Cygnus X-1
Encyclopedia
Cygnus X-1 is a well-known galactic X-ray source
in the constellation
Cygnus
. It was discovered in 1964 during a rocket flight
and is one of the strongest
X-ray sources seen from Earth, producing a peak X-ray flux density
of 2.3 W
m−2Hz
−1 (2.3 Jansky
). Cygnus X-1 was the first X-ray source widely accepted to be a black hole
candidate and it remains among the most studied astronomical object
s in its class. It is now estimated to have a mass about 8.7 times the mass of the Sun
and has been shown to be too compact to be any known kind of normal star or other likely object besides a black hole. If so, the radius of its event horizon
is probably about 26 km.
Cygnus X-1 belongs to a high-mass X-ray binary system about 6,100 light years from the Sun
that includes a blue supergiant
variable star
designated HDE 226868 which it orbits at about 0.2 AU, or 20% of the distance from the Earth to the Sun. A stellar wind
from the star provides material for an accretion disk around the X-ray source. Matter in the inner disk is heated to millions of kelvin
s (K), generating the observed X-rays. A pair of jets
, arranged perpendicular
to the disk, are carrying part of the infalling material away into interstellar space.
This system may belong to a stellar association called Cygnus OB3, which would mean that Cygnus X-1 is about five million years old and formed from a progenitor star that had more than 40 solar mass
es. The majority of the star's mass was shed, most likely as a stellar wind. If this star had then exploded as a supernova
, the resulting force would most likely have ejected the remnant from the system. Hence the star may have instead collapsed directly into a black hole.
Cygnus X-1 was the subject of a friendly scientific wager
between physicists Stephen Hawking
and Kip Thorne
in 1974, with Hawking betting that it was not a black hole. He conceded the bet in 1990 after observational data had strengthened the case that there was indeed a black hole
in the system.
s to study celestial phenomena involving gas with temperatures in the millions of kelvin
s. However, because X-ray emissions are blocked by the Earth's atmosphere
, observation of celestial X-ray sources
is not possible without lifting instruments to altitudes where the X-rays can penetrate. Cygnus X-1 was discovered using X-ray instruments
that were carried aloft by a sounding rocket launched
from White Sands Missile Range
in New Mexico
. As part of an ongoing effort to map these sources, a survey was conducted in 1964 using two Aerobee suborbital rockets. The rockets carried Geiger counters to measure X-ray emission in wavelength
range 1–15 Å
across an 8.4° section of the sky. These instruments swept across the sky as the rockets rotated, producing a map of closely spaced scans.
As a result of these surveys, eight new sources of cosmic X-rays were discovered, including Cyg XR-1 (later Cyg X-1) in the constellation Cygnus. The celestial coordinates
of this source were estimated as right ascension
19h53m and declination
34.6°. It was not associated with any especially prominent radio
or optical
source at that position.
Seeing a need for longer duration studies, in 1963 Riccardo Giacconi
and Herb Gursky proposed the first orbital satellite to study X-ray sources. NASA
launched their Uhuru satellite
in 1970, which led to the discovery of 300 new X-ray sources. Extended Uhuru observations of Cygnus X-1 showed fluctuations in the X-ray intensity that occurs several times a second. This rapid variation meant that the energy generation must take place over a relatively small region of roughly 105 km, as the speed of light
restricts communication between more distant regions. For a size comparison, the diameter of the Sun
is about 1.4 km.
In April–May 1971, Luc Braes and George K. Miley from Leiden Observatory
, and independently Robert M. Hjellming and Campbell Wade at the National Radio Astronomy Observatory
, detected radio emission from Cygnus X-1, and their accurate radio position pinpointed the X-ray source to the star AGK2 +35 1910 = HDE 226868. On the celestial sphere
, this star lies about half a degree
from the 4th magnitude
star Eta Cygni
. It is a supergiant star that is, by itself, incapable of emitting the observed quantities of X-rays. Hence, the star must have a companion that could heat gas to the millions of degrees needed to produce the radiation source for Cygnus X-1.
