Ultra-luminous X-ray source

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Ultra-luminous X-ray sources ( English ultra-luminous X-ray source , abbreviated to ULX ) are observed as point-like X-ray sources in neighboring galaxies that are not positioned in the centers of the galaxies and have a luminosity that exceeds the Eddington limit for spherical accretion on a compact object of 10 solar masses with 10 38  erg / s in the range from 0.3 to 10 keV. They don't get most of their energy from thermonuclear reactions . No ultra-luminous X-ray source is known in the Milky Way . It is believed that the X-ray radiation is caused by the accretion of matter onto black holes in X-ray binary stars .

properties

When identifying ultra-luminous X-ray sources, other objects are initially excluded, such as active galactic nuclei behind the galaxy, young supernova remnants in the galaxy, and foreground objects such as cataclysmic variables and chromospherically active stars in the Milky Way. The remaining X-ray sources have the following properties:

  • The X-ray luminosity can reach up to 2 · 10 42 erg / s , whereby these sources are sometimes also referred to as hyper-luminous X-ray sources and ultra-luminous X-ray sources are only spoken of at luminosities of 10 39 -10 41 erg / s .
  • The X-rays are variable with fluctuations in intensity and spectrum. As with other candidates for accretive black holes such as X-ray novae , transitions between a hard, low-intensity spectrum and a soft, high-intensity spectrum are observed. The variability takes place on time scales from seconds to years.
  • The spectrum in the range of X-rays can be described as a first approximation by a simple power law . A small excess compared to the power law is often observed below 2 keV. This excess is interpreted either as a result of a discharge in the form of jets or as an emission from the outer cold area of ​​an accretion disc. Above 4 keV the validity of the power law usually breaks down.
  • Some ULX with maximum X-ray luminosities of a few 10 39  ergs show periodic luminosity changes in the order of magnitude of a few hours, such as X-ray binary stars of the type LXMB. In the case of ULX with higher luminosity, however, no long-term stable changes in brightness have been observed. As with all types of X-ray binary stars, dips are also detected with ultra-luminous X-ray sources in which the X-ray brightness drops by up to a factor of 10 within a few hours or days and returns to the previous brightness in the same period of time. In the case of X-ray binary stars, this behavior is interpreted as shadowing of the X-ray radiation by structures in the disk or in the stellar wind of the mass-donating companion star.
  • Quasi-periodic oscillations of the X-ray radiation show frequencies in the range from one to 150  mHz , as they are also observed in X-ray binary stars with black holes. The distribution of the frequencies of the QPOs suggests a high mass of the black holes.
  • The ultraviolet radiation from the location of the ultraviolet x-ray sources sometimes shows signs of accretion of stellar wind from a Wolf-Rayet star and sometimes signs of mass transfer when a star in a binary system crosses its Roche limit.
  • Optical radiation from the location of ultra-luminous X-ray sources usually has a blue color index . However, this does not have to be interpreted as a massive blue star as the source of the accreted gas, since it can also be radiation from the accretion disk, which is heated by the X-rays. Since the reprocessed radiation of the accretion disk exceeds the optical brightness of the star by up to 5 magnitudes , only companions with the spectral type O can be excluded.
  • Ionized nebulae have been found around some ultra- luminous X-ray sources , the size and luminosity of which clearly exceed those of supernova remnants. The lines of He II and N V are also observed in active galactic nuclei, where these are interpreted as the result of photoionization by hard X-rays.
  • 75 percent of the ultra-luminous X-ray sources are found in or near areas with active star formation in the galaxies. Accordingly, these X-ray sources are less frequently discovered in elliptical galaxies . In recent years, at the location of the ultra-luminous X-ray sources, young open star clusters or globular star clusters with an age of around 10 million years have been found. Since the star formation rates in interacting galaxies are quite high, a correspondingly high number of ultra-luminous X-ray sources are observed there. The stars in the vicinity of this ULX are usually on the order of 10 million years old.
  • Ultra-luminous X-ray sources are more likely to occur in metal-poor galaxies with a proportion of heavy elements that is less than five percent of the value of the sun.
  • A subgroup of the ultra-luminous X-ray sources are the super-soft, ultra-luminous X-ray sources, whose X-ray spectrum is dominated by radiation with energies below 2000 electron volts . These spectra can be described both as accretion disks with high inclination around black holes with stellar masses and as cool disks around black holes with masses of 1,000 to 10,000 solar masses.

Classification

Ultra-luminous X-ray sources are classified as follows based on their luminosity in the X-ray range:

  • In the 10 39 –2 * 10 40 erg / s range as standard ultra-luminous X-ray sources or sULX
  • Up to 10 41  erg / s as extremely ultra-luminous X-ray sources or eULX
  • With values ​​beyond 10 42  erg / s as hyper-luminous ULX or HLX

In addition to their luminosity, the classes also differ in their X-ray spectrum and in their quasi-periodic variability.

