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Superluminal motion
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==History== The apparent superluminal motion in the faint nebula surrounding Nova Persei was first observed in 1901 by [[Charles Dillon Perrine]].<ref>{{cite journal |last1=Perrine |first1=Charles |title=Motion in the faint nebula surrounding Nova Persei |journal=Astrophysical Journal |date=Dec 1901 |volume=14 |pages=359β362|doi=10.1086/140877 |bibcode=1901ApJ....14..359P |doi-access=free }}</ref> βMr. Perrineβs photograph of November 7th and 8th, 1901, secured with the Crossley Reflector, led to the remarkable discovery that the masses of nebulosity were apparently in motion, with a speed perhaps several hundred times as great as hitherto observed.β<ref>{{cite journal |last1=Campbell |first1=William|url=https://books.google.com/books?id=1gYNAQAAIAAJ&q=%22lick+observatory%22&pg=PA326 |title=The Lick Observatory and Its Problems |journal=Overland Monthly |date=1902 |volume=XL |issue=3 |pages=326β327}}</ref> βUsing the 36-in. telescope (Crossley), he discovered the apparent superluminal motion of the expanding light bubble around Nova Persei (1901). Thought to be a nebula, the visual appearance was actually caused by light from the nova event reflected from the surrounding interstellar medium as the light moved outward from the star. Perrine studied this phenomenon using photographic, spectroscopic, and polarization techniques.β<ref>{{cite journal |last1=Teare S.W. |title=Charles Dillon Perrine |journal=Biographical Encyclopedia of Astronomers |date=2014 |doi=10.1007/978-1-4419-9917-7_1074}}</ref> Superluminal motion was first observed in 1902 by [[Jacobus Kapteyn]] in the ejecta of the [[nova]] [[GK Persei]], which had exploded in 1901.<ref>{{cite journal |doi=10.1086/381529|title=Echoes of an Explosive Past: Solving the Mystery of the First Superluminal Source |year=2004 |last1=Bode |first1=M. F. |last2=O'Brien |first2=T. J. |last3=Simpson |first3=M. |journal=The Astrophysical Journal |volume=600 |issue=1 |pages=L63βL66 |bibcode=2004ApJ...600L..63B |s2cid=121645094 |doi-access=free }}</ref> His discovery was published in the [[German language|German]] journal ''[[Astronomische Nachrichten]]'', and received little attention from English-speaking astronomers until many decades later.<ref>[http://adsabs.harvard.edu/abs/1902AN....157..201K Kapteyn's paper]</ref><ref>[http://adsabs.harvard.edu/cgi-bin/nph-ref_query?bibcode=1901AN....157..201K&refs=CITATIONS&db_key=AST Index of citations to Kapteyn's paper]</ref> In 1966, [[Martin Rees]] pointed out that "an object moving relativistically in suitable directions may appear to a distant observer to have a transverse velocity much greater than the velocity of light".<ref name="Rees">{{Cite journal | last1 = Rees | first1 = M. J. | author-link = Martin Rees, Baron Rees of Ludlow| title = Appearance of Relativistically Expanding Radio Sources | doi = 10.1038/211468a0 | journal = Nature | volume = 211 | issue = 5048 | pages = 468β470 | year = 1966 | bibcode=1966Natur.211..468R| s2cid = 41065207 }}</ref> In 1969 and 1970 such sources were found as very distant astronomical radio sources, such as radio galaxies and quasars,<ref name="Gubbay">{{cite journal|last1=Gubbay|first1=J.S.|last2=Legg|first2=A.J.|last3=Robertson|first3=D.S.|last4=Moffet|first4=A.T.|last5=Ekers|first5=R.D.|last6=Seidel|first6=B.|title=Variations of Small Quasar Components at 2,300 MHz|journal=Nature|date=1969|volume=224|issue=5224|pages=1094β1095|bibcode=1969Natur.224.1094G|doi=10.1038/2241094b0|s2cid=4196846}}</ref><ref>{{cite journal|last1=Cohen|journal=The Astrophysical Journal|date=1971|bibcode=1971ApJ...170..207C|title=The Small-Scale Structure of Radio Galaxies and Quasi-Stellar Sources at 3.8 Centimeters|last2=Cannon|first2=W.|last3=Purcell|first3=G. H.|last4=Shaffer|first4=D. B.|last5=Broderick|first5=J. J.|last6=Kellermann|first6=K. I.