Louise Webster and Paul Murdin, at the Royal Greenwich Observatory, and Charles Thomas Bolton
, working independently at the University of Toronto
's David Dunlap Observatory
, announced the discovery of a massive hidden companion to HDE 226868 in 1971. Measurements of the Doppler shift of the star's spectrum demonstrated the companion's presence and allowed its mass to be estimated from the orbital parameters. Based on the high predicted mass of the object, they surmised that it may be a black hole
as the largest possible neutron star
cannot exceed three times the mass of the Sun
.
With further observations strengthening the evidence, by the end of 1973 the astronomical community generally conceded that Cygnus X-1 was most likely a black hole. More precise measurements of Cygnus X-1 demonstrated variability down to a single millisecond
. This interval is consistent with turbulence
in a disk of accreted matter surrounding a black hole—the accretion disk. X-ray bursts that last for about a third of a second match the expected time frame of matter falling toward a black hole.
Cygnus X-1 has since been studied extensively using observations by orbit
ing and ground-based instruments. The similarities between the emissions of X-ray binaries such as HDE 226868/Cygnus X-1 and active galactic nuclei suggests a common mechanism of energy generation involving a black hole, an orbiting accretion disk and associated jets
. For this reason, Cygnus X-1 is identified among a class of objects called microquasars; an analog of the quasar
s, or quasi-stellar radio sources, now known to be distant active galactic nuclei. Scientific studies of binary systems such as HDE 226868/Cygnus X-1 may lead to further insights into the mechanics of active galaxies.
star form a binary system
that orbit around their center of mass
every . 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 inclination of the orbital plane to the line of sight
from the Earth remains uncertain, with predictions ranging from 27–65°. A 2007 study estimated the inclination is , which would mean that the semi-major axis
is about 0.2 AU
, or 20% of the distance from the Earth to the Sun. The orbital eccentricity
is thought to be only ; a nearly circular orbit. Earth's distance to this system is about 1860 pc.
The HDE 226868/Cygnus X-1 system shares a common motion through space with an association of massive stars named Cygnus OB3, which is located at roughly 2,000 parsecs from the Sun. This implies that HDE 226868, Cygnus X-1 and this OB association may have formed at the same time and location. If so, then the age of the system is about . The motion of HDE 226868 with respect to Cygnus OB3 is ; a typical value for random motion within a stellar association. HDE 226868 is about 60 parsecs from the center of the association, and could have reached that separation in about —which roughly agrees with estimated age of the association.
With a galactic latitude of 4 degrees and galactic longitude 71 degrees, this system lies inward along the same Orion Spur in which the Sun is located within the Milky Way
, near where the spur approaches the Sagittarius Arm
. Cygnus X-1 has been described as belonging to the Sagittarius Arm, though the structure of the Milky Way is not well established.
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 26 km for Cygnus X-1. Anything (including matter
and photon
s) that passes through this boundary is unable to escape.
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.
The space-based Chandra X-ray Observatory
was used to measure the spectral signature of iron
atoms orbiting near the object. A rotating black hole
drags the nearby space around with it—a process known as frame-dragging
, which allows atoms to orbit closer to the event horizon. In the case of Cygnus X-1, none of the atoms were found orbiting closer than 160 km. Hence, if this object is a black hole, then this data shows it is not rotating to any significant degree.
enrichment of HDE 226868's outer atmosphere may be evidence for this mass transfer. 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.
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).
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
—and a cooler, less ionized outer region that extends to an estimated 500 times the Schwarzschild radius, or about 15,000 km.
Though highly and erratically variable, Cygnus X-1 is typically the brightest persistent source of hard X-rays—those with energies from about 30 up to several hundred keV—in the sky. 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. An alternative possibility is that the X-rays may be Compton scattered by the base of a jet instead of a disk corona.
The X-ray emission from Cygnus 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 X-1, providing a cross-check with other mass derivations.
Pulsations with a stable period, similar to those resulting from the spin of a neutron star, have never been seen from Cygnus X-1. The pulsations from neutron stars are caused by the neutron star's magnetic field and 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. Cygnus X-1 has also never displayed X-ray bursts similar to those seen from neutron stars. Cygnus 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 150 km), accompanied by cooling or ejection of the corona. When a new corona is generated, Cygnus X-1 transitions back to the hard state.
