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{{Utente:HeNRyKus/SBintestazione}}

</noinclude>

; [[Cygnus X-1]]

<nowiki>{{tradotto da|en|Cygnus X-1|564388137|13 luglio 2013}}</nowiki>

{{Nota disambigua|il brano musicale del gruppo [[rock]] [[canada|canadese]] [[Rush]]|[[Cygnus X-1 (brano musicale)]]}}

{{corpo celeste

|tipo = Stella

|nome = HDE 226868

|distanza_anniluce =

|sigla_costellazione = Cyg

|costellazione = [[Cigno (costellazione)|Cigno]]

|immagine = HDE 226868.jpg

|didascalia = HDE 226868, la controparte ottica di Cygnus X-1, è la stella più brillante della coppia al centro del campo stellare.

|dimensione_immagine = 280px

|categoria = [[Supergigante blu]]

|epoca = [[J2000.0]]

|ar = {{RA|19|58|21,67595}}<ref name=aaa474_2_653>{{cita pubblicazione | autore=F. van Leeuwen | titolo=Validation of the new Hipparcos reduction | rivista=Astronomy and Astrophysics | volume=474 | numero=2 | pagine=653–664 | data=novembre 2007 | doi=10.1051/0004-6361:20078357 | url=http://adsabs.harvard.edu/abs/2007A&A...474..653V |arxiv = 0708.1752 }}</ref>

|declinaz = {{DEC|+35|12|05,7783}}<ref name=aaa474_2_653/>

|lat_galattica = +03,0668°<ref name=SIMBAD>{{cita web | data=3 marzo 2003 | url=http://simbad.u-strasbg.fr/simbad/sim-id?protocol=html&Ident=HDE+226868 | titolo=V* V1357 Cyg -- High Mass X-ray Binary | editore=SIMBAD, Centre de Données astronomiques de Strasbourg | accesso=3 marzo 2008}}</ref>

|long_galattica = +71,3350°<ref name=SIMBAD/>

|classe_spettrale = O9,7Iab<ref name=SIMBAD/>

|redshift=

|raggio_sole = 20–22<ref name=MNRAS358_3_851>{{Cita pubblicazione | autore=J. Ziółkowski | titolo=Evolutionary constraints on the masses of the components of HDE 226868/Cyg X-1 binary system | rivista=Monthly Notices of the Royal Astronomical Society | anno=2005 | volume=358 | numero=3 | pagine=851–859 | arxiv=astro-ph/0501102 | doi=10.1111/j.1365-2966.2005.08796.x | accesso=4 marzo 2008 | url=http://adsabs.harvard.edu/abs/2005MNRAS.358..851Z}} Nota: per il raggio e la luminosità, si veda la Tabella 1 con ''d''=2&nbsp;kpc.</ref>

|indice_di_colore = +0,81<ref name=lob647>{{cita pubblicazione | autore=J. Bregman ''et al.'' | titolo=Colors, magnitudes, spectral types and distances for stars in the field of the X-ray source Cyg X-1 | rivista=Lick Observatory Bulletin | anno=1973 | pagine=1 | volume=647 | url=http://adsabs.harvard.edu/abs/1973LicOB..24....1B}}</ref>

|schiacciamento =

|massa_sole = 14-16<ref name=orosz2011>{{cita web | autore=J. Orosz | data=1º dicembre 2011 | titolo=The Mass of the Black Hole In Cygnux X-1 | url=http://iopscience.iop.org/0004-637X/742/2/84 | accesso=24-03-2012}}</ref>

|densità =

|velocità_rotaz_note =

|accel_gravità = log''g''=3,31 ± 0,07 <ref name=hadrava>{{cita conferenza | autore=P. Hadrava | titolo=Optical spectroscopy of Cyg X-1 | conferenza=Proceedings of RAGtime 9: Workshops on black holes and neutron stars | data=15-21 settembre 2007 | città=Opava, Repubblica Ceca | arxiv=0710.0758}}</ref>

|temp_med = 31&nbsp;000 [[kelvin|K]]<ref name=eas030610>{{Cita web | data=10 giugno 2003 | url=http://hubble.esa.int/science-e/www/object/index.cfm?fobjectid=32700 | titolo=Integral's view of Cygnus X-1 | editore=ESA | accesso=20 marzo 2008 }}</ref>

|luminosità_sole = 3–4 × 10⁵<ref name=MNRAS358_3_851/>

|radianza =

|età = 5 × 10⁶ anni<ref name=science300_5622_1119>{{Cita pubblicazione | autore=I. F. Mirabel, I. Rodrigues | titolo=Formation of a Black Hole in the Dark | rivista=Science | anno=2003 | volume=300 | numero=5622 | pagine=1119–1120 | url=http://www.sciencemag.org/cgi/content/full/300/5622/1119 | accesso=15 marzo 2008 | doi=10.1126/science.1083451 | id=PMID 12714674 |arxiv = astro-ph/0305205 |bibcode = 2003Sci...300.1119M }}</ref>

|magn_app = 8,95<ref name=SIMBAD/>

|magn_ass = −6,5&nbsp;±&nbsp;0,2<ref name=apj321>{{Cita pubblicazione | autore=Z. Ninkov, G. A. H. Walker, S. Yang | titolo=The primary orbit and the absorption lines of HDE 226868 (Cygnus X-1) | rivista=[[Astrophysical Journal]] | anno=1987 | volume=321 | pagine=425–437 | url=http://adsabs.harvard.edu/abs/1987ApJ...321..425N | doi=10.1086/165641}}</ref>

|metallicità =

|parallasse = 0,539 ± 0,0330.58<ref name=apj742_2>{{cita pubblicazione| autore=M. J. Reid ''et al'' | titolo=The Trigonometric Parallax of Cygnus&nbsp;X-1 | rivista=The Astrophysical Journal | volume=742 | numero=2 | data=dicembre 2011 | doi=10.1088/0004-637X/742/2/83 | url=http://adsabs.harvard.edu/abs/2011ApJ...742...83R |arxiv = 1106.3688 }}</ref> [[milliarcosecondo|mas]]

|moto_proprio = {{moto proprio|ar= −3,82<ref name=SIMBAD/>|dec= −7,62<ref name=SIMBAD/>}}

|velocità_radiale = −13<ref name=SIMBAD/> km/s

|designazioni_alternative_stellari= [[Astronomische Gesellschaft Katalog|AG (o AGK2)]]+35&nbsp;1910, [[Bonner Durchmusterung|BD]]+34 3815, [[Henry Draper catalogue|HD (or HDE)]]&nbsp;226868, [[Hipparcos catalogue|HIP]]&nbsp;98298, [[Smithsonian Astrophysical Observatory|SAO]]&nbsp;69181, V1357&nbsp;Cyg.<ref name=SIMBAD/>

