Observational evidence for accretion discs in the Universe
From Scholarpedia
| This article has not been peer-reviewed or accepted for publication yet; It may be unfinished, contain inaccuracies, or unapproved changes. | ||||||||||||||||||||
Author: Dr. Marek A. Abramowicz, Physics Department, Göteborg University, Sweden and N. Copernicus Astronomical Center, PAN, Warsaw, Poland
Author: Miss Odele Straub, N. Copernicus Astronomical Center PAN, Warsaw, Poland
[edit] Accretion discs in young stellar objects (YSOs)
[edit] Accretion discs in cataclysmic variables (CVs)
[edit] Accretion discs in Quasars and other active galactic nuclei (AGNs)Most galaxies have supermassive (millions to billions solar masses) black holes at their centers (nuclei). In AGNs, the black hole accretion produces radiative power that usually outshines its host galaxy. The accretion disc is surrounded by a hot corona that contains clouds of gas. These which move fast produce broad lines (BRL) and these which move slow produce narrow lines (NRL) in the AGNs spectra. The large torus of gas and dust partially obscures the central part, which is important for the observed appearance of an AGN, as Figure 3 explains.
AGNs are generally divided into two families, "radio loud" and "radio quiet", depending on whether they exhibit jets or not. The viewing angle gives rise to several AGN types that are distinguished by their emission properties. Among them, the spiral galaxies with broad and narrow emission lines (Seyfert 2 galaxies) or just narrow emission lines (Seyfert 1 galaxies, where the dust torus obscures the BLR); the counterparts in the radio loud family are broad (BLRG viewing angle above ~60°) and narrow line radio galaxies (NLRG viewing angle below ~60°) with jets and radio lobes perpendicular to the accretion disc; and radio loud and radio quiet quasars (i.e. quasi-stellar objects). The latter are the most luminous beacons in the universe and observed up to highest redshifts, implying cosmological distance and gigantic energy output. Despite their name, quasi-stellar objects are thus anything but stars. However, because quasars shine at such large distances, it is not possible to resolve the bright core. Recent observations detect jets and nebulosity around some of them. [edit] Accretion discs in Microquasars and X-ray binariesMain-sequence secondary stars in orbit with accreting neutron stars or black holes (neutron star binaries or black hole binaries, respectively) are common objects in the Galaxy. Neutron stars are often magnetised, especially young ones, such that their accretion discs are disrupted by magnetic fields or do not exist at all. Matter in these cases is lead by partial or total column accretion to the compact object. In comparison to CVs their spectral energy distribution is observed up to the X-ray regime, since neutron stars and black holes own a much stronger gravitational potential. X-ray binaries are, depending on the mass of the companion star, roughly divided into two categories, the low-mass X-ray binaries (LMXBs) and the high-mass X-ray binaries (HMXB), where soft X-ray transients and X-ray pulsars are respective sub-classes. Soft X-ray transients with both NSs and BHs show quasi-periodic outbursts. Many, if not all, black hole X-ray binaries exhibit in addition relativistic twin jets that propagate along the rotational axis of the compact object and are called microquasars. [edit] Accretion discs in gamma ray bursts (GRBs)The most energetic explosions in the universe are gamma-ray bursts. Models that describe GRBs as a result of merging compact objects or failed supernova (collapsars) generally predict a similar configuration of the end products: the formation of a solar-mass black hole surrounded by a massive debris disc with a huge accretion rate. |
of the rotating binary system. When the white dwarf
is only weakly magnetized, the matter forms an accretion disc around
it and eventually reaches its surface. "Dwarf novae" (DN) are CVs that
show outbursts lasting for about a week and separated by weeks to months
of quiescence. U Gem is the prototype of dwarf novae. The brightness
in the visible light of U Gem increases 100-fold every 120 days or so,
and returns to the original level after a week or two. The DN phenomenon
is due to a specific accretion disc limit-cycle instability, tidal torques,
and fluctuations in the mass-transfer rate from the secondary.
The geometry of accretion is very different in magnetic CVs, where
accretion disks are truncated or absent, and accretion occur along
magnetic field lines.
| Invited by: | Dr. Eugene M. Izhikevich, Editor-in-Chief of Scholarpedia, the peer-reviewed open-access encyclopedia |
