approximately equally massive by observing these pulse fluctuations, which led them to believe the other  object was also a neutron star. Pulses from this system are now tracked to within 15 [[microsecond|μs]].<ref name="wet2010">{{cite journal|last1=Weisberg|first1=J. M.|year=2010|title=Timing Measurements of the Relativistic Binary Pulsar PSR B1913+16|journal=Astrophysical Journal|volume=722| pages=1030–1034|doi=10.1088/0004-637X/722/2/1030|arxiv =1011.0718|bibcode=2010ApJ...722.1030W |last3=Taylor|first3=J. H.|last2=Nice|first2=D. J.|issue=2|s2cid=118573183}}</ref> (Note: [[Centaurus_X-3|Cen X-3]] was actually the first "binary pulsar" discovered in 1971, followed by [[Hercules_X-1|Her X-1]] in 1972.)

The study of the PSR B1913+16 binary pulsar also led to the first accurate determination of neutron star masses, using relativistic timing effects.<ref name="wt82">{{cite journal|last1=Taylor|first1=J. H.|last2=Weisberg|first2=J. M.|year=1982| title=A new test of general relativity - Gravitational radiation and the binary pulsar PSR 1913+16|journal=Astrophysical Journal|volume=253| pages=908–920|doi=10.1086/159690|bibcode=1982ApJ...253..908T }}</ref> When the two bodies are in close proximity, the gravitational field is stronger, the passage of time is slowed – and the time between pulses (or ticks) is lengthened. Then as the pulsar clock travels more slowly through the weakest part of the field it regains time. A special relativistic effect, time dilation, acts around the orbit in a similar fashion.  This relativistic time delay is the difference between what one would expect to see if the pulsar were moving at a constant distance and speed around its companion in a circular orbit, and what is actually observed.

Prior to the [[first observation of gravitational waves]] in 2015 and the operation of [[LIGO|Advanced LIGO]],<ref name="PRL-20160211">{{cite journal|collaboration=LIGO Scientific Collaboration and Virgo Collaboration|last1=Abbott|first1=Benjamin P.|title=Observation of Gravitational Waves from a Binary Black Hole Merger|url=http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102|journal=[[Phys. Rev. Lett.]]|volume=116|issue=6|pages=061102|year=2016|doi=10.1103/PhysRevLett.116.061102|arxiv=1602.03837|bibcode=2016PhRvL.116f1102A|pmid=26918975| s2cid=124959784 }}</ref> binary pulsars were the only  tools scientists had to detect evidence of [[gravitational waves]]; Einstein's theory of [[general relativity]] predicts that two neutron stars would emit gravitational waves as they orbit a common center of mass, which would carry away orbital energy and cause the two stars to draw closer together and shorten their orbital period. A  10-parameter model incorporating information about the pulsar timing, the [[Kepler's laws|Keplerian]] orbits and three post-Keplerian corrections (the rate of [[periastron]] advance,  a factor for [[gravitational redshift]] and [[time dilation]], and a rate of change of the orbital period from [[gravitational radiation]] emission) is sufficient to completely model the binary pulsar timing.<ref>{{cite journal|last1=Weisberg|first1=J. M.|date=October 1981| title=Gravitational waves from an orbiting pulsar|journal=Scientific American|volume=245| pages=74–82|doi=10.1038/scientificamerican1081-74|bibcode=1981SciAm.245d..74W| last2=Taylor|first2=J. H.|last3=Fowler|first3=L. A.|issue=4}}</ref><ref name="haynes2007">{{cite web|url=http://www.astro.cornell.edu/academics/courses/astro201/psr1913.htm|title=Prof. Martha Haynes Astro 201 Binary Pulsar PSR 1913+16 Website}}</ref>

The measurements made of the orbital decay of the PSR B1913+16 system were a near perfect match to Einstein's equations. Relativity predicts that over time a binary system's orbital energy will be converted to [[gravitational radiation]]. Data collected by Taylor and Joel M. Weisberg and their colleagues of the orbital period of PSR B1913+16 supported this relativistic prediction; they reported in 1982<ref name="wt82"/> and subsequently<ref name="wet2010"/><ref>{{cite journal|last1=Taylor|first1=J. H.|last2=Weisberg|first2=J. M.|year=1989| title=Further experimental tests of relativistic gravity using the binary pulsar PSR 1913 + 16|journal=Astrophysical Journal|volume=345| pages=434–450|doi=10.1086/167917|bibcode=1989ApJ...345..434T |s2cid=120688730 }}</ref> that there was a difference in the observed minimum separation of the two pulsars compared to that expected if the orbital separation had remained constant. In the decade following its discovery, the system's orbital period had decreased by about 76 millionths of a second per year, indicating that the pulsar was approaching its maximum separation more than a second earlier than it would have if the orbit had remained the same. Subsequent observations continue to show this decrease.