}}

'''Cygnus&nbsp;X-1''' (abbreviato in '''Cyg&nbsp;X-1''')<ref name=science3656>{{cita pubblicazione| autore= S. Bowyer ''et al.'' | anno=1965 | titolo=Cosmic X-ray Sources | rivista=Science | volume=147 | numero=3656 | pagine=394–398 | doi=10.1126/science.147.3656.394 | pmid=17832788 | url=http://adsabs.harvard.edu/abs/1965Sci...147..394B }}</ref> è una nota sorgente di [[raggi X]]<ref>{{cita web | data=5 novembre 2004 | editore=ESA | titolo=Observations: Seeing in X-ray wavelengths | url=http://www.esa.int/esaSC/SEMTA2T1VED_index_0.html | accesso=12 agosto 2008}}</ref> localizzata nella [[costellazione]] del [[Cigno (costellazione)|Cigno]]. Scoperta in seguito ad una campagna osservativa nel [[1964]], è una delle sorgenti di raggi X più forti visibili dalla [[Terra]], con un picco di [[flusso|densità di flusso]] pari a {{TA|2,3×10⁻²³ [[Watt|W]][[Metro|m]]⁻²[[Hertz|Hz]]⁻¹}} ({{TA|2,3×10³ [[Jansky|Jy]]}}).<ref>{{cita pubblicazione| autore=W. Lewin, M. Van Der Klis | anno=2006 | titolo=Compact Stellar X-ray Sources | rivista=Cambridge University Press | pagine=159 | id=ISBN 0-521-82659-4 }}</ref><ref>{{cita pubblicazione| titolo=2010 X-Ray Sources | rivista=The Astronomical Almanac, U.S. Naval Observatory | url=http://asa.usno.navy.mil/SecH/Xray.html | accesso=4 agosto 2009}} dà un range di 235–1320 μJy alle energie di 2–10 [[elettronvolt|kEv]]; 1Jy = 10⁻²⁶Wm⁻² Hz⁻¹.</ref> Distante circa 6070 [[anno luce|anni luce]] dal [[Sole]], la sorgente è un [[stella degenere|oggetto compatto]] identificato con molta probabilità con un [[buco nero stellare|buco nero]], la cui [[massa (fisica)|massa]], secondo le stime più recenti, ammonterebbe a 14,8 volte [[massa solare|quella del Sole]] (M_⊙),<ref name=orosz2011/> mentre il [[raggio (astronomia)|raggio]] dell'[[orizzonte degli eventi]] si aggirerebbe sui 44 [[chilometro|km]].<ref>{{cita pubblicazione| autore=T. Harko | data=28 giugno 2006 | url=http://www.physics.hku.hk/~astro/harko_science.html | titolo=Black Holes | editore=University of Hong Kong | accesso=28 marzo 2008}}</ref>

Cygnus X-1 fa parte di una [[Stella binaria a raggi X#Binaria a raggi X di grande_massa|binaria a raggi X di grande massa]] formata, oltre che dall'oggetto compatto, da una [[supergigante blu]] [[stella variabile|variabile]] (designata come '''HDE&nbsp;226868'''); la stella e l'oggetto compatto orbitano attorno al [[centro di massa|baricentro]] del sistema ad una distanza di 0,2 [[unità astronomica|UA]]. Il [[vento stellare|vento]] emesso dalla stella spiraleggia attorno al buco nero alimentando un [[disco di accrescimento]]<ref name=apj304_371>{{cita pubblicazione| autore=D. R. Gies, C. T. Bolton | titolo=The optical spectrum of HDE&nbsp;226868 = Cygnus&nbsp;X-1. II&nbsp;— Spectrophotometry and mass estimates | rivista=The Astrophysical Journal, Part 1 | anno=1986 | volume=304 | pagine=371–393 | url=http://adsabs.harvard.edu/abs/1986ApJ...304..371G | doi=10.1086/164171 }}</ref> le cui regioni più interne sono scaldate a milioni di [[kelvin]] dando luogo all'emissione di [[raggi X]] osservata.<ref>{{cita web| autore=S. Nayakshin, J. B. Dove | data=3 novembre 1998 | titolo=X-rays From Magnetic Flares In Cygnus&nbsp;X-1: The Role Of A Transition Layer | rivista=eprint [[arXiv]]:astro-ph/9811059 | url=http://adsabs.harvard.edu/abs/1998astro.ph.11059N }}</ref><ref name=mnras325_3_1045>{{cita pubblicazione | autore=A. J. Young ''et al.'' | titolo=A Complete Relativistic Ionized Accretion Disc in Cygnus&nbsp;X-1 | rivista=Monthly Notices of the Royal Astronomical Society | anno=2001 | volume=325 | numero=3 | pagine=1045–1052 | arxiv=astro-ph/0103214 | doi=10.1046/j.1365-8711.2001.04498.x |url = http://adsabs.harvard.edu/abs/2001MNRAS.325.1045Y }}</ref> Perpendicolarmente al disco si dipartono due [[getto relativistico|getti relativistici]], che espellono nello spazio interstellare una parte della materia che va ad affluire verso il buco nero.<ref>{{cita pubblicazione | autore=E. Gallo, R. Fender | anno=2005 | titolo=Accretion modes and jet production in black hole X-ray binaries | rivista=Memorie della Società Astronomica Italiana | volume=76 | pagine=600–607 | arxiv=astro-ph/0509172 | url=http://adsabs.harvard.edu/abs/2005MmSAI..76..600G }}</ref>

Il sistema appartiene con probabilità all'[[associazione OB]] Cygnus&nbsp;OB3, il che dà informazioni sull'età del sistema e sul progenitore della sorgente X: infatti, Cygnus&nbsp;X-1 avrebbe un'età di circa 5 milioni di anni e si sarebbe formata a partire da una stella di 40 M_⊙. Secondo i [[modello fisico|modelli]], la gran parte della massa della stella sarebbe stata persa in seguito al forte vento stellare, quindi il [[struttura stellare|nucleo]] sarebbe [[collasso gravitazionale|collassato]] direttamente in un buco nero, senza la preliminare esplosione in [[supernova]]: infatti, qualora la stella fosse esplosa, la forza rilasciata avrebbe determinato l'espulsione del residuo compatto e la disgregazione del sistema.<ref name=science300_5622_1119/>

Cygnus&nbsp;X-1 è stata l'oggetto nel [[1974]] di una scommessa amichevole tra i fisici [[Stephen Hawking]] e [[Kip Thorne]]: Hawking scommise che la sorgente non fosse originata da un buco nero, al contrario di Thorne. Hawking decise però di abbandonare la scommessa quando, a partire dal 1990, i dati osservativi rinforzarono l'ipotesi dell'esistenza del buco nero, ipotesi oggi in larga parte confermata.<ref>{{cita web| data 27 febbraio 2004 | titolo=Galaxy Entree or Main Course? | editore=Swinburne University | url=http://astronomy.swin.edu.au/sao/astronomynews/astronews2004s1.xml | accesso=31 marzo 2008}}</ref>

