Accretion disk

An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction causes orbiting material in the disk to spiral inward towards the central body. Gravitational and frictional forces compress and raise the temperature of the material, causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object's mass. Accretion disks of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the X-ray part of the spectrum. The study of oscillation modes in accretion disks is referred to as diskoseismology.[1][2]

Black hole - Messier 87 crop max res
Image of the disk of the black hole in the center of the supergiant elliptical galaxy Messier 87

Manifestations

Question, Web Fundamentals.svg Unsolved problem in physics:
Accretion disk jets: Why do the disks surrounding certain objects, such as the nuclei of active galaxies, emit jets along their polar axes? These jets are invoked by astronomers to do everything from getting rid of angular momentum in a forming star to reionizing the universe (in active galactic nuclei), but their origin is still not well understood.
(more unsolved problems in physics)

Accretion disks are a ubiquitous phenomenon in astrophysics; active galactic nuclei, protoplanetary disks, and gamma ray bursts all involve accretion disks. These disks very often give rise to astrophysical jets coming from the vicinity of the central object. Jets are an efficient way for the star-disk system to shed angular momentum without losing too much mass.

The most spectacular accretion disks found in nature are those of active galactic nuclei and of quasars, which are thought to be massive black holes at the center of galaxies. As matter enters the accretion disc, it follows a trajectory called a tendex line, which describes an inward spiral. This is because particles rub and bounce against each other in a turbulent flow, causing frictional heating which radiates energy away, reducing the particles' angular momentum, allowing the particle to drift inwards, driving the inward spiral. The loss of angular momentum manifests as a reduction in velocity; at a slower velocity, the particle wants to adopt a lower orbit. As the particle falls to this lower orbit, a portion of its gravitational potential energy is converted to increased velocity and the particle gains speed. Thus, the particle has lost energy even though it is now travelling faster than before; however, it has lost angular momentum. As a particle orbits closer and closer, its velocity increases, as velocity increases frictional heating increases as more and more of the particle's potential energy (relative to the black hole) is radiated away; the accretion disk of a black hole is hot enough to emit X-rays just outside the event horizon. The large luminosity of quasars is believed to be a result of gas being accreted by supermassive black holes.[3] Elliptical accretion disks formed at tidal disruption of stars can be typical in galactic nuclei and quasars.[4] Accretion process can convert about 10 percent to over 40 percent of the mass of an object into energy as compared to around 0.7 percent for nuclear fusion processes.[5] In close binary systems the more massive primary component evolves faster and has already become a white dwarf, a neutron star, or a black hole, when the less massive companion reaches the giant state and exceeds its Roche lobe. A gas flow then develops from the companion star to the primary. Angular momentum conservation prevents a straight flow from one star to the other and an accretion disk forms instead.

Accretion disks surrounding T Tauri stars or Herbig stars are called protoplanetary disks because they are thought to be the progenitors of planetary systems. The accreted gas in this case comes from the molecular cloud out of which the star has formed rather than a companion star.

Star with accretion disk
Artist's view of a star with accretion disk
This animation of supercomputer data takes you to the inner zone of the accretion disk of a stellar-mass black hole.
This video shows an artist’s impression of the dusty wind emanating from the black hole at the centre of galaxy NGC 3783.

Accretion disk physics

Accretion disk
Artist's conception of a black hole drawing matter from a nearby star, forming an accretion disk.

In the 1940s, models were first derived from basic physical principles.[6] In order to agree with observations, those models had to invoke a yet unknown mechanism for angular momentum redistribution. If matter is to fall inwards it must lose not only gravitational energy but also lose angular momentum. Since the total angular momentum of the disk is conserved, the angular momentum loss of the mass falling into the center has to be compensated by an angular momentum gain of the mass far from the center. In other words, angular momentum should be transported outwards for matter to accrete. According to the Rayleigh stability criterion,

where represents the angular velocity of a fluid element and its distance to the rotation center, an accretion disk is expected to be a laminar flow. This prevents the existence of a hydrodynamic mechanism for angular momentum transport.

On one hand, it was clear that viscous stresses would eventually cause the matter towards the center to heat up and radiate away some of its gravitational energy. On the other hand, viscosity itself was not enough to explain the transport of angular momentum to the exterior parts of the disk. Turbulence-enhanced viscosity was the mechanism thought to be responsible for such angular-momentum redistribution, although the origin of the turbulence itself was not well understood. The conventional -model (discussed below) introduces an adjustable parameter describing the effective increase of viscosity due to turbulent eddies within the disk.[7][8] In 1991, with the rediscovery of the magnetorotational instability (MRI), S. A. Balbus and J. F. Hawley established that a weakly magnetized disk accreting around a heavy, compact central object would be highly unstable, providing a direct mechanism for angular-momentum redistribution.[9]

α-Disk Model

Shakura and Sunyaev (1973)[7] proposed turbulence in the gas as the source of an increased viscosity. Assuming subsonic turbulence and the disk height as an upper limit for the size of the eddies, the disk viscosity can be estimated as where is the sound speed, is the FWHM of the disk, and is a free parameter between zero (no accretion) and approximately one. In a turbulent medium , where is the velocity of turbulent cells relative to the mean gas motion, and is the size of the largest turbulent cells, which is estimated as and , where is the Keplerian orbital angular velocity, is the radial distance from the central object of mass .[10] By using the equation of hydrostatic equilibrium, combined with conservation of angular momentum and assuming that the disk is thin, the equations of disk structure may be solved in terms of the parameter. Many of the observables depend only weakly on , so this theory is predictive even though it has a free parameter.