|last7=Jauncey|first7=D. L.|volume=170|pages=207|doi=10.1086/151204|first1=M. H.}}</ref><ref>{{cite journal|last1=Whitney|journal=Science|date=1971|bibcode=1971Sci...173..225W|title=Quasars Revisited: Rapid Time Variations Observed Via Very-Long-Baseline Interferometry|last2=Shapiro|first2=Irwin I.|last3=Rogers|first3=Alan E. E.|last4=Robertson|first4=Douglas S.|last5=Knight|first5=Curtis A.|last6=Clark|first6=Thomas A.|last7=Goldstein|first7=Richard M.|last8=Marandino|first8=Gerard E.|last9=Vandenberg|first9=Nancy R.|volume=173|pages=225β30|doi=10.1126/science.173.3993.225|pmid=17741416|first1=AR|issue=3993|s2cid=20152786}}</ref> and were called superluminal sources. The discovery was the result of a new technique called [[Very Long Baseline Interferometry]], which allowed astronomers to set limits to the angular size of components and to determine positions to better than [[Minute of arc|milli-arcsecond]]s, and in particular to determine the change in positions on the sky, called [[proper motion]]s, in a timespan of typically years. The apparent velocity is obtained by multiplying the observed proper motion by the distance, which could be up to 6 times the speed of light. In the introduction to a workshop on superluminal radio sources, Pearson and Zensus reported <blockquote>The first indications of changes in the structure of some sources were obtained by an American-Australian team in a series of transpacific VLBI observations between 1968 and 1970 (Gubbay et al. 1969).<ref name="Gubbay"/> Following the early experiments, they had realised the potential of the NASA tracking antennas for VLBI measurements and set up an interferometer operating between California and Australia. The change in the source visibility that they measured for [[3C 279]], combined with changes in total flux density, indicated that a component first seen in 1969 had reached a diameter of about 1 milliarcsecond, implying expansion at an apparent velocity of at least twice the speed of light. Aware of Rees's model,<ref name="Rees" /> (Moffet et al. 1972)<ref>{{cite book |last1=Moffet |first1=A.T. |last2=Gubbay |first2=J. |last3=Robertson |first3=D.S. |last4=Legg |first4=A.J. |title=External Galaxies and Quasi-Stellar Objects : IAU Symposium 44, held in Uppsala, Sweden 10-14 August 1970 |date=1972 |publisher=Reidel |location=Dordrecht |isbn=9027701997 |page=228 |editor=Evans, D.S}}</ref> concluded that their measurement presented evidence for relativistic expansion of this component. This interpretation, although by no means unique, was later confirmed, and in hindsight it seems fair to say that their experiment was the first interferometric measurement of superluminal expansion.<ref>{{cite book |last1=Pearson |first1=Timothy J. |last2=Zensus |first2=J. Anton |editor=J. Anton Zensus |editor2=Timothy J Pearson |title=Superluminal Radio Sources: proceedings of a workshop in honor of Professor Marshall H. Cohen, held at Big Bear Solar Observatory, California, October 28-30, 1986 |journal=Superluminal Radio Sources |publisher=[[Cambridge University Press]] |location=Cambridge; New York |date=1987 |page=3 |bibcode=1987slrs.work....1P |isbn=9780521345606}}</ref></blockquote> In 1994, a galactic speed record was obtained with the discovery of a superluminal source in the [[Milky Way]], the [[cosmic x-ray source]] [[GRS 1915+105]]. The expansion occurred on a much shorter timescale. Several separate blobs were seen to expand in pairs within weeks by typically 0.5 [[Minute of arc#Symbols.2C abbreviations and subdivisions|arcsec]].<ref>{{cite journal |last1=Mirabel |first1=I.F. |last2=Rodriguez |first2=L.F. |title=A superluminal source in the Galaxy |journal=Nature |date=1994 |volume=371 |issue=6492 |pages=46β48 |bibcode=1994Natur.371...46M |doi=10.1038/371046a0 |s2cid=4347263}}</ref> Because of the analogy with quasars, this source was called a [[microquasar]].
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