== Scoperta e osservazione ==

[[astronomia a raggi X|Osservare l'emissione di raggi X]] da parte dei corpi celesti permette agli astronomi di studiare i fenomeni che coinvolgono i gas riscaldati a temperature dell'ordine dei milioni di [[kelvin]]; tuttavia, dal momento che la radiazione X è bloccata dall'[[atmosfera terrestre]], l'osservazione a queste lunghezze d'onda è possibile soltanto tramite strumenti lanciati a quote alle quali la radiazione X non è ancora assorbita.<ref>{{cita libro| autore=H. Friedman | anno=2002 | capitolo=From the ionosphere to high energy astronomy&nbsp;– a personal experience | titolo=The Century of Space Science | editore=Springer | id=ISBN 0-7923-7196-8 }}</ref><ref name=apj611_2_1084>{{cita pubblicazione | autore=C. Z. Liu, T. P. Li | titolo=X-Ray Spectral Variability in Cygnus&nbsp;X-1 | rivista=The Astrophysical Journal | anno=1999 | volume=611 | numero=2 | pagine=1084–1090 | doi=10.1086/422209 | url=http://adsabs.harvard.edu/abs/2004ApJ...611.1084L|arxiv = astro-ph/0405246 }}</ref> Cygnus&nbsp;X-1 fu infatti scoperta nel [[1964]] nell'ambito di una campagna osservativa (''survey'') mirata ad analizzare le sorgenti X e condotta tramite dei [[contatore Geiger|contatori Geiger]] collocati su una serie di [[razzo|razzi]] lanciati in [[volo suborbitale]]. I rilevatori erano impostati per misurare l'emissione X alle lunghezze d'onda di 0,1-1,5 [[nanometro|nm]] attraverso una porzione di cielo estesa per 8,4°; ad ogni modo gli strumenti analizzarono tutto il cielo man mano che i razzi ruotavano, producendo una mappa di scansioni ravvicinate.<ref name=science3656/>

[[File:Cygnus constellation map with approximate postion of cygnus-x1.png|thumb|left|270px|Mappa della costellazione del Cigno che mostra la posizione di Cyg X-1, nei pressi della stella {{ST|Eta|Cyg}}.]]

Grazie a questa survey furono scoperte otto sorgenti, tra cui Cygnus X-1, inizialmente denominata Cygnus&nbsp;XR-1; quest'ultima è stata individuata alle [[sistema di coordinate equatoriali|coordinate]] 19^h53^m di [[ascensione retta]] e 34,6° di [[declinazione (astronomia)|declinazione]]. La sorgente X non era associata ad alcuna controparte [[radioastronomia|radio]] o [[stella|ottica]] particolarmente brillante localizzata a tali coordinate.<ref name=science3656/>

Vista la necessità di studi sul lungo periodo, [[Riccardo Giacconi]] ed [[Herb Gursky]] proposero nel [[1963]] di lanciare in orbita un [[satellite artificiale|satellite]] in grado di studiare le sorgenti X. La [[NASA]] soddisfece la proposta lanciando nel [[1970]] il satellite [[Uhuru]],<ref>{{cita web | data=26 giugno 2003 | url=http://heasarc.gsfc.nasa.gov/docs/uhuru/uhuru.html | titolo=The Uhuru Satellite | editore=NASA | accesso=9 maggio 2008 }}</ref> attraverso il quale furono scoperte altre 300 sorgenti X.<ref>{{cita web | autore=[[Riccardo Giacconi|R. Giacconi]] | data=8 dicembre 2002 | url=http://nobelprize.org/nobel_prizes/physics/laureates/2002/giacconi-lecture.html | titolo=The Dawn of X-Ray Astronomy | editore=The Nobel Foundation | accesso=24 marzo 2008}}</ref> Le osservazioni condotte dal satellite permisero di scoprire che Cygnus&nbsp;X-1 possedeva delle fluttuazioni nell'intensità dei raggi X che si verificano diverse volte in un secondo;<ref>{{cita pubblicazione | autore=M. Oda ''et al.'' | titolo=X-Ray Pulsations from Cygnus&nbsp;X-1 Observed from UHURU | rivista=The Astrophysical Journal | anno=1999 | volume=166 | pagine=L1–L7 | url=http://adsabs.harvard.edu/abs/1971ApJ...166L...1O | doi=10.1086/180726 }}</ref> questa peculiarità fu spiegata assumendo che l'energia venisse generata in una regione di spazio relativamente piccola, stimata in 10⁵ km (equivalente alla distanza percorsa dalla luce in 0,3 secondi) dal momento che la [[velocità della luce]] limita la comunicazione tra le regioni più lontane.

Nell'aprile-maggio del 1971, Luc Braes e George K. Miley dell'[[Osservatorio di Leida]] e, indipendentemente, Robert M. Hjellming e Campbell Wade del [[National Radio Astronomy Observatory]],<ref>{{cita pubblicazione | autore=J. Kristian ''et al.'' | anno=1971 | titolo=On the Optical Identification of Cygnus&nbsp;X-1 | url=http://adsabs.harvard.edu/abs/1971ApJ...168L..91K | rivista=The Astrophysical Journal | volume=168 | pagine=L91–L93 | doi=10.1086/180790 }}</ref> riuscirono ad individuare l'emissione nelle [[onde radio]] di Cygnus&nbsp;X-1, notando che coincideva con una stella denominata AGK2&nbsp;+35 1910 (conosciuta anche come HDE&nbsp;226868),<ref>{{cita pubblicazione | autore=L. L. E. Braes, G. K. Miley | data=23 luglio 1971 | titolo=Physical Sciences: Detection of Radio Emission from Cygnus&nbsp;X-1 | rivista=Nature | volume=232 | numero=5308 | pagine=246 | url=http://adsabs.harvard.edu/abs/1971Natur.232Q.246B | doi=10.1038/232246a0 | pmid=16062947 }}</ref><ref>{{cita pubblicazione | autore=L. L. E. Braes, G. K. Miley| anno=1971 | titolo=Variable Radio Emission from X-Ray Sources | rivista=Veröffentlichungen Remeis-Sternwarte Bamberg | volume=9 | numero=100 | pagine=173 | url=http://adsabs.harvard.edu/abs/1972VeBam.100...... }}</ref> visibile a circa mezzo grado di distanza da {{STL|Eta|Cyg}}, una stella di quarta [[magnitudine apparente|magnitudine]].<ref>{{cita libro | autore=B. Abrams, M. Stecker | anno=1999 | titolo=Structures in Space: Hidden Secrets of the Deep Sky | pagine=91 | editore=Springer | id=ISBN 1-85233-165-8 | quote=Eta Cygni è posta a 25' ad ESE da questa stella. }}</ref> Si tratta di una [[stella supergigante]] la cui temperatura superficiale non giustifica l'importante emissione di raggi X osservata; pertanto è stata ipotizzata la presenza di un compagno in grado di riscaldare i gas fino ai milioni di kelvin necessari per produrre l'intensa radiazione X osservata per Cygnus&nbsp;X-1.