Using Kramers' law for the opacity it is found that

where and are the mid-plane temperature and density respectively. is the accretion rate, in units of , is the mass of the central accreting object in units of a solar mass, , is the radius of a point in the disk, in units of , and , where is the radius where angular momentum stops being transported inwards.

The Shakura-Sunyaev α-Disk model is both thermally and viscously unstable. An alternative model, known as the -disk, which is stable in both senses assumes that the viscosity is proportional to the gas pressure .[11][12] In the standard Shakura-Sunyaev model, viscosity is assumed to be proportional to the total pressure since .

The Shakura-Sunyaev model assumes that the disk is in local thermal equilibrium, and can radiate its heat efficiently. In this case, the disk radiates away the viscous heat, cools, and becomes geometrically thin. However, this assumption may break down. In the radiatively inefficient case, the disk may "puff up" into a torus or some other three-dimensional solution like an Advection Dominated Accretion Flow (ADAF). The ADAF solutions usually require that the accretion rate is smaller than a few percent of the Eddington limit. Another extreme is the case of Saturn's rings, where the disk is so gas poor that its angular momentum transport is dominated by solid body collisions and disk-moon gravitational interactions. The model is in agreement with recent astrophysical measurements using gravitational lensing.[13][14][15][16]

Magnetorotational instability

Protoplanetary disk HH-30
HH-30, a Herbig–Haro object surrounded by an accretion disk

Balbus and Hawley (1991)[9] proposed a mechanism which involves magnetic fields to generate the angular momentum transport. A simple system displaying this mechanism is a gas disk in the presence of a weak axial magnetic field. Two radially neighboring fluid elements will behave as two mass points connected by a massless spring, the spring tension playing the role of the magnetic tension. In a Keplerian disk the inner fluid element would be orbiting more rapidly than the outer, causing the spring to stretch. The inner fluid element is then forced by the spring to slow down, reduce correspondingly its angular momentum causing it to move to a lower orbit. The outer fluid element being pulled forward will speed up, increasing its angular momentum and move to a larger radius orbit. The spring tension will increase as the two fluid elements move further apart and the process runs away.[17]

It can be shown that in the presence of such a spring-like tension the Rayleigh stability criterion is replaced by

Most astrophysical disks do not meet this criterion and are therefore prone to this magnetorotational instability. The magnetic fields present in astrophysical objects (required for the instability to occur) are believed to be generated via dynamo action.[18]

Magnetic fields and jets

Accretion disks are usually assumed to be threaded by the external magnetic fields present in the interstellar medium. These fields are typically weak (about few micro-Gauss), but they can get anchored to the matter in the disk, because of its high electrical conductivity, and carried inward toward the central star. This process can concentrate the magnetic flux around the centre of the disk giving rise to very strong magnetic fields. Formation of powerful astrophysical jets along the rotation axis of accretion disks requires a large scale poloidal magnetic field in the inner regions of the disk.[19]

Such magnetic fields may be advected inward from the interstellar medium or generated by a magnetic dynamo within the disk. Magnetic fields strengths at least of order 100 Gauss seem necessary for the magneto-centrifugal mechanism to launch powerful jets. There are problems, however, in carrying external magnetic flux inward towards the central star of the disk.[20] High electric conductivity dictates that the magnetic field is frozen into the matter which is being accreted onto the central object with a slow velocity. However, the plasma is not a perfect electric conductor, so there is always some degree of dissipation. The magnetic field diffuses away faster than the rate at which it is being carried inward by accretion of matter.[21]

A simple solution is assuming a viscosity much larger than the magnetic diffusivity in the disk. However, numerical simulations, and theoretical models, show that the viscosity and magnetic diffusivity have almost the same order of magnitude in magneto-rotationally turbulent disks.[22] Some other factors may possibly affect the advection/diffusion rate: reduced turbulent magnetic diffusion on the surface layers; reduction of the Shakura-Sunyaev viscosity by magnetic fields;[23] and the generation of large scale fields by small scale MHD turbulence –a large scale dynamo.

Analytic models of sub-Eddington accretion disks (thin disks, ADAFs)

When the accretion rate is sub-Eddington and the opacity very high, the standard thin accretion disk is formed. It is geometrically thin in the vertical direction (has a disk-like shape), and is made of a relatively cold gas, with a negligible radiation pressure. The gas goes down on very tight spirals, resembling almost circular, almost free (Keplerian) orbits. Thin disks are relatively luminous and they have thermal electromagnetic spectra, i.e. not much different from that of a sum of black bodies. Radiative cooling is very efficient in thin disks. The classic 1974 work by Shakura and Sunyaev on thin accretion disks is one of the most often quoted papers in modern astrophysics. Thin disks were independently worked out by Lynden-Bell, Pringle and Rees. Pringle contributed in the past thirty years many key results to accretion disk theory, and wrote the classic 1981 review that for many years was the main source of information about accretion disks, and is still very useful today.