The X-ray flux from Cygnus X-1 varies periodically every 5.6 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 226868. There is a roughly 300 day periodicity in the emission that could be caused by the precession
of the accretion disk.
. Part of this released energy is dissipated by jets
of particles, aligned perpendicular
to the accretion disk, that flow outward with 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.
The Cygnus 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 precessing
. 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 optical wavelengths
. To produce this nebula, the jet must have an estimated average power of , or . This is more than 1,000 times the power emitted by the Sun. There is no corresponding ring in the opposite direction because that jet is facing a lower density region of the ISM.
In 2006, Cygnus 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 100 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 226868.
HDE 226868 is a supergiant star with a spectral class of O9.7 Iab, which is on the borderline between class O and class B stars. It has an estimated surface temperature of 31,000 kelvin
s and mass approximately 20–40 times the 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 20–22 times the solar radius
and is approximately 300,000–400,000 times the luminosity of the Sun. For comparison, the compact object is estimated to be orbiting HDE 226868 at a distance of about 40 solar radii, or twice the radius of this star.
The surface of HDE 226868 is being 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. The "ellipsoidal" pattern of light variation results from the limb darkening
and gravity darkening
of the star's surface.
When the spectrum of HDE 226868 is compared to the similar star Epsilon Orionis
, the former shows an overabundance of helium
and an underabundance of carbon
in its atmosphere. The ultraviolet
and Hydrogen alpha spectral lines of HDE 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 1500 kilometers per second
.
Like other stars of its spectral type, HDE 226868 is thought to be shedding mass in a stellar wind
at an estimated rate of masses per year. This is the equivalent of losing a mass equal to the Sun's every 400,000 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. 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, the radio emission, and the X-rays themselves all vary.
The Roche lobe
of HDE 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 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.
The gas and dust between the Sun and HDE 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
(AV) is 3.3 magnitudes
. Without the intervening matter, HDE 226868 would be a fifth magnitude star and thus visible to the unaided eye.
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
,
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") 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.
Astrophysical X-ray source
Astrophysical X-ray sources are astronomical objects with physical properties which result in the emission of X-rays.There are a number of types of astrophysical objects which emit X-rays, from galaxy clusters, through black holes in active galactic nuclei to galactic objects such as supernova...
in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Cygnus
Cygnus (constellation)
Cygnus is a northern constellation lying on the plane of the Milky Way. Its name is the Latinized Hellenic word for swan. One of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross...
. It was discovered in 1964 during a rocket flight
Sub-orbital spaceflight
A sub-orbital space flight is a spaceflight in which the spacecraft reaches space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it does not complete one orbital revolution....
and is one of the strongest
Signal strength
In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
X-ray sources seen from Earth, producing a peak X-ray flux density
Flux density
-Formal Statement:The flux density is simply defined as the amount of flux passing through a unit-area. -Mathematical Statement:The flux density would essentially be the number of field lines passing through a defined unit-area...
of 2.3 W
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
m−2Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
−1 (2.3 Jansky
Jansky
The flux unit or jansky is a non-SI unit of spectral flux density equivalent to 10−26 watts per square metre per hertz...
). Cygnus X-1 was the first X-ray source widely accepted to be a black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
candidate and it remains among the most studied astronomical object
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
s in its class. It is now estimated to have a mass about 8.7 times the mass of the Sun
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
and has been shown to be too compact to be any known kind of normal star or other likely object besides a black hole. If so, the radius of its event horizon
Event horizon
In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible. The most common case...
is probably about 26 km.
Cygnus X-1 belongs to a high-mass X-ray binary system about 6,100 light years from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
that includes a blue supergiant
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...
variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
designated HDE 226868 which it orbits at about 0.2 AU, or 20% of the distance from the Earth to the Sun. A stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
from the star provides material for an accretion disk around the X-ray source. Matter in the inner disk is heated to millions of kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s (K), generating the observed X-rays. A pair of jets
Relativistic jet
Relativistic jets are extremely powerful jets of plasma which emerge from presumed massive objects at the centers of some active galaxies, notably radio galaxies and quasars. Their lengths can reach several thousand or even hundreds of thousands of light years...