[[File:Cygnus x1 xray.jpg|right|thumb|Un'immagine nei raggi X di Cygnus&nbsp;X-1 ripresa dall<nowiki>'</nowiki>''High Energy Replicated Optics'' (HERO) della NASA, un telescopio montato su un [[pallone sonda]].]]

Louise Webster e Paul Murdin del [[Royal Greenwich Observatory]],<ref>{{cita pubblicazione | autore=B. L. Webster, P. Murdin | anno=1972 | titolo=Cygnus&nbsp;X-1—a Spectroscopic Binary with a Heavy Companion? | rivista=Nature | volume=235 | numero=2 | pagine=37–38 | doi=10.1038/235037a0 | url = http://adsabs.harvard.edu/abs/1972Natur.235...37W }}</ref> e, indipendentemente, [[Charles Thomas Bolton]] del [[David Dunlap Observatory]] dell'[[Università di Toronto]]<ref>{{cita pubblicazione | autore=C. T. Bolton | anno=1972 | titolo=Identification of Cygnus&nbsp;X-1 with HDE&nbsp;226868 | rivista=Nature | volume=235 | numero=2 | pagine=271–273 | doi=10.1038/235271b0 | url = http://adsabs.harvard.edu/abs/1972Natur.235..271B }}</ref> annunciarono nel 1971 la scoperta di un invisibile compagno massiccio attorno ad HDE&nbsp;226868 misurando l'[[effetto Doppler]] dello [[spettro elettromagnetico|spettro]] stellare.<ref>{{cita libro | autore= J.-P. Luminet | anno=1992 | titolo=Black Holes | editore=Cambridge University Press | id=ISBN 0-521-40906-3 }}</ref> Sulla base dell'elevato valore della massa dedotto per l'oggetto, gli astronomi hanno ipotizzato che potesse trattarsi di un [[buco nero stellare|buco nero]], dal momento che la [[stella di neutroni]] più massiccia teoricamente possibile non può superare una massa pari a tre volte [[massa solare|quella del Sole]].<ref>{{cita pubblicazione | autore=I. Bombaci | titolo=The maximum mass of a neutron star | rivista=Astronomy and Astrophysics | anno=1996 | volume=305 | pagine=871–877 | url=http://adsabs.harvard.edu/abs/1996A&A...305..871B | arxiv=astro-ph/9608059 }}</ref>

Le osservazioni successive hanno rinforzato quest'ipotesi, tant'è che dalla fine del 1973 la comunità astronomica ritiene elevata la probabilità che in Cygnus X-1 sia presente un buco nero.<ref>{{cita web | autore=B. Rolston | data=10 novembre 1997 | url=http://news.utoronto.ca/bin/bulletin/nov10_97/art4.htm | titolo= The First Black Hole | editore=University of Toronto | accesso=11 marzo 2008 | urlarchivio = http://web.archive.org/web/20080307181205/http://www.news.utoronto.ca/bin/bulletin/nov10_97/art4.htm | dataarchivio = 7 marzo 2008 }}</ref><ref>{{cita pubblicazione | autore=H. L. Shipman | anno=1975 | titolo=The implausible history of triple star models for Cygnus&nbsp;X-1 Evidence for a black hole | rivista=Astrophysical Letters | volume=16 | numero=1 | pagine=9–12 | url=http://adsabs.harvard.edu/abs/1975ApL....16....9S | doi=10.1016/S0304-8853(99)00384-4 }}</ref> Misure più precise hanno inoltre mostrato una variabilità che si estrinseca nell'arco di un [[millisecondo]], un intervallo coerente con le [[turbolenze]] che caratterizzano i [[disco di accrescimento|dischi di accrescimento]] che circondano i buchi neri. I lampi di raggi X della durata di circa 0,3 secondi sono invece dovuti alla materia in caduta verso il buco nero.<ref>{{cita pubblicazione | autore=R. E. Rothschild ''et al.'' | titolo=Millisecond Temporal Structure in Cygnus&nbsp;X-1 | rivista=The Astrophysical Journal | anno=1974 | volume=189 | pagine=77–115 | url=http://adsabs.harvard.edu/abs/1974ApJ...189L..13R | doi=10.1086/181452 }}</ref>

Cygnus&nbsp;X-1 è stata poi ampiamente studiata tramite strumenti a terra e in orbita,<ref name=SIMBAD/> che hanno mostrato delle somiglianze tra l'emissione delle binarie a raggi X e quella delle [[galassia attiva|galassie attive]], suggerendo un comune meccanismo di liberazione dell'energia che coinvolge un buco nero, un disco di accrescimento e i [[getto relativistico|getti relativistici]] associati.<ref>{{cita pubblicazione | autore=E. Koerding, S. Jester, R. Fender | titolo=Accretion states and radio loudness in Active Galactic Nuclei: analogies with X-ray binaries | rivista=Monthly Notices of the Royal Astronomical Society | anno=2006 | volume=372 | numero=3 | pagine=1366–1378 | arxiv=astro-ph/0608628 | doi=10.1111/j.1365-2966.2006.10954.x | url=http://adsabs.harvard.edu/abs/2006MNRAS.372.1366K }}</ref> Perciò, lo studio delle binarie a raggi X può permettere una maggiore comprensione dei fenomeni che caratterizzano le galassie attive.<ref>{{cita web | autore=J. Brainerd | data=10 luglio 2005 | url=http://www.astrophysicsspectator.com/topics/observation/XRayAGN.html | titolo=X-rays from AGNs | editore=The Astrophysics Spectator | accesso=24 marzo 2008}}</ref> Inoltre, per via dell'emissione radio, Cygnus&nbsp;X-1 è annoverata tra i [[microquasar]], oggetti simili ai [[quasar]] ma di dimensioni notevolmente inferiori.