CNRSblackhole
Simulation by J.A. Marck of optical appearance of Schwarzschild black hole with thin (Keplerian) disk.

A fully general relativistic treatment, as needed for the inner part of the disk when the central object is a black hole, has been provided by Page and Thorne,[24] and used for producing simulated optical images by Luminet[25] and Marck,[26] in which it is to be noted that, although such a system is intrinsically symmetric its image is not, because the relativistic rotation speed needed for centrifugal equilibrium in the very strong gravitational field near the black hole produces a strong Doppler redshift on the receding side (taken here to be on the right) whereas there will be a strong blueshift on the approaching side. It is also to be noted that due to light bending, the disk appears distorted but is nowhere hidden by the black hole (in contrast with what is shown in the misinformed artist's impression presented below).

When the accretion rate is sub-Eddington and the opacity very low, an ADAF is formed. This type of accretion disk was predicted in 1977 by Ichimaru. Although Ichimaru's paper was largely ignored, some elements of the ADAF model were present in the influential 1982 ion-tori paper by Rees, Phinney, Begelman and Blandford. ADAFs started to be intensely studied by many authors only after their rediscovery in the mid-1990 by Narayan and Yi, and independently by Abramowicz, Chen, Kato, Lasota (who coined the name ADAF), and Regev. Most important contributions to astrophysical applications of ADAFs have been made by Narayan and his collaborators. ADAFs are cooled by advection (heat captured in matter) rather than by radiation. They are very radiatively inefficient, geometrically extended, similar in shape to a sphere (or a "corona") rather than a disk, and very hot (close to the virial temperature). Because of their low efficiency, ADAFs are much less luminous than the Shakura-Sunyaev thin disks. ADAFs emit a power-law, non-thermal radiation, often with a strong Compton component.

Black Holes - Monsters in Space
Black hole with corona, an X-ray source (artist's concept).[27]
PIA18467-NuSTAR-Plot-BlackHole-BlursLight-20140812
Blurring of X-rays near Black hole (NuSTAR; 12 August 2014).[27]

Analytic models of super-Eddington accretion disks (slim disks, Polish doughnuts)

The theory of highly super-Eddington black hole accretion, MMEdd, was developed in the 1980s by Abramowicz, Jaroszynski, Paczyński, Sikora and others in terms of "Polish doughnuts" (the name was coined by Rees). Polish doughnuts are low viscosity, optically thick, radiation pressure supported accretion disks cooled by advection. They are radiatively very inefficient. Polish doughnuts resemble in shape a fat torus (a doughnut) with two narrow funnels along the rotation axis. The funnels collimate the radiation into beams with highly super-Eddington luminosities.

Slim disks (name coined by Kolakowska) have only moderately super-Eddington accretion rates, MMEdd, rather disk-like shapes, and almost thermal spectra. They are cooled by advection, and are radiatively ineffective. They were introduced by Abramowicz, Lasota, Czerny and Szuszkiewicz in 1988.

Question, Web Fundamentals.svg Unsolved problem in physics:
Accretion disk QPO's: Quasi-Periodic Oscillations happen in many accretion disks, with their periods appearing to scale as the inverse of the mass of the central object. Why do these oscillations exist? Why are there sometimes overtones, and why do these appear at different frequency ratios in different objects?
(more unsolved problems in physics)

Excretion disk

The opposite of an accretion disk is an excretion disk where instead of material accreting from a disk on to a central object, material is excreted from the center outwards on to the disk. Excretion disks are formed when stars merge.[28]

