Editing Cygnus X-1 (section)

====Accretion disk====
[[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 15,000&nbsp;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 kiloelectronvolts—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 [[stellar corona|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 [[pulsar|pulsations from neutron stars]] are caused by the neutron star's rotating magnetic field, but the [[no-hair theorem]] guarantees that the magnetic field of a black hole is exactly aligned with its rotation axis and thus is static. For example, the X-ray binary [[V&nbsp;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 spectral transition of Cygnus X-1 can be explained using a two-component [[advection|advective]] flow solution, as proposed by Chakrabarti and Titarchuk.<ref name="CHTI_1"/> A hard state is generated by the inverse Comptonisation of seed photons from the Keplarian disk and likewise synchrotron photons produced by the hot electrons in the centrifugal-pressure–supported boundary layer ([[CENBOL]]).<ref name="CHM_1"/>

The X-ray flux from Cygnus&nbsp;X-1 varies periodically every 5.6&nbsp;days, especially during [[Conjunction (astronomy)|superior conjunction]] when the orbiting objects are most closely aligned with 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 300-day periodicity in the emission, which could be caused by the [[precession]] of the accretion disk.<ref name=apj531_1_546/>