== Ambiente galattico ==

La regione galattica attorno a Cygnus X-1 e all'associazione Cygnus&nbsp;OB3, di cui probabilmente fa parte, è una delle più studiate del cielo, a causa della presenza lungo la sua linea di vista di un esteso sistema di [[nube molecolare gigante|nubi molecolari giganti]] in cui hanno luogo estesi processi di [[formazione stellare]], probabilmente fra i più attivi di tutta la galassia.<ref name="Odenwald1993">{{cita pubblicazione|autore=Odenwald, Sten F.; Schwartz, Phil R.|titolo=An IRAS survey of star-forming regions toward Cygnus|url=http://adsabs.harvard.edu/abs/1993ApJ...405..706O|rivista=Astrophysical Journal|volume=405|numero=2|pagine=706-719|doi=10.1086/172398|anno=1993|mese=marzo|accesso=28 luglio 2013}}</ref> Alla regione di Cygnus&nbsp;X appartengono diversi oggetti notevoli, come l'estesa nebulosa [[IC 1318]] e un gran numero di associazioni&nbsp;OB, fra le quali spicca [[Cygnus OB2]], una delle più massicce conosciute e famosa per ospitare la stella supermassiccia [[Cygnus&nbsp;OB2-12]].<ref name="Kosma">{{cita pubblicazione|autore=Schneider, N.; Bontemps, S.; Simon, R.; Jakob, H.; Motte, F.; Miller, M.; Kramer, C.; Stutzki, J.|titolo=A new view of the Cygnus X region. KOSMA ¹³CO 2 to 1, 3 to 2, and ¹²CO 3 to 2 imaging|url=http://adsabs.harvard.edu/abs/2006A&A...458..855S|rivista=Astronomy and Astrophysics|volume=458|pagine=855-871|numero=3|anno=2006|mese=novembre|doi=10.1051/0004-6361:20065088|accesso=28 luglio 2013}}</ref> Cygnus&nbsp;OB3 si trova tuttavia a una distanza leggermente maggiore della maggior parte dei [[complesso nebuloso molecolare del Cigno|complessi del Cigno]], così da trovarsi sul tratto iniziale del [[Braccio di Orione]] dopo la sua diramazione dal [[Braccio del Sagittario]].

Con una distanza stimata mediamente attorno ai 2000&nbsp;parsec, Cygnus OB3 si accompagna ad altre due associazioni OB: una, situata a una distanza angolare minore e probabilmente fisicamente più vicina ad essa, è Cygnus&nbsp;OB8, mentre nella direzione opposta si trova il [[regione di Vulpecula OB1|complesso di Vulpecula&nbsp;OB1]], circondato dalla [[regione H II]] [[NGC 6820|Sh2-86]] e da una [[superbolla]] in espansione.<ref name="Vulpecula">{{cita pubblicazione|autore=Billot, N.; Noriega-Crespo, A.; Carey, S.; Guieu, S.; Shenoy, S.; Paladini, R.; Latter, W.|titolo=Young Stellar Objects and Triggered Star Formation in the Vulpecula OB Association|url=http://adsabs.harvard.edu/abs/2010ApJ...712..797B|rivista=The Astrophysical Journal|volume=712|numero=2|pagine=797-812|anno=2010|mese=aprile|doi=10.1088/0004-637X/712/2/797|accesso=28 luglio 2013}}</ref> Secondo alcuni studi, l'origine del Braccio di Orione andrebbe ricercata proprio nei pressi di Vulpecula&nbsp;OB1, in particolare in corrispondenza della sorgente di [[radiazione infrarossa]] [[IRAS]]&nbsp;19410+2336.<ref name="Xu2009">{{cita pubblicazione|autore=Xu, Y.; Reid, M. J.; Menten, K. M.; Brunthaler, A.; Zheng, X. W.; Moscadelli, L.|titolo=Trigonometric Parallaxes of Massive Star-Forming Regions: III. G59.7+0.1 and W 51 IRS2|url=http://adsabs.harvard.edu/abs/2009ApJ...693..413X|rivista=The Astrophysical Journal|volume=693|numero=1|pagine=413-418|anno=2009|mese=marzo|doi=10.1088/0004-637X/693/1/413|accesso=28 luglio 2013}}</ref> Contrapposta a questa teoria vi è quella secondo cui il punto di origine del Braccio di Orione sia da ricercarsi a una distanza maggiore, fino a 5500&nbsp;parsec, dove si trova il [[complesso nebuloso molecolare W51|sistema nebuloso di W51]], indicato come possibile punto di biforcazione in base a studi sulla parallasse.<ref name="Sato2010">{{cita pubblicazione|autore=Sato, Mayumi; Hirota, Tomoya; Reid, Mark J.; Honma, Mareki; Kobayashi, Hideyuki; Iwadate, Kenzaburo; Miyaji, Takeshi; Shibata, Katsunori M.|titolo=Distance to G14.33-0.64 in the Sagittarius Spiral Arm: H2O Maser Trigonometric Parallax with VERA|url=http://adsabs.harvard.edu/abs/2010PASJ...62..287S|rivista=Publications of the Astronomical Society of Japan|volume=62|numero=2|pagine=287-299|anno=2010|mese=aprile|accesso=28 giugno 2013}}</ref>

A circa 2700&nbsp;parsec, dunque a una distanza forse un po' superiore rispetto a Cygnus OB3, si troverebbe la [[Nebulosa Tulipano]] (Sh2-101), che dalla prospettiva terrestre è visibile proprio in prossimità di Cygnus&nbsp;X-1; si tratta di una grande nebulosa ionizzata probabilmente dalla radiazione della [[gigante blu]] HD&nbsp;227018 e ospitante alcuni fenomeni di formazione stellare, data la presenza al suo interno di quattro sorgenti infrarosse identificate dall'IRAS.<ref name="Johnson1982">{{cita pubblicazione|autore=Johnson, P. G.; Mack, P.; Songsathaporn, R.|titolo=A radial velocity survey of the H II region S101|url=http://adsabs.harvard.edu/abs/1982Ap%26SS..87...13J|rivista=Astrophysics and Space Science|volume=87|numero=1-2|pagine=13-20|anno=1982|mese=ottobre|doi=10.1007/BF00648903|accesso=28 luglio 2013}}</ref>

In corrispondenza di Cygnus&nbsp;OB3, ma a una [[sistema di coordinate galattiche|latitudine galattica]] più elevata, si trova infine l'[[ammasso aperto]] [[NGC 6819]]; è uno dei più antichi conosciuti, con un'età stimata attorno ai 2,5 miliardi di anni, pertanto non condivide le origini con Cygnus&nbsp;OB3, che ha invece un'età di pochi milioni di anni al massimo. Si tratta di un ammasso molto concentrato, sopravvissuto alla progressiva disgregazione cui va incontro la quasi totalità degli ammassi aperti nel giro di qualche centinaio di milioni di anni al massimo; al suo interno sono state osservate delle [[stella vagabonda blu|''blue stragglers'']], oggetti tipici di ambienti ad elevata densità stellare.<ref name="Talamantes2010">{{cita pubblicazione|autore=Talamantes, Antonio; Sandquist, Eric L.; Clem, James L.; Robb, Russell M.; Balam, David D.; Shetrone, Matthew|titolo=Bright Variable Stars in NGC 6819: An Open Cluster in The Kepler Field|url=http://adsabs.harvard.edu/abs/2010AJ....140.1268T|rivista=The Astronomical Journal|volume=140|numero=5|pagine=1268-1281|anno=2010|mese=novembre|doi=10.1088/0004-6256/140/5/1268|accesso=28 giugno 2013}}</ref>