See also

References

  1. ^ Nowak, Michael A.; Wagoner, Robert V. (1991). "Diskoseismology: Probing accretion disks. I - Trapped adiabatic oscillations". Astrophysical Journal. 378: 656–664. Bibcode:1991ApJ...378..656N. doi:10.1086/170465.
  2. ^ Wagoner, Robert V. (2008). "Relativistic and Newtonian diskoseismology". New Astronomy Reviews. 51 (10–12): 828–834. Bibcode:2008NewAR..51..828W. doi:10.1016/j.newar.2008.03.012.
  3. ^ Lynden-Bell, D. (1969). "Galactic Nuclei as Collapsed Old Quasars". Nature. 280 (5207): 690–694. Bibcode:1969Natur.223..690L. doi:10.1038/223690a0.
  4. ^ Gurzadyan, V. G.; Ozernoy, L. M. (1979). "Accretion on massive black holes in galactic nuclei". Nature. 280 (5719): 214–215. Bibcode:1979Natur.280..214G. doi:10.1038/280214a0.
  5. ^ Massi, Maria. "Accretion" (PDF). Retrieved 2018-07-22.
  6. ^ Weizsäcker, C. F. (1948). "Die Rotation Kosmischer Gasmassen" [The rotation of cosmic gas masses]. Zeitschrift für Naturforschung A (in German). 3: 524–539. Bibcode:1948ZNatA...3..524W. doi:10.1515/zna-1948-8-1118 (inactive 2019-04-11).
  7. ^ a b Shakura, N. I.; Sunyaev, R. A. (1973). "Black Holes in Binary Systems. Observational Appearance". Astronomy and Astrophysics. 24: 337–355. Bibcode:1973A&A....24..337S.
  8. ^ Lynden-Bell, D.; Pringle, J. E. (1974). "The evolution of viscous discs and the origin of the nebular variables". Monthly Notices of the Royal Astronomical Society. 168 (3): 603–637. Bibcode:1974MNRAS.168..603L. doi:10.1093/mnras/168.3.603.
  9. ^ a b Balbus, Steven A.; Hawley, John F. (1991). "A powerful local shear instability in weakly magnetized disks. I – Linear analysis". Astrophysical Journal. 376: 214–233. Bibcode:1991ApJ...376..214B. doi:10.1086/170270.
  10. ^ Landau, L. D.; Lishitz, E. M. (1959). Fluid Mechanics. 6 (Reprint 1st ed.). Pergamon Press. ISBN 978-0-08-009104-4.
  11. ^ Lightman, Alan P.; Eardley, Douglas M. (1974). "Black Holes in Binary Systems: Instability of Disk Accretion". The Astrophysical Journal. 187: L1. Bibcode:1974ApJ...187L...1L. doi:10.1086/181377.
  12. ^ Piran, T. (1978). "The role of viscosity and cooling mechanisms in the stability of accretion disks". The Astrophysical Journal. 221: 652. Bibcode:1978ApJ...221..652P. doi:10.1086/156069.
  13. ^ Poindexter, Shawn; Morgan, Nicholas; Kochanek, Christopher S. (2008). "The Spatial Structure of An Accretion Disk". The Astrophysical Journal. 673 (1): 34–38. arXiv:0707.0003. Bibcode:2008ApJ...673...34P. doi:10.1086/524190.
  14. ^ Eigenbrod, A.; Courbin, F.; Meylan, G.; Agol, E.; Anguita, T.; Schmidt, R. W.; Wambsganss, J. (2008). "Microlensing variability in the gravitationally lensed quasar QSO 2237+0305=the Einstein Cross. II. Energy profile of the accretion disk". Astronomy & Astrophysics. 490 (3): 933–943. arXiv:0810.0011. Bibcode:2008A&A...490..933E. doi:10.1051/0004-6361:200810729.
  15. ^ Mosquera, A. M.; Muñoz, J. A.; Mediavilla, E. (2009). "Detection of chromatic microlensing in Q 2237+0305 A". The Astrophysical Journal. 691 (2): 1292–1299. arXiv:0810.1626. Bibcode:2009ApJ...691.1292M. doi:10.1088/0004-637X/691/2/1292.
  16. ^ Floyd, David J. E.; Bate, N. F.; Webster, R. L. (2009). "The accretion disc in the quasar SDSS J0924+0219". Monthly Notices of the Royal Astronomical Society. 398 (1): 233–239. arXiv:0905.2651. Bibcode:2009MNRAS.398..233F. doi:10.1111/j.1365-2966.2009.15045.x.
  17. ^ Balbus, Steven A. (2003), "Enhanced Angular Momentum Transport in Accretion Disks", Annu. Rev. Astron. Astrophys. (Submitted manuscript), 41 (1): 555–597, arXiv:astro-ph/0306208, Bibcode:2003ARA&A..41..555B, doi:10.1146/annurev.astro.41.081401.155207
  18. ^ Rüdiger, Günther; Hollerbach, Rainer (2004), The Magnetic Universe: Geophysical and Astrophysical Dynamo Theory, Wiley-VCH, ISBN 978-3-527-40409-4
  19. ^ Blandford, Roger; Payne, David (1982). "Hydromagnetic flows from accretion discs and the production of radio jets". Monthly Notices of the Royal Astronomical Society. 199 (4): 883–903. Bibcode:1982MNRAS.199..883B. doi:10.1093/mnras/199.4.883.
  20. ^ Beckwith, Kris; Hawley, John F.; Krolik, Julian H. (2009). "Transport of large-scale poloidal flux in black hole accretion". Astrophysical Journal. 707 (1): 428–445. arXiv:0906.2784. Bibcode:2009ApJ...707..428B. doi:10.1088/0004-637x/707/1/428.
  21. ^ Park, Seok Jae; Vishniac, Ethan (1996). "The Variability of Active Galactic Nuclei and the Radial Transport of Vertical Magnetic Flux". Astrophysical Journal. 471: 158–163. arXiv:astro-ph/9602133. Bibcode:1996ApJ...471..158P. doi:10.1086/177959.
  22. ^ Guan, Xiaoyue; Gammie, Charles F. (2009). "The turbulent magnetic Prandtl number of MHD turbulence in disks". Astrophysical Journal. 697 (2): 1901–1906. arXiv:0903.3757. Bibcode:2009ApJ...697.1901G. doi:10.1088/0004-637x/697/2/1901.
  23. ^ Shakura, N. I.; Sunyaev, R. A (1973). "Black holes in binary systems. Observational appearance". Astronomy and Astrophysics. 24: 337–355. Bibcode:1973A&A....24..337S.
  24. ^ Page, D. N.; Thorne, K. S. (1974). "Disk-Accretion onto a Black Hole. Time-Averaged Structure of Accretion Disk". Astrophys. J. 191 (2): 499–506. Bibcode:1974ApJ...191..499P. doi:10.1086/152990.
  25. ^ Luminet, J. P. (1979). "Image of a spherical black hole with thin accretion disk". Astron. Astrophys. 75 (1–2): 228–235. Bibcode:1979A&A....75..228L.
  26. ^ Marck, J. A. (1996). "Short-cut method of solution of geodesic equations for Schwarzchild black hole". Class. Quantum Grav. 13 (3): 393–. arXiv:gr-qc/9505010. Bibcode:1996CQGra..13..393M. doi:10.1088/0264-9381/13/3/007.
  27. ^ a b Clavin, Whitney; Harrington, J.D. (12 August 2014). "NASA's NuSTAR Sees Rare Blurring of Black Hole Light". NASA. Retrieved 12 August 2014.
  28. ^ Poindexter, Shawn; Morgan, Nicholas; Kochanek, Christopher S (2011). "A binary merger origin for inflated hot Jupiter planets". Astronomy & Astrophysics. 535: A50. arXiv:1102.3336. Bibcode:2011A&A...535A..50M. doi:10.1051/0004-6361/201116907.
  • Frank, Juhan; Andrew King; Derek Raine (2002), Accretion power in astrophysics (Third ed.), Cambridge University Press, ISBN 978-0-521-62957-7
  • Krolik, Julian H. (1999), Active Galactic Nuclei, Princeton University Press, ISBN 978-0-691-01151-6