, arranged perpendicular
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
to the disk, are carrying part of the infalling material away into interstellar space.
This system may belong to a stellar association called Cygnus OB3, which would mean that Cygnus X-1 is about five million years old and formed from a progenitor star that had more than 40 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es. The majority of the star's mass was shed, most likely as a stellar wind. If this star had then exploded as a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
, the resulting force would most likely have ejected the remnant from the system. Hence the star may have instead collapsed directly into a black hole.
Cygnus X-1 was the subject of a friendly scientific wager
Scientific wager
A scientific wager is a wager whose outcome is settled by scientific method. They typically consist of an offer to pay a certain sum of money on the scientific proof or disproof of some currently uncertain statement. Some wagers have specific date restrictions for collection, but many are open...
between physicists Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...
and Kip Thorne
Kip Thorne
Kip Stephen Thorne is an American theoretical physicist, known for his prolific contributions in gravitation physics and astrophysics and for having trained a generation of scientists...
in 1974, with Hawking betting that it was not a black hole. He conceded the bet in 1990 after observational data had strengthened the case that there was indeed a black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
in the system.
Discovery and observation
Observation of X-ray emissions allows astronomerAstronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
s to study celestial phenomena involving gas with temperatures in the millions of kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s. However, because X-ray emissions are blocked by the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
, observation of celestial X-ray sources
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
is not possible without lifting instruments to altitudes where the X-rays can penetrate. Cygnus X-1 was discovered using X-ray instruments
X-ray astronomy detector
X-ray astronomy detectors are instruments that detect X-rays for use in the study of X-ray astronomy.X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray emission from celestial objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to...
that were carried aloft by a sounding rocket launched
Sub-orbital spaceflight
A sub-orbital space flight is a spaceflight in which the spacecraft reaches space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it does not complete one orbital revolution....
from White Sands Missile Range
White Sands Missile Range
White Sands Missile Range is a rocket range of almost in parts of five counties in southern New Mexico. The largest military installation in the United States, WSMR includes the and the WSMR Otera Mesa bombing range...
in New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
. As part of an ongoing effort to map these sources, a survey was conducted in 1964 using two Aerobee suborbital rockets. The rockets carried Geiger counters to measure X-ray emission in wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
range 1–15 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
across an 8.4° section of the sky. These instruments swept across the sky as the rockets rotated, producing a map of closely spaced scans.
As a result of these surveys, eight new sources of cosmic X-rays were discovered, including Cyg XR-1 (later Cyg X-1) in the constellation Cygnus. The celestial coordinates
Celestial coordinate system
In astronomy, a celestial coordinate system is a coordinate system for mapping positions on the celestial sphere.There are different celestial coordinate systems each using a system of spherical coordinates projected on the celestial sphere, in analogy to the geographic coordinate system used on...
of this source were estimated as right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
19h53m and declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
34.6°. It was not associated with any especially prominent radio
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
or optical
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
source at that position.
Seeing a need for longer duration studies, in 1963 Riccardo Giacconi
Riccardo Giacconi
Riccardo Giacconi is an Italian/American Nobel Prize-winning astrophysicist who laid the foundations of X-ray astronomy. He is currently a professor at the Johns Hopkins University.- Biography :...
and Herb Gursky proposed the first orbital satellite to study X-ray sources. NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
launched their Uhuru satellite
Uhuru (satellite)
Uhuru was the first satellite launched specifically for the purpose of X-ray astronomy. It was also known as the X-ray Explorer Satellite, SAS-A , SAS 1, or Explorer 42.The observatory was launched on 12 December 1970 into an initial orbit of about 560 km apogee, 520 km...
in 1970, which led to the discovery of 300 new X-ray sources. Extended Uhuru observations of Cygnus X-1 showed fluctuations in the X-ray intensity that occurs several times a second. This rapid variation meant that the energy generation must take place over a relatively small region of roughly 105 km, as the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
restricts communication between more distant regions. For a size comparison, the diameter of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
is about 1.4 km.