== Il sistema ==

The [[compact object]] and [[blue supergiant]] star form a [[binary star|binary system]] in which they orbit around their [[center of mass]] every {{val|5.599829|0.000016|u=days}}.<ref name=aaa343_861/> From the perspective of the Earth, the compact object never goes behind the other star; in other words, the system does not [[eclipse]]. However, the [[orbital inclination|inclination of the orbital plane]] to the [[Line-of-sight propagation|line of sight]] from the Earth remains uncertain, with predictions ranging from 27–65°. A 2007 study estimated the inclination is {{val|48.0|6.8|s=°}}, which would mean that the [[semi-major axis]] is about {{val|0.2|u=[[Astronomical Unit|AU]]}}, or 20% of the distance from the Earth to the Sun. The [[orbital eccentricity]] is thought to be only {{val|0.0018|0.002}}; a nearly circular orbit.<ref name=orosz2011/><ref name=apj200/> Earth's distance to this system is about {{Convert|1860|+/-|120|pc|ly|abbr=off|lk=on}}.<ref name=apj742_2/>

The HDE&nbsp;226868/Cygnus&nbsp;X-1 system shares a common motion through space with an association of massive stars named Cygnus&nbsp;OB3, which is located at roughly {{val|2000|ul=parsecs}} from the Sun. This implies that HDE&nbsp;226868, Cygnus&nbsp;X-1 and this [[stellar association|OB association]] may have formed at the same time and ___location. If so, then the age of the system is about {{val|5|1.5|u=million year}}. The motion of HDE&nbsp;226868 with respect to Cygnus&nbsp;OB3 is {{val|9|3|ul=km/s}}; a typical value for random motion within a stellar association. HDE&nbsp;226868 is about {{val|60|u=parsecs}} from the center of the association, and could have reached that separation in about {{val|7|2|u=million year}}—which roughly agrees with estimated age of the association.<ref name=science300_5622_1119/>

With a [[galactic latitude]] of 4 degrees and [[galactic longitude]] 71 degrees,<ref name=SIMBAD/> this system lies inward along the same [[Orion Spur]] in which the Sun is located within the [[Milky Way]],<ref name=apj167_L15/> near where the spur approaches the [[Sagittarius Arm]]. Cygnus&nbsp;X-1 has been described as belonging to the Sagittarius Arm,<ref name=goebel/> though the structure of the Milky Way is not well established.

=== L'oggetto compatto ===

There is some uncertainty about the mass of the compact object. Stellar evolutionary models suggest a mass of {{val|20|5|u=solar masses}},<ref name=MNRAS358_3_851/> while other techniques resulted in 10 solar masses. Measuring periodicities in the X-ray emission near the object has yielded a more precise value of {{val|14.8|1|u=solar masses}}. In all cases, the object is most likely a black hole<ref name=orosz2011/><ref name=esa070516/>—a region of space with a [[gravity|gravitational field]] that is strong enough to prevent the escape of [[electromagnetic radiation]] from the interior. The boundary of this region is called the [[event horizon]] and has an effective radius called the [[Schwarzschild radius]], which is about {{val|26|u=km}} for Cygnus&nbsp;X-1.<ref name=csf9_7_1025/> Anything (including [[matter]] and [[photon]]s) that passes through this boundary is unable to escape.<ref name=mit20060109/>

Evidence of just such an event horizon may have been detected in 1992 using [[ultraviolet]] (UV) observations with the [[High Speed Photometer]] on the [[Hubble Space Telescope]]. As self-luminous clumps of matter spiral into a black hole, their radiation will be emitted in a series of pulses that are subject to [[gravitational redshift]] as the material approaches the horizon. That is, the [[wavelength]]s of the radiation will steadily increase, as predicted by [[General Relativity]]. Matter hitting a solid, compact object would emit a final burst of energy, whereas material passing through an event horizon would not. Two such "dying pulse trains" were observed, which is consistent with the existence of a black hole.<ref name=pasp113/>

[[File:Chandra_image_of_Cygnus_X-1.jpg|thumb|right|Chandra image of Cygnus&nbsp;X-1]]

Past analysis of data from the space-based [[Chandra X-ray Observatory]] suggested that Cygnus&nbsp;X-1 was not rotating to any significant degree.<ref name=mgmgr10/><ref name=roy_watzke2003/> However, evidence announced in 2011 suggests it is rotating extremely rapidly, approximately 790 times per second.<ref name="rapid rotation"/>

====Formazione====

The largest star in the Cygnus&nbsp;OB3 association has a mass 40 times that of the Sun. As more massive stars evolve more rapidly, this implies that the progenitor star for Cygnus&nbsp;X-1 had more than 40 solar masses. Given the current estimated mass of the black hole, the progenitor star must have lost over 30 solar masses of material. Part of this mass may have been lost to HDE&nbsp;226868, while the remainder was most likely expelled by a strong stellar wind. The [[helium]] enrichment of HDE&nbsp;226868's outer atmosphere may be evidence for this mass transfer.<ref name=mnras341_2_385/> Possibly the progenitor may have evolved into a [[Wolf-Rayet star]], which ejects a substantial proportion of its atmosphere using just such a powerful stellar wind.<ref name=science300_5622_1119/>

If the progenitor star had exploded as a [[supernova]], then observations of similar objects show that the remnant would most likely have been ejected from the system at a relatively high velocity. As the object remained in orbit, this indicates that the progenitor may have collapsed directly into a black hole without exploding (or at most produced only a relatively modest explosion).<ref name=science300_5622_1119/>

==== Disco di accrescimento ====

[[File:Cygx1 spectrum.jpg|right|thumb|A [[Chandra X-ray Observatory|Chandra]] X-ray spectrum of Cygnus&nbsp;X-1 showing a characteristic peak near {{val|6.4|ul=keV}} due to [[ion]]ized [[iron]] in the accretion disk, but the peak is gravitationally red-shifted, broadened by the [[Doppler effect]], and skewed toward lower energies.<ref name=chandra20060830/>]]

The compact object is thought to be orbited by a thin, flat disk of accreting matter known as an [[accretion disk]]. This disk is intensely heated by friction between ionized gas in faster-moving inner orbits and that in slower outer ones. It is divided into a hot inner region with a relatively high level of ionization—forming a [[Plasma (physics)|plasma]]—and a cooler, less ionized outer region that extends to an estimated 500 times the Schwarzschild radius,<ref name=mnras325_3_1045/> or about {{val|15000|u=km}}.