External links

A0620-00

A0620-00 (abbreviated from 1A 0620-00) is a binary star system in the constellation of Monoceros.

A0620-00 consists of two objects. The first object is a K-type main-sequence star with a spectral type of K5 V. The second object cannot be seen, but based on its calculated mass of 6.6 M☉, it is too massive to be a neutron star and must therefore be a stellar-mass black hole. At a distance of about roughly 3,300 light-years (1,000 parsecs) away, this would make A0620-00 the nearest-known black hole to the Solar System, closer than GRO J1655-40. The two objects orbit each other every 7.75 hours.A0620-00 has undergone two X-ray outbreaks. The first one was in 1917. The second time, in 1975, the burst was detected by the Ariel 5 satellite. During that time, A0620-00 was the brightest X-ray point source. It is now classified as an X-ray nova.The black hole in A0620-00 pulls matter from the K-type star into an accretion disk. The accretion disk emits significant amounts of visible light and X-rays. Because the K-type star has been pulled into an ellipsoidal shape, the amount of surface area visible, and thus the apparent brightness, changes from the Earth's perspective. A0620-00 also bears the variable star designation V616 Monocerotis.

AM Canum Venaticorum

AM Canum Venaticorum (AM CVn) is a hydrogen-deficient cataclysmic variable binary star in the constellation of Canes Venatici. It is the type star of its class of variables, the AM Canum Venaticorum stars. The system consists of a white dwarf gaining matter via an accretion disk from a semi-degenerate or white dwarf companion.

BG Geminorum

BG Geminorum is an eclipsing binary star system in the constellation Gemini. It consists of a K0 supergiant with a more massive but unseen companion. The companion is likely to be either a black hole or class B star. Material from the K0 star is being transferred to an accretion disk surrounding the unidentified object.

CP Lacertae

CP Lacertae (also known as Nova Lacertae 1936 or CP Lac) was a nova, which lit up on June 18, 1936 in the constellation Lacerta. It was discovered independently by several observers including Leslie Peltier in the US and E. Loreta in Italy. The nova reached a peak brightness of 2.1 mag, making it readily visible to the naked eye during night time. Following the outbreak, the brightness of CP Lacertae decreased thereafter, falling 3 magnitudes after nine days.

Located at an estimated distance of 5.4 ± 2.0 kly (1.67 ± 0.61 kpc), this is a close binary system with a degenerate white dwarf primary in orbit with a cool red dwarf secondary over a period of 0.145143 days. Matter from the red dwarf is being drawn off onto an accretion disk orbiting the white dwarf. The mean brightness of the system varies with an amplitude of 0.5 magnitude from day to day. The observational data shows a general period of 0.037 days, which may be related to the rotation period of the white dwarf component.

Cataclysmic variable star

Cataclysmic variable stars (CV) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae, from the Latin 'new', since ones with an outburst brightness visible to the naked eye and an invisible quiescent brightness appeared as new stars in the sky.

Cataclysmic variable stars are binary stars that consist of two components; a white dwarf primary, and a mass transferring secondary. The stars are so close to each other that the gravity of the white dwarf distorts the secondary, and the white dwarf accretes matter from the companion. Therefore, the secondary is often referred to as the donor star. The infalling matter, which is usually rich in hydrogen, forms in most cases an accretion disk around the white dwarf. Strong UV and X-ray emission is often seen from the accretion disc, powered by the loss of gravitational potential energy from the infalling material.

