Editing Quasar (section)

==Multiple quasars==
A grouping of two or more quasars on the sky can result from a chance alignment, where the quasars are not physically associated, from actual physical proximity, or from the effects of gravity bending the light of a single quasar into two or more images by [[gravitational lensing]].

When two quasars appear to be very close to each other as seen from Earth (separated by a few [[arcsecond]]s or less), they are commonly referred to as a "double quasar". When the two are also close together in space (i.e. observed to have similar redshifts), they are termed a "quasar pair", or as a "binary quasar" if they are close enough that their host galaxies are likely to be physically interacting.<ref>{{Cite journal |last=Myers |first=Adam D. |display-authors=et al |year=2008 |title=Quasar Clustering at 25 ''h''<sup>−1</sup> kpc from a Complete Sample of Binaries |journal=The Astrophysical Journal |language=en |volume=678 |issue=2 |pages=635–646 |arxiv=0709.3474 |bibcode=2008ApJ...678..635M |doi=10.1086/533491 |issn=0004-637X |s2cid=15747141}}</ref>

As quasars are overall rare objects in the universe, the probability of three or more separate quasars being found near the same physical location is very low, and determining whether the system is closely separated physically requires significant observational effort. The first true triple quasar was found in 2007 by observations at the [[W. M. Keck Observatory]] in [[Mauna Kea]], [[Hawaii]].<ref>{{cite news |first=Paul |last=Rincon |author-link=Paul Rincon |date=2007-01-09 |title=Astronomers see first quasar trio |work=BBC News |url=http://news.bbc.co.uk/2/hi/science/nature/6243361.stm }}</ref> [[LBQS 1429-008]] (or QQQ J1432-0106) was first observed in 1989 and at the time was found to be a double quasar. When [[astronomer]]s discovered the third member, they confirmed that the sources were separate and not the result of gravitational lensing. This triple quasar has a redshift of ''z'' = 2.076.<ref>{{cite web |title=Triple quasar QQQ 1429-008 |publisher=ESO |access-date=2009-04-23 |url=http://www.eso.org/gallery/v/ESOPIA/Quasars/phot-02-07.jpg.html |url-status=dead |archive-url=https://web.archive.org/web/20090208213256/http://eso.org/gallery/v/ESOPIA/Quasars/phot-02-07.jpg.html |archive-date=2009-02-08}}</ref> The components are separated by an estimated 30–50&nbsp;[[kiloparsecs]] (roughly 97,000–160,000 light-years), which is typical for interacting galaxies.<ref>{{Cite journal |last=Djorgovski |first=S. G. |author-link=Stanislav George Djorgovski |display-authors=etal |date=2007 |title=Discovery of a Probable Physical Triple Quasar |journal=The Astrophysical Journal |language=en |volume=662 |issue=1 |pages=L1–L5 |arxiv=astro-ph/0701155 |bibcode=2007ApJ...662L...1D |doi=10.1086/519162 |issn=0004-637X |s2cid=22705420}}</ref> In 2013, the second true triplet of quasars, QQQ&nbsp;J1519+0627, was found with a redshift ''z'' = 1.51, the whole system fitting within a physical separation of 25&nbsp;kpc (about 80,000 light-years).<ref>{{cite web |date=12 March 2013 |title=Extremely rare triple quasar found |url=https://phys.org/news/2013-03-extremely-rare-triple-quasar.html |access-date=1 May 2025 |website=phys.org}}</ref><ref>{{Cite journal |last=Farina |first=E. P. |display-authors=et al |year=2013 |title=Caught in the act: discovery of a physical quasar triplet |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=431 |issue=2 |pages=1019–1025 |arxiv=1302.0849 |bibcode=2013MNRAS.431.1019F |doi=10.1093/mnras/stt209 |issn=1365-2966 |s2cid=54606964 |doi-access=free}}</ref>

The first true quadruple quasar system was discovered in 2015 at a redshift ''z''&nbsp;=&nbsp;2.0412 and has an overall physical scale of about 200&nbsp;kpc (roughly 650,000 light-years).<ref>{{Cite journal |last=Hennawi |first=Joseph F. |display-authors=et al |year=2015 |title=Quasar quartet embedded in giant nebula reveals rare massive structure in distant universe |journal=Science |language=en |volume=348 |issue=6236 |pages=779–783 |arxiv=1505.03786 |bibcode=2015Sci...348..779H |doi=10.1126/science.aaa5397 |issn=0036-8075 |pmid=25977547 |s2cid=35281881}}</ref>

A multiple-image quasar is a quasar whose light undergoes [[gravitational lensing]], resulting in double, triple or quadruple images of the same quasar. The first such gravitational lens to be discovered was the double-imaged quasar [[Q0957+561]] (or Twin Quasar) in 1979.<ref>{{Cite journal |last1=Blandford |first1=R. D. |author-link=Roger Blandford |last2=Narayan |first2=R. |date=September 1992 |title=Cosmological Applications of Gravitational Lensing |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=30 |issue=1 |pages=311–358 |bibcode=1992ARA&A..30..311B |doi=10.1146/annurev.aa.30.090192.001523 |issn=0066-4146}}</ref>
An example of a triply lensed quasar is PG1115+08.<ref>{{Cite journal |last1=Henry |first1=J. Patrick |last2=Heasley |first2=J. N. |date=May 1986 |title=High-resolution imaging from Mauna Kea: the triple quasar in 0.3-arc s seeing |journal=Nature |language=en |volume=321 |issue=6066 |pages=139–142 |bibcode=1986Natur.321..139H |doi=10.1038/321139a0 |issn=0028-0836 |s2cid=4244246}}</ref>
Several quadruple-image quasars are known, including the [[Einstein Cross]] and the [[Cloverleaf Quasar]], with the first such discoveries happening in the mid-1980s.