In April–May 1971, Luc Braes and George K. Miley from Leiden Observatory
Leiden Observatory
Leiden Observatory is an astronomical observatory in the city of Leiden, Netherlands. It was established by Leiden University in 1633, to house the quadrant of Snellius, and is the oldest operating University observatory in the world Leiden Observatory (Sterrewacht Leiden in Dutch) is an...
, and independently Robert M. Hjellming and Campbell Wade at the National Radio Astronomy Observatory
National Radio Astronomy Observatory
The National Radio Astronomy Observatory is a Federally Funded Research and Development Center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc for the purpose of radio astronomy...
, detected radio emission from Cygnus X-1, and their accurate radio position pinpointed the X-ray source to the star AGK2 +35 1910 = HDE 226868. On the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
, this star lies about half a degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
from the 4th magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
star Eta Cygni
Eta Cygni
Eta Cygni is a star in theconstellation Cygnus. It is located about 137 light years from the Sun and is categorized as an orange giant star of spectral class K0III....
. It is a supergiant star that is, by itself, incapable of emitting the observed quantities of X-rays. Hence, the star must have a companion that could heat gas to the millions of degrees needed to produce the radiation source for Cygnus X-1.
Louise Webster and Paul Murdin, at the Royal Greenwich Observatory, and Charles Thomas Bolton
Charles Thomas Bolton
Charles Thomas or Tom Bolton is an American astronomer who was the first astronomer to present strong evidence of the existence of a black hole....
, working independently at the University of Toronto
University of Toronto
The University of Toronto is a public research university in Toronto, Ontario, Canada, situated on the grounds that surround Queen's Park. It was founded by royal charter in 1827 as King's College, the first institution of higher learning in Upper Canada...
's David Dunlap Observatory
David Dunlap Observatory
The David Dunlap Observatory is a large astronomical observatory site once owned by the University of Toronto, located just north of the city in Richmond Hill, Ontario within a estate. Its primary instrument is a 74-inch reflector telescope, at one time the second largest telescope in the world,...
, announced the discovery of a massive hidden companion to HDE 226868 in 1971. Measurements of the Doppler shift of the star's spectrum demonstrated the companion's presence and allowed its mass to be estimated from the orbital parameters. Based on the high predicted mass of the object, they surmised that it may be a black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
as the largest possible neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
cannot exceed three times the mass of the Sun
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
.
With further observations strengthening the evidence, by the end of 1973 the astronomical community generally conceded that Cygnus X-1 was most likely a black hole. More precise measurements of Cygnus X-1 demonstrated variability down to a single millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
. This interval is consistent with turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
in a disk of accreted matter surrounding a black hole—the accretion disk. X-ray bursts that last for about a third of a second match the expected time frame of matter falling toward a black hole.
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
ing and ground-based instruments. The similarities between the emissions of X-ray binaries such as HDE 226868/Cygnus X-1 and active galactic nuclei suggests a common mechanism of energy generation involving a black hole, an orbiting accretion disk and associated jets
Relativistic jet
Relativistic jets are extremely powerful jets of plasma which emerge from presumed massive objects at the centers of some active galaxies, notably radio galaxies and quasars. Their lengths can reach several thousand or even hundreds of thousands of light years...
. For this reason, Cygnus X-1 is identified among a class of objects called microquasars; an analog of the quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s, or quasi-stellar radio sources, now known to be distant active galactic nuclei. Scientific studies of binary systems such as HDE 226868/Cygnus X-1 may lead to further insights into the mechanics of active galaxies.