== Intermediate mass binary pulsar ==

An {{visible anchor|intermediate-mass binary pulsar|Intermediate-mass binary pulsar|IMBP}} (IMBP) is a pulsar-white dwarf binary system with a relatively long spin period of around 10–200&nbsp;ms consisting of a white dwarf with a relatively high mass of approximately <math>\,\geq 0.4 M_{\odot}.</math><ref name="ChenLi2011">{{cite journal|last1=Chen|first1=W.-C.|last2=Li|first2=X.-D.|last3=Xu|first3=R.-X.|title=He star evolutionary channel to intermediate-mass binary pulsar PSR J1802-2124|journal=Astronomy & Astrophysics|volume=530|year=2011|pages=A104|issn=0004-6361|doi=10.1051/0004-6361/201116532|arxiv=1105.1046|bibcode=2011A&A...530A.104C |s2cid=119183244 }}</ref> 
The spin periods, magnetic field strengths, and orbital eccentricities of IMBPs are significantly larger than those of low mass binary pulsars (LMBPs).<ref name="ChenLi2011" /> As of 2014, there are fewer than 20 known IMBPs.<ref name="KaplanBoyles2014">{{cite journal|last1=Kaplan|first1=David L.|last2=Boyles|first2=Jason|last3=Dunlap|first3=Bart H.|last4=Tendulkar|first4=Shriharsh P.|last5=Deller|first5=Adam T.|last6=Ransom|first6=Scott M.|last7=McLaughlin|first7=Maura A.|last8=Lorimer|first8=Duncan R.|last9=Stairs|first9=Ingrid H.|title=A 1.05 M<sub>⊙</sub> Companion to PSR J2222-0137: The Coolest Known White Dwarf?|journal=The Astrophysical Journal|volume=789|issue=2|year=2014|pages=119|issn=0004-637X|doi=10.1088/0004-637X/789/2/119|arxiv=1406.0488v1|bibcode=2014ApJ...789..119K |s2cid=19986066 }}</ref> Examples of IMBPs include [[PSR J1802−2124]]<ref name="ChenLi2011" /> and [[PSR J2222−0137]].<ref name="KaplanBoyles2014" /> 

The binary system PSR J2222−0137 has an orbital period of about 2.45 days and is found at a distance of 267{{su|b=-0.9|p=+1.2}} [[Parsec|pc]] (approximately 870 light-years), making it the second closest known binary pulsar systems (as of 2014) and one of the closest pulsars and neutron stars.<ref name="KaplanBoyles2014" /> The relatively high-mass pulsar (1.831 <math>\pm</math> 0.010 <math>M_{\odot}</math>) has a companion star PSR J2222−0137 B with a minimum mass of approximately 1.3 solar masses (1.319 <math>\pm</math> 0.004 <math>M_{\odot}</math>).<ref name=Guo>{{cite journal|doi=10.1051/0004-6361/202141450|bibcode=2021A&A...654A..16G|arxiv=2107.09474|title=PSR J2222−0137. I. Improved physical parameters for the system|year=2021|last1=Guo|first1=Y. J.|last2=Freire|first2=P. C. C.|last3=Guillemot|first3=L.|last4=Kramer|first4=M.|last5=Zhu|first5=W. W.|last6=Wex|first6=N.|last7=McKee|first7=J. W.|last8=Deller|first8=A.|last9=Ding|first9=H.|last10=Kaplan|first10=D. L.|last11=Stappers|first11=B.|last12=Cognard|first12=I.|last13=Miao|first13=X.|last14=Haase|first14=L.|last15=Keith|first15=M.|last16=Ransom|first16=S. M.|last17=Theureau|first17=G.|journal=Astronomy & Astrophysics|volume=654|pages=A16|s2cid=236134389}}</ref> This meant the companion is a massive white dwarf (only about 8% of white dwarfs have a mass <math>\,\geq 0.9 M_{\odot}</math>), which would make the system an IMBP. Although initial measurements gave a mass of about 1 solar mass for the PSR J2222−0137 B,<ref name="KaplanBoyles2014" /> later observations showed that it is actually a high-mass white dwarf<ref name=Guo/> and also one of the coolest known white dwarfs, with a temperature less than 3,000 K.<ref name="KaplanBoyles2014" /> 
PSR J2222−0137 B is likely crystallized, leading to this Earth-sized white dwarf being described as a "diamond-star",<ref name="NatGeo24June2014">{{cite news|title=Astronomers Find Slow-Cooked Diamond the Size of Earth|last=Drake|first=Nadia|author-link=Nadia Drake|work=National Geographic|date=24 June 2014|url=https://www.nationalgeographic.com/science/article/diamond-the-size-of-earth|archive-url=https://web.archive.org/web/20210321004316/https://www.nationalgeographic.com/science/article/diamond-the-size-of-earth|url-status=dead|archive-date=March 21, 2021}}</ref> similar to the white dwarf companion of [[PSR J1719-1438]], which lies about 4,000 light-years away.<ref name="NatGeo26Aug2011">{{cite news|title="Diamond" Planet Found; May Be Stripped Star|last=Fazekas|first=Andrew|work=National Geographic|date=26 August 2011|url=https://www.nationalgeographic.com/science/article/110825-new-planet-diamond-pulsar-dwarf-star-space-science|archive-url=https://web.archive.org/web/20210517200230/https://www.nationalgeographic.com/science/article/110825-new-planet-diamond-pulsar-dwarf-star-space-science|url-status=dead|archive-date=May 17, 2021}}</ref>

