Accretion discs/1. Observational evidence for accretion discs in the Universe

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    Accretion discs in young stellar objects (YSOs)

    Figure 1: Accretion disc and jet in a proto-star HH30 observed by the Hubble Space Telescope: the jet (in red) is perpendicular to the accretion disc, seen edge-on (a dark region between two bright lobes), © Burrows, STSci/ESA, WFPC2, NASA)
    *K. Wood, M.J. Wolff, J.E. Bjorkman, B. Whitney The HH30 Spectrum: Constraining Circumstellar Dust
    Figure 2: A proto-star star in NGC 1333. Reconstruction of a possible look of a proto-planetery disk based on a Spitzer Space Telescope image. An evidence was found for water vapor in the surrounding area, which appears to be one of the key moments in the development of a planetary system around such a star: icy material is falling from the envelope that birthed the star onto a dense, surrounding disc. Credit: NASA/JPL-Caltech/R. Gutermuth (Harvard-Smithsonian Center for Astrophysics)

    During star formation, the central part of a dense molecular cloud collapses to a proto-star with a gaseous envelope that finally settles to a rotating proto-planetary accretion disc. Sedimentation and self-gravity in such discs trigger the formation of planets and planetary systems. Proto-stars are heavily embedded in surrounding gas and dust and for this reason visible only in the infrared, millimetre or sub-millimetre wavelength bands. Most of the material that goes into forming a star is accreted through a circumstellar disc and in this process the proto-stellar system drives an energetic bipolar jet and outflow into its surroundings. The least evolved proto-stars are surrounded by remnant proto-planetary accretion discs.

    Based on the spectral energy distribution in the infrared and visible light, YSOs are divided into five classes (0-IV), associated with their evolutionary stages. Class 0 refers to collapsing molecular clouds, proto-planetary discs exist in classes I-III, and class IV contains the zero-age main-sequence star.

    A related issue: the extrasolar planets

    Accretion discs in cataclysmic variables (CVs)

    Figure 3: U Gem system, as it would be seen from the Earth. In reality, the image of the system is unresolved, and only the total flux from the secondary (red) and accretion disc (blue) is observed. The primary white dwarf is located at the centre of the accretion disc (too small to be seen). The secondary and the accretion disc periodically eclipse each other, which results in periodic variations in the observed flux (photometry) and spectral features (spectroscopy). From these variations Smak (1971) reconstructed size and shape of the accretion disc.

    CVs are binary star systems consisting of a white dwarf ("primary") and a normal star ("secondary", or "companion"). Typically, the CVs have sizes comparable to the Earth-Moon system, and orbital periods of a few hours. When the outer layers of the companion overflow the "Roche lobe", the companion loses matter through the first Lagrangian point \(L_1\) of the rotating binary system. When the white dwarf is only weakly magnetised, 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 discs are either truncated or absent and accretion occurs along the magnetic field lines.

    There is a solid observational evidence for accretion discs in CVs based on very accurate photometry and spectroscopy.

    Accretion discs in Quasars and other active galactic nuclei (AGN)

    Most galaxies have supermassive (millions to billions of solar masses) black holes at their centres (nuclei). In AGN, the black hole accretion produces radiative power that usually outshines its host galaxy. The accretion disc is surrounded by a hot corona which contains clouds of gas. Fast moving clouds produce broad lines (BRL) and slow moving clouds produce narrow lines (NRL) in the AGN spectra. A large torus of gas and dust partially obscures the central part, which affects the observed appearance of an AGN, as Figure 4 explains.

    Figure 4: AGN unification scheme. Green arrows indicate the AGN type that is seen from a certain viewing angle
    Property Quasars Seyfert Galaxies

    Radio Galaxies

    Galaxy type Spiral, Elliptical Spiral Giant Elliptical Elliptical
    Appearance compact, blue compact, bright nucleus elliptical bright, star-like
    Maximum luminosity 100-1,000 Milky Way comparable to bright Spirals strong radio 10,000 Milky Way
    Continuum spectrum non-thermal non-thermal non-thermal non-thermal
    Absorbtion lines yes none yes none
    Variability days to weeks days to weeks days hours
    Radio emission some weak strong weak
    Redshifts z > 0.5 z ~ 0.5 z < 0.05 z ~ 0.1

    AGN 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) or galaxies with just narrow emission lines (Seyfert 1, where the dust torus obscures the BLR) and their counterparts in the radio loud family, the broad line (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, as well as 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. Therefore, despite their name, quasi-stellar objects are 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.

    Accretion discs in Microquasars and X-ray binaries

    Figure 5: Microquasars found in several X-ray binaries in our Galaxy are scaled down versions of quasars, as pointed out by Felix Mirabel, who also coined the name "microquasar". This figure first appeared in several of Mirabel's articles.
    Figure 6: Black hole X-ray binaries in our Galaxy. The figure shows the companion star and the accretion disc drawn to scale. For a comparison, the Sun - Mercury distance is shown. (Figure by Jerome Arthur Orosz).

    Main-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 have 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 (in figure 6, systems with coloured companion), and the high-mass X-ray binaries, HMXB (in figure 6, systems with white companion), where soft X-ray transients and X-ray pulsars are respective sub-classes. Soft X-ray transients with NSs or 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.

    Accretion discs in gamma ray bursts (GRBs)

    Figure 7: Afterglow of the gamma-ray burst captured in X-ray (left) and UV/optical (right) on 19.03.2008. At peak, this GRB was visible with the naked eye. (Credit: NASA/Swift/Stefan Immler)

    The most energetic explosions seen in the universe are gamma-ray bursts. They are short, collimated flares of low-energy \(\gamma\)-rays with relativistic emission simultaneously also at longer wavelengths (e.g., X-ray flashes, radio jets). Observations indicate that GRBs are cosmological and followed by slowly fading afterglows. The duration of GRB prompt emission can last from 0.01 - 2 seconds (short bursts) up to 2 - 500 seconds (long bursts) and may be explained by merging compact objects or failed supernovae (collapsars), respectively. Afterglows on the other hand are observed and monitored from a couple of days up to several years. All evidence on the origin of the inner engines (i.e., mergers, collapses, pulsars) of GRBs is deduced indirectly. Energetic requirements suggest, however, a similar configuration of the end products: the formation of a solar-mass black hole surrounded by a massive debris disc (~ 0.1 Msun) with a huge accretion rate. The time scale of the burst is determined by the accretion time of this disc. According to these time scales accretion discs in GRBs are most likely hyper-accreting. This means, the temperatures and densities at the required accretion rates are such, that neutrino production is switched on and the electrons are mildly relativistic and degenerate. Generally, GRBs seem to show similarities to radio-loud AGN and galactic microquasars, since all these systems eject strongly collimated, more or less relativistic flows of matter and involve accretion onto a black hole.


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