Star system
The compact object and blue supergiantBlue supergiant
Blue supergiants are supergiant stars of spectral type O or B.They are extremely hot and bright, with surface temperatures of 30,000-50,000 K. They typically have 10 to 50 solar masses on the Hertzsprung-Russell diagram, and can have radii up to about 25 solar radii...
star form a binary system
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
that orbit around their center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
every . From the perspective of the Earth, the compact object never goes behind the other star; in other words, the system does not eclipse
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
. However, the inclination of the orbital plane to the line of sight
Line-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...
from the Earth remains uncertain, with predictions ranging from 27–65°. A 2007 study estimated the inclination is , which would mean that the semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
is about 0.2 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
, or 20% of the distance from the Earth to the Sun. The orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
is thought to be only ; a nearly circular orbit. Earth's distance to this system is about 1860 pc.
The HDE 226868/Cygnus X-1 system shares a common motion through space with an association of massive stars named Cygnus OB3, which is located at roughly 2,000 parsecs from the Sun. This implies that HDE 226868, Cygnus X-1 and this OB association may have formed at the same time and location. If so, then the age of the system is about . The motion of HDE 226868 with respect to Cygnus OB3 is ; a typical value for random motion within a stellar association. HDE 226868 is about 60 parsecs from the center of the association, and could have reached that separation in about —which roughly agrees with estimated age of the association.
With a galactic latitude of 4 degrees and galactic longitude 71 degrees, this system lies inward along the same Orion Spur in which the Sun is located within the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
, near where the spur approaches the Sagittarius Arm
Sagittarius Arm
The Carina–Sagittarius Arm is generally thought to be a minor spiral arm of our home galaxy, the Milky Way. Each spiral arm is a long, diffuse curving streamer of stars that radiates out from the galactic center...
. Cygnus X-1 has been described as belonging to the Sagittarius Arm, though the structure of the Milky Way is not well established.
Compact object
There is some uncertainty about the mass of the compact object. Stellar evolutionary models suggest a mass of , 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 . In all cases, the object is most likely a black hole—a region of space with a gravitational field that is strong enough to prevent the escape of electromagnetic radiationElectromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
from the interior. The boundary of this region is called the event horizon
Event horizon
In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible. The most common case...
and has an effective radius called the Schwarzschild radius
Schwarzschild radius
The Schwarzschild radius is the distance from the center of an object such that, if all the mass of the object were compressed within that sphere, the escape speed from the surface would equal the speed of light...
, which is about 26 km for Cygnus X-1. Anything (including matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
and photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s) that passes through this boundary is unable to escape.
Evidence of just such an event horizon may have been detected in 1992 using ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(UV) observations with the High Speed Photometer
High Speed Photometer
The High Speed Photometer was a scientific instrument installed on the Hubble Space Telescope. The HSP was designed to measure the brightness and polarity of rapidly varying celestial objects. It could observe in ultraviolet, visible light, and near infrared at a rate of one measurement per 10...
on the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
. 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
Gravitational redshift
In astrophysics, gravitational redshift or Einstein shift describes light or other forms of electromagnetic radiation of certain wavelengths that originate from a source that is in a region of a stronger gravitational field that appear to be of longer wavelength, or redshifted, when seen or...
as the material approaches the horizon. That is, the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s of the radiation will steadily increase, as predicted by General Relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
. 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.
The space-based Chandra X-ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
was used to measure the spectral signature of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
atoms orbiting near the object. A rotating black hole
Rotating black hole
A rotating black hole is a black hole that possesses spin angular momentum.-Types of black holes:There are four known, exact, black hole solutions to Einstein's equations, which describe gravity in General Relativity. Two of these rotate...
drags the nearby space around with it—a process known as frame-dragging
Frame-dragging
Einstein's general theory of relativity predicts that non-static, stationary mass-energy distributions affect spacetime in a peculiar way giving rise to a phenomenon usually known as frame-dragging...
, which allows atoms to orbit closer to the event horizon. In the case of Cygnus X-1, none of the atoms were found orbiting closer than 160 km. Hence, if this object is a black hole, then this data shows it is not rotating to any significant degree.
Formation
The largest star in the Cygnus 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 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 226868, while the remainder was most likely expelled by a strong stellar wind. The heliumHelium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
enrichment of HDE 226868's outer atmosphere may be evidence for this mass transfer. Possibly the progenitor may have evolved into a Wolf-Rayet star
Wolf-Rayet star
Wolf–Rayet stars are evolved, massive stars , which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s...