Material at the inner edge of disc falls onto the surface of the white dwarf primary. A classical nova outburst occurs when the density and temperature at the bottom of the accumulated hydrogen layer rise high enough to ignite runaway hydrogen fusion reactions, which rapidly convert the hydrogen layer to helium. If the accretion process continues long enough to bring the white dwarf close to the Chandrasekhar limit, the increasing interior density may ignite runaway carbon fusion and trigger a Type Ia supernova explosion, which would completely destroy the white dwarf.

The accretion disc may be prone to an instability leading to dwarf nova outbursts, when the outer portion of the disc changes from a cool, dull mode to a hotter, brighter mode for a time, before reverting to the cool mode. Dwarf novae can recur on a timescale of days to decades.

Cygnus X-1

Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus, and the first such source widely accepted to be a black hole. 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 2.3×10−23 Wm−2 Hz−1 (2.3×103 Jansky). It remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 14.8 times the mass of the Sun and has been shown to be too small to be any known kind of normal star, or other likely object besides a black hole. If so, the radius of its event horizon has 300 km "as upper bound to the linear dimension of the source region" of occasional X-ray bursts lasting only for about 1 ms.Cygnus X-1 belongs to a high-mass X-ray binary system, located about 6,070 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 degrees, generating the observed X-rays. A pair of jets, arranged perpendicular to the disk, are carrying part of the energy 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 masses. 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. This hypothesis lacks direct empirical evidence but has generally been accepted from indirect evidence.

DI Lacertae

DI Lacertae or Nova Lacertae 1910 was a nova in constellation Lacerta, announced by Thomas Henry Espinell Compton Espin on December 30, 1910. It reached a brightness of 4.6 mag. Its brightness decreased in 37 days by 3 mag. Today its brightness is 14 mag.

Recent modeling analysis of ultraviolet spectra from the Far Ultraviolet Spectroscopic Explorer and International Ultraviolet Explorer spacecraft, find the best fit for DI Lacertae to be a accretion disk with a mass accretion rate of 10−10 solar masses per year with a 30,000 Kelvin white dwarf.

GRO J1719-24

GRO J1719-24 (GRS 1716-249, V2293 Oph, X-Ray Nova Ophiuchi 1993) is supposed to be a low-mass X-ray binary. Its name derives from an X-ray transient, detected in 1993. The system consists of a black hole candidate and a low mass companion, estimated to be a main sequence star of the spectral type K05-V.The rotation period is uncertain, estimated at 14.7h. The light curve possibly exhibits some faster fluctuations as well, which are hypothesized to be produced by blobs of matter in the accretion disk.

HD 61005

HD 61005 is a G8Vk class star in the constellation of Puppis with an associated accretion disk that has helped astronomers understand the process of planetary formation. The particle size and asymmetrical shape of the accretion cloud, have forced a re-evaluation of traditional models of planet formation.

Microquasar

A microquasar, the smaller version of a quasar, is a compact region surrounding a black hole with a mass several times that of our sun, and its companion star. The matter being pulled from the companion star forms an accretion disk around the black hole. This accretion disk may become so hot, due to friction, that it begins to emit X-rays. The disk also projects narrow streams or "jets" of subatomic particles at near-light speed, generating a strong radio wave emission.

NGC 5548

NGC 5548 is a Type I Seyfert galaxy with a bright, active nucleus. This activity is caused by matter flowing onto a 65 million solar mass (M☉) supermassive black hole at the core. Morphologically, this is an unbarred lenticular galaxy with tightly-wound spiral arms, while shell and tidal tail features suggest that it has undergone a cosmologically-recent merger or interaction event. NGC 5548 is approximately 245 million light years away and appears in the constellation Boötes. The apparent visual magnitude of NGC 5548 is approximately 13.3 in the V band.In 1943, this galaxy was one of twelve nebulae listed by American astronomer Carl Keenan Seyfert that showed broad emission lines in their nuclei. Members of this class of objects became known as Seyfert galaxies, and they were noted to have a higher than normal surface brightness in their nuclei. Observation of NGC 5548 during the 1960s with radio telescopes showed an enhanced level of radio emission. Spectrograms of the nucleus made in 1966 showed that the energized region was confined to a volume a few parsecs across, where temperature were around 14000 K and the plasma had a dispersion velocity of ±450 km/s.Among astronomers, the accepted explanation for the active nucleus in NGC 5548 is the accretion of matter onto a supermassive black hole (SMBH) at the core. This object is surrounded by an orbiting disk of accreted matter drawn in from the surroundings. As material is drawn into the outer parts of this disk, it becomes photoionized, producing broad emission lines in the optical and ultraviolet bands of the electromagnetic spectrum. A wind of ionized matter, organized in filamentary structures at distances of 1–14 light days from the center, is flowing outward in the direction perpendicular to the accretion disk plane.The mass of the central black hole can be estimated based on the properties of the emission lines in the core region. Combined measurements yield an estimated mass of 6.54+0.26−0.25×107 M☉. In other words, it is some 65 million times the mass of the Sun. This result is consistent with other methods of estimating the mass of the SMBH in the nucleus of NGC 5548. Matter is falling onto this black hole at the estimated rate of 0.03 M☉ per year, whereas mass is flowing outward from the core at or above the rate of 0.92 M☉ each year. The inner part of the accretion disk surrounding the SMBH forms a thick, hot corona spanning several light hours that is emitting X-rays. When this radiation reaches the optically thick part of the accretion disk at a radius of around 1–2 light days, the X-rays are converted into heat.