Hypotheses

The interest in the ultra-luminous X-ray sources is due to the crossing of the Eddington limit for known companions in X-ray binary stars. Each star can then gain maximum luminosity from accretion before the radiation pressure accelerates the incident matter away from the compact object and thus sets an upper limit for the accretion rate and luminosity for the respective mass. Therefore, the following hypotheses have been developed for the ultra-luminous X-ray sources:

  • They are accreting black holes with masses greater than 100 solar masses. This hypothesis is often viewed with skepticism, as there are no other signs of black holes with a mass above 20 solar masses that can arise directly from a nuclear collapse supernova . Only black holes with masses well above 100,000 or more solar masses have been found in the active cores of galaxies. Medium-mass black holes could have resulted from the merger of black holes with stellar masses.
  • They are accreting black holes with masses of 5–20 solar masses, whose X-rays are not emitted isotropically , but whose emission is strongly directed.
  • They are accretive black holes with masses of 5–20 solar masses, whose radiation exceeds the Eddington limit. A slight excess of the Eddington luminosity is known from the galactic microquasars V4641 Sgr and GRS 1915 + 105 .
  • They are accreting black holes with masses of up to 80 solar masses, which have arisen from extremely low-hydrogen stars and emit just above their Eddington luminosity. With a low metallicity , only small stellar winds arise , and therefore more massive stars lose less matter in their final stellar phases before they explode as a core collapse supernova and a black hole remains.
  • A hybrid model from these alternatives:

In addition to these hypotheses about unusual X-ray binary stars, it is also discussed whether they are

    • a former central black hole of a smaller galaxy that has since merged with the galaxy could be,
    • a supernova of the type IIn could act (although an optical counterpart of a supernova has never been observed at the location of an ultra-luminous X-ray source),
    • a smaller black hole that ricocheted off the central black hole of the galaxy and accelerated to speeds of a few 1000 km / s,
    • young rotation-driven x-ray pulsars could act some 100 to 1000 years after the supernova explosion . A pulsar wind nebula can also contribute to the X-ray brightness . It is estimated that around three percent of the ULX is powered by the rotational energy of young neutron stars .

Ultra-luminous X-ray sources are probably not a homogeneous group. In particular, the hyper-luminous X-ray sources show different properties such as a harder X-ray spectrum and greater variability in the range from seconds to hours. They are considered to be good candidates for accretive black holes with mean masses in the range of a few hundred to 10,000 solar masses.

Examples

  • M82 X-2 and M82 X-1
  • XMMUJ004243.6 + 412519 in M31
  • HLX-1 in ESO 243-49
  • CXOU133705.1-295207 in M83

Individual evidence

  1. ↑ P. Esposito, SE Motta, F. Pintore, L. Zampieri, L. Tomasella: Swift observations of the ultraluminous X-ray source XMMUJ004243.6 + 412519 in M31 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.5099 .
  2. ↑ PG Jonker et al. a .: THE NATURE OF THE BRIGHT ULX X-2 IN NGC 3921: A Chandra POSITION AND HST CANDIDATE COUNTERPART . In: Astrophysics. Solar and Stellar Astrophysics . 2012 arxiv : 1208.4502 .
  3. ^ NA Webb u. a .: THE ACCRETION DISC, JETS AND ENVIRONMENT OF THE INTERMEDIATE MASS BLACK HOLE CANDIDATE ESO 243-49 HLX-1 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.0831 .
  4. ^ MD Caballero-Garcia et al. a .: The aperiodic variability of the Ultraluminous X-ray source in NGC 5408 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.3965 .
  5. ↑ David Cseh et al. a .: BLACK HOLE POWERED NEBULAE AND A CASE STUDY OF THE ULTRALUMINOUS X-RAY SOURCE IC342 X-1 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1201.4473 .
  6. ↑ Hua Feng, Roberto Soria: Evolution of the spectral curvature in the ULX Holmberg II X-1 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1202.1102 .
  7. ↑ F. Grise et al. a .: A long-term X-ray monitoring of the ultraluminous X-ray source NGC 5408 X-1 with Swift reveals the presence of dips but no orbital period . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.1810v1 .
  8. ↑ JJEKajava et al. a .: Ultraluminous X-ray Sources in the Chandra and XMM-Newton Era . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1109.1610 .
  9. ↑ Joel N. Bregman et al. a .: Ultraviolet Spectra of ULX Systems . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.0424 .
  10. ^ Roberto Soria et al. a .: The Birth of an Ultra-Luminous X-ray Source in M83 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.2335 .
  11. ↑ Jeanette C. G Ladstone et al. a .: OPTICAL COUNTERPARTS OF THE NEAREST ULTRALUMINOUS X-RAY SOURCES . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.1213v1 .
  12. ↑ CT Berghea, RP Dudik: Spitzer Observations of MF 16 Nebula and the associated Ultraluminous X-ray Source . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.4276 .
  13. ↑ TP Roberts et al. a .: A VARIABLE ULTRALUMINOUS X-RAY SOURCE IN A GLOBULAR CLUSTER IN NGC 4649 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.5163 .
  14. ^ S. Mineo et al. a .: SPATIALLY RESOLVED STAR FORMATION IMAGE AND THE ULX POPULATION IN NGC2207 / IC2163 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.4084 .
  15. ↑ AH Prestwich et al. a .: Ultra-Luminous X-Ray Sources in the Most Metal Poor Galaxies . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.6203v1 .
  16. ↑ Lian Tao et al. a .: CHANDRA AND HST OBSERVATIONS OF THE SUPERSOFT ULX IN NGC 247: CANDIDATE FOR STANDARD DISK EMISSION . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.1148 .
  17. ^ Jeanette C. Gladstone: The sub-classes of ultraluminous X-ray sources . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.6886v1 .
  18. ↑ Xu Han et al. a .: Confirming the 115.5-day periodicity in the X-ray light curve of ULX NGC 5408 X-1 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.3883 .
  19. ↑ M. Heida et al. a .: Accurate positions for the ULXs NGC 7319-X4 and NGC 5474-X1 and limiting magnitudes for their optical counterparts . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.0597 .
  20. ↑ Aleksei S. Medvedev, Juri Poutanen: Young rotation-powered pulsars as ultra-luminous X-ray sources . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.6079v1 .
  21. ↑ Andrew D. Sutton et al. a .: The most extreme ultraluminous X-ray sources: evidence for intermediate-mass black holes? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.4100 .