Editing Black hole (section)

==== X-ray binaries ====
{{See also|X-ray binary}}
[[File:Chandra image of Cygnus X-1.jpg|thumb|A [[Chandra X-Ray Observatory]] image of [[Cygnus X-1]], which was the first strong black hole candidate discovered]]

[[X-ray binaries]] are binary star systems that emit a majority of their radiation in the [[X-ray]] part of the spectrum. These X-ray emissions are generally thought to result when one of the stars (compact object) accretes matter from another (regular) star. The presence of an ordinary star in such a system provides an opportunity for studying the central object and to determine if it might be a black hole.<ref name="CMS1999" />

If such a system emits signals that can be directly traced back to the compact object, it cannot be a black hole. The absence of such a signal does, however, not exclude the possibility that the compact object is a neutron star. By studying the companion star it is often possible to obtain the orbital parameters of the system and to obtain an estimate for the mass of the compact object. If this is much larger than the Tolman–Oppenheimer–Volkoff limit (the maximum mass a star can have without collapsing) then the object cannot be a neutron star and is generally expected to be a black hole.<ref name="CMS1999">{{Cite journal |last1=Celotti |first1=A. |last2=Miller |first2=J. C. |last3=Sciama |first3=D. W. |title=Astrophysical evidence for the existence of black holes |journal=Classical and Quantum Gravity |volume=16 |issue=12A |pages=A3–A21 |date=1999 |arxiv=astro-ph/9912186 |doi=10.1088/0264-9381/16/12A/301 |url=https://cds.cern.ch/record/411555/files/9912186.pdf |bibcode=1999CQGra..16A...3C |s2cid=17677758 |access-date=27 July 2018 |archive-date=27 July 2018 |archive-url=https://web.archive.org/web/20180727052939/https://cds.cern.ch/record/411555/files/9912186.pdf |url-status=live }}</ref>

The first strong candidate for a black hole, [[Cygnus X-1]], was discovered in this way by [[Charles Thomas Bolton]],<ref name="Bolton1972"/> [[Louise Webster]], and [[Paul Murdin]]<ref name="Webster1972"/> in 1972.<ref>{{cite web |last=Rolston |first=B. |date=10 November 1997 |url=http://news.utoronto.ca/bin/bulletin/nov10_97/art4.htm |archive-url=https://web.archive.org/web/20080502230214/http://news.utoronto.ca/bin/bulletin/nov10_97/art4.htm |archive-date=2 May 2008 |title=The First Black Hole |website=The bulletin |publisher=University of Toronto |access-date=11 March 2008}}</ref><ref name="Shipman1975"/> Some doubt remained, due to the uncertainties that result from the companion star being much heavier than the candidate black hole. Currently, better candidates for black holes are found in a class of X-ray binaries called soft X-ray transients. In this class of system, the companion star is of relatively low mass allowing for more accurate estimates of the black hole mass. These systems actively emit X-rays for only several months once every 10–50 years. During the period of low X-ray emission, called quiescence, the accretion disk is extremely faint, allowing detailed observation of the companion star during this period. One of the best such candidates is [[V404 Cygni]].<ref name="CMS1999" />

===== Quasi-periodic oscillations =====
{{Main|Quasi-periodic oscillation}}

The X-ray emissions from accretion disks sometimes flicker at certain frequencies. These signals are called [[quasi-periodic oscillation]]s and are thought to be caused by material moving along the inner edge of the accretion disk (the innermost stable circular orbit). As such their frequency is linked to the mass of the compact object. They can thus be used as an alternative way to determine the mass of candidate black holes.<ref>{{cite press release |title=NASA scientists identify smallest known black hole |publisher=[[Goddard Space Flight Center]] |date=1 April 2008 |url=http://www.eurekalert.org/pub_releases/2008-04/nsfc-nsi040108.php |access-date=14 March 2009 |archive-url=https://web.archive.org/web/20081227195554/http://www.eurekalert.org/pub_releases/2008-04/nsfc-nsi040108.php |archive-date=27 December 2008 }}</ref>