, which ejects a substantial proportion of its atmosphere using just such a powerful stellar wind.
If the progenitor star had exploded as a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
, 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).
Accretion disk
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
—and a cooler, less ionized outer region that extends to an estimated 500 times the Schwarzschild radius, or about 15,000 km.
Though highly and erratically variable, Cygnus X-1 is typically the brightest persistent source of hard X-rays—those with energies from about 30 up to several hundred keV—in the sky. The X-rays are produced as lower energy photons in the thin inner accretion disk, then given more energy through Compton scattering
Compton scattering
In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...
with very high temperature electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s in a geometrically thicker, but nearly transparent corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
enveloping it, as well as by some further reflection from the surface of the thin disk. An alternative possibility is that the X-rays may be Compton scattered by the base of a jet instead of a disk corona.
The X-ray emission from Cygnus X-1 can vary in a somewhat repetitive pattern called quasi-periodic oscillations
Quasi-periodic oscillations
In X-ray astronomy, quasi-periodic oscillation is the manner in which the X-ray light from an astronomical object flickers about certain frequencies...
(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 X-1, providing a cross-check with other mass derivations.
Pulsations with a stable period, similar to those resulting from the spin of a neutron star, have never been seen from Cygnus X-1. The pulsations from neutron stars are caused by the neutron star's magnetic field and the no hair theorem
No hair theorem
The no-hair theorem postulates that all black hole solutions of the Einstein-Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum...
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. Cygnus X-1 has also never displayed X-ray bursts similar to those seen from neutron stars. Cygnus 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 150 km), accompanied by cooling or ejection of the corona. When a new corona is generated, Cygnus X-1 transitions back to the hard state.
The X-ray flux from Cygnus X-1 varies periodically every 5.6 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 226868. There is a roughly 300 day periodicity in the emission that could be caused by the precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
of the accretion disk.
Jets
As accreted matter falls toward the compact object, it loses gravitational potential energyPotential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
. Part of this released energy is dissipated by jets
Relativistic jet
Relativistic jets are extremely powerful jets of plasma which emerge from presumed massive objects at the centers of some active galaxies, notably radio galaxies and quasars. Their lengths can reach several thousand or even hundreds of thousands of light years...
of particles, aligned perpendicular
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
to the accretion disk, that flow outward with relativistic
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
velocities. (That is, the particles are moving at a significant fraction of the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
.) This pair of jets provide a means for an accretion disk to shed excess energy and angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
. They may be created by magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s within the gas that surrounds the compact object.
The Cygnus X-1 jets are inefficient radiators and so release only a small proportion of their energy in the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. That is, they appear "dark". The estimated angle of the jets to the line of sight is 30° and they may be precessing
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
. One of the jets is colliding with a relatively dense part of the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
(ISM), forming an energized ring that can be detected by its radio emission. This collision appears to be forming a nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
that has been observed in the optical wavelengths
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
. To produce this nebula, the jet must have an estimated average power of , or . This is more than 1,000 times the power emitted by the Sun. There is no corresponding ring in the opposite direction because that jet is facing a lower density region of the ISM.
In 2006, Cygnus X-1 became the first stellar mass black hole candidate found to display evidence of gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
emission in the very high energy band, above 100 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 226868.
HDE 226868
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s and mass approximately 20–40 times the mass of the Sun
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
. Based on a stellar evolutionary model, at the estimated distance of 2,000 parsecs this star may have a radius equal to about 20–22 times the solar radius
and is approximately 300,000–400,000 times the luminosity of the Sun. For comparison, the compact object is estimated to be orbiting HDE 226868 at a distance of about 40 solar radii, or twice the radius of this star.
The surface of HDE 226868 is being tidally
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...
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. The "ellipsoidal" pattern of light variation results from the limb darkening
Limb darkening
Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or "limb" of the image...
and gravity darkening
Gravity darkening
Gravity darkening, also referred to as Gravity Brightening, is an astronomical phenomenon where a star rotates so rapidly that it has a detectably oblate spheroid shape, such as in Regulus in the Leo constellation....
of the star's surface.