NGC 6814

NGC 6814 is an intermediate spiral galaxy in constellation Aquila. It is located at a distance of about 75 million light years from Earth, which, given its apparent dimensions, means that NGC 6814 is about 85,000 light years across. NGC 6814 has an extremely bright nucleus and is a type 1.5 Seyfert galaxy. The galaxy is also a highly variable source of X-ray radiation. The ultraviolet and optical emission also varies, although more smoothly, with time lag of two days. The cause of the lag and the smoothing of light curves is considered to be the reprocessing of the X-rays in the accretion disk. The cause of the active galactic nucleus is suspected to be a supermassive black hole with a mass about 18 million times that of the Sun. Many regions of ionised gas are studded along the dusty spiral arms.

QS Telescopii

QS Telescopii is a faint binary star system in the constellation Telescopium. It is composed of a white dwarf and main sequence donor star, in this case the two are so close and fused into orbit facing one another. Known as polars, material from the donor star does not form an accretion disk around the white dwarf, but rather streams directly onto it. This is due to the presence of the white dwarf's strong magnetic field.

Reverberation mapping

Reverberation mapping is an astrophysical technique for measuring the structure of the broad emission-line region (BLR) around a supermassive black hole at the center of an active galaxy, and thus estimating the hole's mass. It is considered a "primary" mass estimation technique, i.e., the mass is measured directly from the motion that its gravitational force induces in the nearby gas.

Newton's law of gravity defines a direct relation between the mass of a central object and the speed of a smaller object in orbit around the central mass. Thus, for matter orbiting a black hole, the black hole mass is related by the formula

to the RMS velocity ΔV of gas moving near the black hole in the broad emission-line region, measured from the Doppler broadening of the gaseous emission lines. In that formula, RBLR is the radius of the broad-line region; G is the constant of gravitation; and f is a poorly known "form factor" that depends on the shape of the BLR.

While ΔV can be measured directly using spectroscopy, the necessary determination of RBLR is much less straightforward. This is where reverberation mapping comes into play. It utilizes the fact that the emission-line fluxes vary strongly in response to changes in the continuum, i.e., the light from the accretion disk near the black hole. Put simply, if the brightness of the accretion disk varies, the emission lines, which are excited in response to the accretion disk's light, will "reverberate", that is, vary in response. But it will take some time for light from the accretion disk to reach the broad-line region. Thus, the emission-line response is delayed with respect to changes in the continuum. Assuming that this delay is solely due to light travel times, the distance traveled by the light, corresponding to the radius of the broad emission-line region, can be measured.

Only a small handful of AGN (less than 40) have been accurately "mapped" in this way. An alternative approach is to use an empirical correlation between RBLR and the continuum luminosity.

Another uncertainty is the value of f. In principle, the response of the BLR to variations in the continuum could be used to map out the three-dimensional structure of the BLR. In practice, the amount and quality of data required to carry out such a deconvolution is prohibitive. Until about 2004, f was estimated ab initio based on simple models for the structure of the BLR. More recently, the value of f has been determined so as to bring the M-sigma relation for active galaxies into the best possible agreement with the M–sigma relation for quiescent galaxies. When f is determined in this way, reverberation mapping becomes a "secondary", rather than "primary," mass estimation technique.

SU Ursae Majoris

SU Ursae Majoris, or SU UMa, is a close binary star in the northern circumpolar constellation of Ursa Major. It is a periodic cataclysmic variable that varies in magnitude from a peak of 10.8 down to a base of 14.96. The distance to this system, as determined from its annual parallax shift of 4.53 mas, is 719 light years. It is moving further from the Earth with a heliocentric radial velocity of +27 km/s.The variable nature of this star was discovered at the Moscow Observatory by Lidiya Tseraskaya (L. Ceraski) in 1908. It was classified as a U Geminorum-type variable, or dwarf nova. Observation since 1926 showed that this variable undergoes two different types of eruptions: a short maxima lasting around two days that ranged in brightess between 11.6–12.9 magnitude, and a longer maxima extending for 13 days that ranged between 10.4–11.8 magnitude. The later event came to be referred to as 'supermaxima'. Similar dwarf novae of this class have since been discovered, and SU UMa is now the prototype for this sub-category of variable stars.This is a single-lined spectroscopic binary with an orbital period of 1.83 hours. It consists of a white dwarf star that is acquiring matter from its close companion via an accretion disk. This disk is unstable and undergoes periodic outbursts which increase the luminosity of the system. For SU UMa, the accretion rate from the companion is 9.8×10−13 M☉·yr−1. X-ray emission has been detected in the vicinity of the white dwarf, which drops by a factor of four during outbursts. This emission is theorized to come from the boundary layer between the white dwarf and its accretion disk.