== Effects ==

Sometimes the relatively normal companion star of a binary pulsar will swell up to the point that it dumps its outer layers onto the pulsar. This interaction can heat the gas being exchanged between the bodies and produce X-ray light which can appear to pulsate, in a process called the [[X-ray binary]] stage. The flow of matter from one stellar body to another often leads to the creation of an [[accretion disk]] about the recipient star.

[[Pulsar]]s also create a  "wind" of relativistically outflowing particles, which in the case of binary pulsars can blow away the [[magnetosphere]] of their companions and have a dramatic effect on the pulse emission.

== See also ==

* {{annotated link|Astronomy}}
* {{annotated link|PSR B1913+16}}
* {{annotated link|PSR J0737-3039}}
* {{annotated link|Square Kilometre Array}}

== References ==
{{reflist}}

== External links ==
{{wiktionary}}
* [http://www.astro.cornell.edu/academics/courses/astro201/psr1913.htm Prof. Martha Haynes Astro 201 Binary Pulsar PSR 1913+16 Website]
* [http://nobelprize.org/nobel_prizes/physics/laureates/1993/illpres/discovery.html Nobel Prize for the binary pulsar discovery]
* [http://www.lsw.uni-heidelberg.de/users/mcamenzi/NS_Mass.html Neutron Star Masses] {{Webarchive|url=https://web.archive.org/web/20121017011931/http://www.lsw.uni-heidelberg.de/users/mcamenzi/NS_Mass.html |date=2012-10-17 }}
* {{Cite journal |last=Lorimer |first=Duncan R. |year=2008 |title=Binary and Millisecond Pulsars |journal=Living Reviews in Relativity |language=en |volume=11 |issue=1 |page=8 |arxiv=0811.0762 |bibcode=2008LRR....11....8L |doi=10.12942/lrr-2008-8 |issn=2367-3613 |pmc=5256074 |pmid=28179824 |doi-access=free}}
* {{Cite journal |last=Will |first=Clifford M. |year=2001 |title=The Confrontation between General Relativity and Experiment |journal=Living Reviews in Relativity |language=en |volume=4 |issue=1 |pages=4 |arxiv=gr-qc/0103036 |bibcode=2001LRR.....4....4W |doi=10.12942/lrr-2001-4 |issn=2367-3613 |pmc=5253802 |pmid=28163632 |doi-access=free}}
* {{Cite journal |last=Stairs |first=I. H. |year=2009 |title=Binary pulsars and tests of general relativity |journal=Proceedings of the International Astronomical Union |language=en |volume=5 |issue=S261 |pages=218–227 |doi=10.1017/S1743921309990433 |issn=1743-9213 |doi-access=free}}

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{{DEFAULTSORT:Binary Pulsar}}
[[Category:Tests of general relativity]]
[[Category:Binary stars|*pulsar]]
[[Category:Pulsars|*binary pulsar]]
[[Category:Radio astronomy]]