Though highly and erratically variable, Cygnus&nbsp;X-1 is typically the brightest persistent source of [[hard X-ray]]s—those with energies from about 30 up to several hundred keV—in the sky.<ref name=apj611_2_1084/> The X-rays are produced as lower energy photons in the thin inner accretion disk, then given more energy through [[Compton scattering]] with very high temperature [[electron]]s in a geometrically thicker, but nearly transparent [[corona]] enveloping it, as well as by some further reflection from the surface of the thin disk.<ref name=apj484_1_375/> An alternative possibility is that the X-rays may be Compton scattered by the base of a jet instead of a disk corona.<ref name=asr38_12_2810/>

The X-ray emission from Cygnus&nbsp;X-1 can vary in a somewhat repetitive pattern called [[quasi-periodic oscillations]] (QPO). The mass of the compact object appears to determine the distance at which the surrounding plasma begins to emit these QPOs, with the emission radius decreasing as the mass decreases. This technique has been used to estimate the mass of Cygnus&nbsp;X-1, providing a cross-check with other mass derivations.<ref name=apj678_2_1230/>

Pulsations with a stable period, similar to those resulting from the spin of a neutron star, have never been seen from Cygnus&nbsp;X-1.<ref name=science297_5583_947/><ref name=wen1998/> The pulsations from neutron stars are caused by the neutron star's magnetic field, however, the [[no hair theorem]] guarantees that black holes do not have magnetic poles. For example, the X-ray binary [[V 0332+53]] was thought to be a possible black hole until pulsations were found.<ref name=apj2le288_L45/> Cygnus&nbsp;X-1 has also never displayed X-ray bursts similar to those seen from neutron stars.<ref name=ag44_6_77/> Cygnus&nbsp;X-1 unpredictably changes between two X-ray states, although the X-rays may vary continuously between those states as well. In the most common state, the X-rays are "hard", which means that more of the X-rays have high energy. In the less common state, the X-rays are "soft", with more of the X-rays having lower energy. The soft state also shows greater variability. The hard state is believed to originate in a corona surrounding the inner part of the more opaque accretion disk. The soft state occurs when the disk draws closer to the compact object (possibly as close as {{val|150|u=km}}), accompanied by cooling or ejection of the corona. When a new corona is generated, Cygnus&nbsp;X-1 transitions back to the hard state.<ref name=apj626_2_1015/>

The X-ray flux from Cygnus&nbsp;X-1 varies periodically every {{val|5.6|u=days}}, especially during [[superior conjunction]] when the orbiting objects are most closely aligned with the Earth and the compact source is the more distant. This indicates that the emissions are being partially blocked by circumstellar matter, which may be the stellar wind from the star HDE&nbsp;226868. There is a roughly {{val|300|u=day}} periodicity in the emission that could be caused by the [[precession]] of the accretion disk.<ref name=apj531_1_546/>

==== Getti ====

As accreted matter falls toward the compact object, it loses gravitational [[potential energy]]. Part of this released energy is dissipated by [[Relativistic jet|jets]] of particles, aligned [[perpendicular]] to the accretion disk, that flow outward with [[Special relativity|relativistic]] velocities. (That is, the particles are moving at a significant fraction of the [[speed of light]].) This pair of jets provide a means for an accretion disk to shed excess energy and [[angular momentum]]. They may be created by [[magnetic field]]s within the gas that surrounds the compact object.<ref name=science300_5627/>

The Cygnus&nbsp;X-1 jets are inefficient radiators and so release only a small proportion of their energy in the [[electromagnetic spectrum]]. That is, they appear "dark". The estimated angle of the jets to the line of sight is 30° and they may be [[Precession|precessing]].<ref name=apj626_2_1015/> One of the jets is colliding with a relatively dense part of the [[interstellar medium]] (ISM), forming an energized ring that can be detected by its radio emission. This collision appears to be forming a [[nebula]] that has been observed in the [[Visible spectrum|optical wavelengths]]. To produce this nebula, the jet must have an estimated average power of 4–{{val|14|e=36|u=[[erg]]/s}}, or {{val|9|5|e=29|ul=W}}.<ref name=mnras376_3_1341/> This is more than 1,000 times the power emitted by the Sun.<ref name=apj418_457/> There is no corresponding ring in the opposite direction because that jet is facing a lower density region of the ISM.<ref name=nature436_7052_819/>

In 2006, Cygnus&nbsp;X-1 became the first stellar mass black hole candidate found to display evidence of [[gamma ray]] emission in the very high energy band, above {{val|100|ul=GeV}}. The signal was observed at the same time as a flare of hard X-rays, suggesting a link between the events. The X-ray flare may have been produced at the base of the jet while the gamma rays could have been generated where the jet interacts with the stellar wind of HDE&nbsp;226868.<ref name=apjl665_1_L51/>

===La supergigante blu: HDE&nbsp;226868===

[[File:Cygnus X-1.png|right|thumb|An artist's impression of the HDE&nbsp;226868–Cygnus&nbsp;X-1 binary system. ''ESA/Hubble illustration.'']]

HDE&nbsp;226868 is a supergiant star with a [[spectral class]] of O9.7&nbsp;Iab,<ref name=SIMBAD/> which is on the borderline between class O and class B stars. It has an estimated surface temperature of {{val|31000|ul=K}}<ref name=eas030610/> and mass approximately 20–40 times the [[solar mass|mass of the Sun]]. Based on a stellar evolutionary model, at the estimated distance of 2,000 parsecs this star may have a radius equal to about 15-17<ref name=orosz2011/> times the [[solar radius]] and is approximately 300,000–400,000 times the [[solar luminosity|luminosity of the Sun]].<ref name=iorio2007/><ref name=MNRAS358_3_851/> For comparison, the compact object is estimated to be orbiting HDE&nbsp;226868 at a distance of about 40 solar radii, or twice the radius of this star.<ref name=apj620_1_398/>

The surface of HDE&nbsp;226868 is being [[Tidal force|tidally]] distorted by the [[gravity]] of the massive companion, forming a tear-drop shape that is further distorted by rotation. This causes the optical brightness of the star to vary by 0.06 magnitudes during each 5.6-day binary orbit, with the minimum magnitude occurring when the system is aligned with the line of sight.<ref name=caballero/> The "ellipsoidal" pattern of light variation results from the [[limb darkening]] and [[gravity darkening]] of the star's surface.<ref name=cox2001/>

When the spectrum of HDE&nbsp;226868 is compared to the similar star [[Epsilon Orionis]], the former shows an overabundance of [[helium]] and an underabundance of [[carbon]] in its atmosphere.<ref name=rmaa31_1_63/> The [[ultraviolet]] and [[Hydrogen alpha]] spectral lines of HDE&nbsp;226868 show profiles similar to the star [[P Cygni]], which indicates that the star is surrounded by a gaseous envelope that is being accelerated away from the star at speeds of about {{val|1500|u=km/s}}.<ref name=aaa63_1/><ref name=apj506_1_424/>

Like other stars of its spectral type, HDE&nbsp;226868 is thought to be shedding mass in a [[stellar wind]] at an estimated rate of {{val|2.5|e=-6}} solar masses per year.<ref name=apj203_438/> This is the equivalent of losing a mass equal to the Sun's every {{val|400000|u=years}}. The gravitational influence of the compact object appears to be reshaping this stellar wind, producing a focused wind geometry rather than a spherically symmetrical wind.<ref name=apj620_1_398/> X-rays from the region surrounding the compact object heat and ionize this stellar wind. As the object moves through different regions of the stellar wind during its 5.6-day orbit, the UV lines,<ref name=baas38_334/> the radio emission,<ref name=mnras302_1_L1/> and the X-rays themselves all vary.<ref name=apj583_1_424/>