Soft X-ray transient

Soft X-ray transients (SXT) are composed of some type of compact object and some type of "normal", low mass star (i.e. a star with a mass of some fraction of the Sun's mass). These objects show changing levels of low-energy, or "soft", X-ray emission, probably produced somehow by variable transfer of mass from the normal star to the compact object. In effect the compact object "gobbles up" the normal star, and the X-ray emission can provide the best view of how this process occurs.Soft X-ray transients Cen X-4 and Aql X-1 were discovered by Hakucho, Japan's first X-ray astronomy satellite to be X-ray bursters.Typical SXTs are usually very faint, or even unobservable, in X-rays and their apparent magnitude in the optical wavelengths is about 20. This is called the "quiescent" state.

In the "outburst" state the brightness of the system increases by a factor of 100-10000 in both X-rays and optical. During outburst, a bright SXT is the brightest object in the X-ray sky, and the apparent magnitude is about 12. The SXTs have outbursts with intervals of decades or longer, as only a few systems have shown two or more outbursts. The system fades back to quiescence in a few months. During the outburst, the X-ray spectrum is "soft" or dominated by low-energy X-rays, hence the name Soft X-ray transients.

SXTs are quite rare, about 100 systems are known. SXTs are a class of low-mass X-ray binaries. A typical SXT contains a K-type subgiant or dwarf that is transferring mass to a compact object through an accretion disk. In some cases the compact object is a neutron star, but black holes are more common. The type of compact object can be determined by observation of the system after an outburst; residual thermal emission from the surface of a neutron star will be seen whereas a black hole will not show residual emission. During "quiescence" mass is accumulating to the disk, and during outburst most of the disk falls into the black hole. The outburst is triggered as the density in the accretion disk exceeds a critical value. High density increases viscosity, which results in heating of the disk. Increasing temperature ionizes the gas, increasing the viscosity, and the instability increases and propagates throughout the disk. As the instability reaches the inner accretion disk, the X-ray luminosity rises and outburst begins. The outer disk is further heated by intense radiation from the inner accretion disk. A similar runaway heating mechanism operates in dwarf novae.

UW Coronae Borealis

UW Coronae Borealis, also known as MS 1603.6+2600, is a low-mass X-ray binary star system in the constellation Corona Borealis. Astronomer Simon Morris and colleagues discovered the X-ray source in 1990 and were able to match it up with a faint star with an average visual magnitude of 19.4. The system is thought to be made up of a neutron star that has an accretion disk that draws material from its companion, a star less massive than the Sun. The disk is asymmetrical. The variability of the system is complex, with several periods identified: the two components orbit each other every 111 minutes, while there is another period of 112.6 minutes. The beat period of these is 5.5 days, which is thought to represent the precession of the asymmetrical accretion disk around the neutron star.

V1315 Aquilae

V1315 Aquilae is a cataclysmic variable star located in the constellation of Aquila. It is a type of cataclysmic variable known as a nova-like variable (NL), specifically a SW Sextantis star. Nova-like variables such as these SW Sextantis stars are characterized as having non-magnetic white dwarfs and do not undergo dwarf-nova eruptions. There is some evidence that SW Sextantis stars may contain a magnetic white dwarf. Being a SW Sextantis star, V1315 Aquilae has a high rate of mass transfer, so it is in steady-state accretion and in a constant state of outburst. It emits the majority of its light in the visible range, which is emitted from the accretion disk. The eclipse depth is 1.8 mag. No description of the donating star has been mentioned.

V603 Aquilae

V603 Aquilae (or Nova Aquilae 1918) was a bright nova occurring in the constellation Aquila in 1918. It is a binary system, comprising a white dwarf and donor low-mass star in close orbit to the point of being only semidetached. The white dwarf sucks matter off its companion, which has filled its Roche lobe, onto its accretion disk and surface until the excess material is blown off in a thermonuclear event. This material then forms an expanding shell, which eventually thins out and disappears.First seen by Zygmunt Laskowski, a medical professor and amateur astronomer, and then confirmed on the night of 8 June 1918 by the UK amateur astronomer Grace Cook, Nova Aquilae reached a peak magnitude of −0.5; it was the brightest nova recorded in the era of the telescope. It was brighter than all stars but Sirius and Canopus. Tycho's and Kepler's supernovae were brighter, but both occurred before the invention of the telescope. Originally a star system with a magnitude of 11.43, it took twelve days to fade three magnitudes and then 18.6 years to fade to quiescence. In 1964 Robert P. Kraft ascertained that it was a binary system, recently determined to be true for several other novae at the time.The star system has settled to an average apparent magnitude of 11.4 since the 1940s, fading by around 1/100 of a magnitude per decade. Spectroscopic analysis conducted by Arenas and colleagues indicated the system consisted of a white dwarf of about 1.2 times as massive as the sun, with an accretion disk, and a companion star with about 20% of the Sun's mass. This second star is most likely a red dwarf. The two stars orbit each other approximately every 3 hours 20 minutes.

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