When the spectrum of HDE 226868 is compared to the similar star Epsilon Orionis
Epsilon Orionis
Alnilam is a large blue supergiant star in the constellation of Orion. Its Flamsteed designation is 46 Orionis....
, the former shows an overabundance of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
and an underabundance of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
in its atmosphere. The ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
and Hydrogen alpha spectral lines of HDE 226868 show profiles similar to the star P Cygni
P Cygni
P Cygni is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in Uranometria as a nova....
, which indicates that the star is surrounded by a gaseous envelope that is being accelerated away from the star at speeds of about 1500 kilometers per second
Metre per second
Metre per second is an SI derived unit of both speed and velocity , defined by distance in metres divided by time in seconds....
.
Like other stars of its spectral type, HDE 226868 is thought to be shedding mass in a stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
at an estimated rate of masses per year. This is the equivalent of losing a mass equal to the Sun's every 400,000 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. 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, the radio emission, and the X-rays themselves all vary.
The Roche lobe
Roche lobe
The Roche lobe is the region of space around a star in a binary system within which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material can escape the gravitational pull of the star. If the star is in a binary system then the material...
of HDE 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 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.
The gas and dust between the Sun and HDE 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
Extinction (astronomy)
Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first...
(AV) is 3.3 magnitudes
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
. Without the intervening matter, HDE 226868 would be a fifth magnitude star and thus visible to the unaided eye.
Stephen Hawking and Kip Thorne
Cygnus X-1 was the subject of the bet between physicists Stephen HawkingStephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...
and Kip Thorne
Kip Thorne
Kip Stephen Thorne is an American theoretical physicist, known for his prolific contributions in gravitation physics and astrophysics and for having trained a generation of scientists...
, 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
A Brief History of Time
A Brief History of Time is a popular science book written by renown physicist Stephen Hawking and first published by the Bantam Dell Publishing Group in 1988. It became a best-seller and has sold more than 10 million copies...
,
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") due to subsequent observational data in favor of black holes. In his own book, Black Holes and Time Warps
Black Holes and Time Warps
Black Holes and Time Warps: Einstein's Outrageous Legacy is a popular science book by Kip Thorne. It provides an illustrated overview of the history and development of black hole theory up until the early 1990s....
, Thorne reports that Hawking conceded the bet by breaking into Thorne's office while he was in Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
, finding the framed bet, and signing it.
See also
- Stellar black holeStellar black holeA stellar black hole is a black hole formed by the gravitational collapse of a massive star. They have masses ranging from about 3 to several tens of solar masses...
- CanadianCanadaCanada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
progressive rockProgressive rockProgressive rock is a subgenre of rock music that developed in the late 1960s and early 1970s as part of a "mostly British attempt to elevate rock music to new levels of artistic credibility." John Covach, in Contemporary Music Review, says that many thought it would not just "succeed the pop of...
band RushRush (band)Rush is a Canadian rock band formed in August 1968, in the Willowdale neighbourhood of Toronto, Ontario. The band is composed of bassist, keyboardist, and lead vocalist Geddy Lee, guitarist Alex Lifeson, and drummer and lyricist Neil Peart...
's Cygnus X-1 duology tells a story of the black hole. - AmericanUnited StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
science fictionScience fictionScience fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
authorAuthorAn author is broadly defined as "the person who originates or gives existence to anything" and that authorship determines responsibility for what is created. Narrowly defined, an author is the originator of any written work.-Legal significance:...
John C. Wright's trilogyTrilogyA trilogy is a set of three works of art that are connected, and that can be seen either as a single work or as three individual works. They are commonly found in literature, film, or video games...
The Golden AgeThe Golden Age (novel series)The Golden Age is a science fiction trilogy by the American writer John C. Wright. It consists of three books, The Golden Age, The Phoenix Exultant and The Golden Transcendence.-Plot introduction:...
tells the story of a manned expedition to Cygnus X-1.