The [[Roche lobe]] of HDE&nbsp;226868 defines the region of space around the star where orbiting material remains gravitationally bound. Material that passes beyond this lobe may fall toward the orbiting companion. This Roche lobe is believed to be close to the surface of HDE&nbsp;226868 but not overflowing, so the material at the stellar surface is not being stripped away by its companion. However, a significant proportion of the stellar wind emitted by the star is being drawn onto the compact object's accretion disk after passing beyond this lobe.<ref name=apj304_371/>

The gas and dust between the Sun and HDE&nbsp;226868 results in a reduction in the apparent magnitude of the star as well as a reddening of the hue—red light can more effectively penetrate the dust in the interstellar medium. The estimated value of the interstellar [[Extinction (astronomy)|extinction]] (A_V) is 3.3 [[Apparent magnitude|magnitudes]].<ref name=apj185_2_L113/> Without the intervening matter, HDE&nbsp;226868 would be a fifth magnitude star<ref name=sut_ir/> and thus visible to the unaided eye.<ref name=kaler/>

== La scommessa tra Stephen Hawking e Kip Thorne ==

{{Doppia immagine|right|Stephen Hawking.StarChild.jpg|120|Kip Thorne at Caltech.jpg|136|Stephen Hawking e Kip Thorne.}}

Cygnus&nbsp;X-1 was the subject of a bet between physicists [[Stephen Hawking]] and [[Kip Thorne]], in which Hawking bet against the existence of black holes in the region. Hawking later described this as an "insurance policy" of sorts. To quote from his book, ''[[A Brief History of Time]]'':

{{quote|This was a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist. But in that case, I would have the consolation of winning my bet, which would win me four years of the magazine ''[[Private Eye]]''. If black holes do exist, Kip will get one year of ''[[Penthouse]]''. When we made the bet in 1975, we were 80% certain that Cygnus&nbsp;X-1 was a black hole. By now [1988], I would say that we are about 95% certain, but the bet has yet to be settled.<ref name=hawking1988/>}}

According to the updated 10th anniversary edition of ''A Brief History of Time'', Hawking has conceded the bet ("to the outrage of Kip's liberated wife")<ref name=hawking1998/> due to subsequent observational data in favor of black holes. In his own book, ''[[Black Holes and Time Warps]]'', Thorne reports that Hawking conceded the bet by breaking into Thorne's office while he was in [[Russia]], finding the framed bet, and signing it.<ref name=thorne1994/>

== Note ==

{{references|2}}

<!--

<ref name=aaa343_861>{{citation | display-authors=1 | last1=Brocksopp | first1=C. | last2=Tarasov | first2=A. E. | last3=Lyuty | first3=V. M. | last4=Roche | first4=P. | title=An Improved Orbital Ephemeris for Cygnus&nbsp;X-1 | journal=Astronomy & Astrophysics | year=1999 | volume=343 | pages=861–864 | arxiv=astro-ph/9812077 | bibcode = 1999A&A...343..861B }}</ref>

<ref name=apj200>{{citation | last=Bolton | first=C. T. | title=Optical observations and model for Cygnus&nbsp;X-1 | journal=The Astrophysical Journal | year=1975 | volume=200 | pages=269–277 | bibcode=1975ApJ...200..269B | doi=10.1086/153785 }}</ref>

<ref name=science300_5622_1119>{{citation | last1=Mirabel | first1=I. Félix | last2=Rodrigues | first2=Irapuan | title=Formation of a Black Hole in the Dark | journal=Science | year=2003 | volume=300 | issue=5622 | pages=1119–1120 | url=http://www.sciencemag.org/cgi/content/full/300/5622/1119 | accessdate=2008-03-15 | doi=10.1126/science.1083451 | pmid=12714674 | arxiv = astro-ph/0305205 | bibcode = 2003Sci...300.1119M }}</ref>

<ref name=apj167_L15>{{citation | display-authors=1 | last1=Gursky | first=H. | year=1971 | last2=Gorenstein | first2=P. | last3=Kerr | first3=F. J. | last4=Grayzeck | first4=E. J. | bibcode=1971ApJ...167L..15G | title=The Estimated Distance to Cygnus&nbsp;X-1 Based on its Low-Energy X-Ray Spectrum | journal=Astrophysical Journal | volume=167 | pages=L15 | doi=10.1086/180751 }}</ref>

<ref name=goebel>{{citation | first=Greg | last=Goebel | url=http://www.vectorsite.net/tastgal_07.html | title=7.0 The Milky Way Galaxy | publisher=In The Public Domain | accessdate=2008-06-29 }}</ref>

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* {{en}} {{cita libro | autore= Bradley W. Carroll, Dale A. Ostlie |anno= 1996| titolo=An Introduction to Modern Astrophysics| editore= Addison Wesley| id= ISBN 0201547309| pagine= pp. 1325}}

* {{en}} {{cita libro|autore=Elske V.P. Smith, Kenneth C. Jacobs| curatore= Michael Zeilik, Stephen A. Gregory| anno= 1997| titolo= Introductory Astronomy and Astrophysics| editore= Thomson Learning| id= ISBN 0030062284| pagine= pp. 650}}

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* {{en}} {{cita libro | autore=Paul Murdin | anno=2000 | titolo=Encyclopedia of Astronomy and Astrophysics | editore=Institute of Physics Publishing | città = Bristol | id= ISBN 0122266900}}

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== Voci correlate ==

* [[Buco nero stellare]]

* [[Complesso nebuloso molecolare del Cigno]]

* [[Cygnus X]]

== Altri progetti ==

{{interprogetto|commons=Category:Cygnus X-1}}

== Collegamenti esterni ==

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*{{en}} {{cita web | data=1º aprile 1996 | url=http://www.oa.uj.edu.pl/research/cygx1.html | titolo=Cygnus&nbsp;X-1, the black hole | editore=Osservatorio astronomico dell'Università Jagellonica | accesso=24 marzo 2008}}

*{{en}} {{cita web | autore=W. Cyrmon ''et al'' | data=18 dicembre 2002 | url=http://www.eso.org/public/outreach/eduoff/cas/cas2002/cas-projects/austria_cygnus_1/ | titolo=Black Hole in Cygnus | editore=ESA | accesso=29 marzo 2008}}

*{{en}} {{cita web | titolo=Possible Jet Blown Shells Near Microquasar Cygnus&nbsp;X-1 | autore=S. Cullen | data=8 giugno 2009 | url=http://apod.nasa.gov/apod/ap090608.html | editore=NASA Astronomical Picture of the Day | accesso=8 giugno 2009}}

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