X-ray nova

from Wikipedia, the free encyclopedia

An X-ray nova is a short-period X - ray binary star of low mass with orbital periods of a few hours. The intensity of the X-rays increases during the rare outbreaks by a factor of 100 to 10,000,000 over a period of days to weeks. In parallel with the increase in X-ray radiation , the optical brightness also increases by 6 to 10  mag . Some X-ray novae are considered to be the best known cases for black holes with stellar masses , since the masses of the primary stars are well above three solar masses. X-ray novae are also known as Soft X-ray Transients (SXT).

properties

The course of the light curve is similar to that of Novae . However, a thousand times more radiation is emitted in the X-ray range than in the optical range, which is why these events were referred to as "X-ray novae". The radiation arises in an accretion disk or in a shock front near a neutron star or a black hole. However, since normal novae also emit X-rays in their late phase, the term "soft X-ray transient" has become established. In the case of novae, the X-rays, in contrast to the X-ray novae, arise as a direct result of the hydrogen burning on the surface of a white dwarf .

X-ray luminosity

The X-ray luminosity of the X-ray novae is between 10 23 and 10 25  W in the rest phases and the double stars spend more than 95 percent of the time at a minimum. Within days to weeks, the luminosity increases to values ​​of 10 29 to 10 32  W and the outbreak lasts for months to years. At the beginning of the eruption, mostly hard X-rays are seen, which become softer and softer as the eruption progresses. In parallel with the increase and decrease in the X-ray brightness, the optical brightness also changes by 6 to 10 mag. In the outbreak, the optical spectrum is dominated by emission lines , while at the minimum the spectrum of a late main sequence star or subgiant with the spectral classes K or M is shown. In the binary star systems, very high radial speeds of up to 800 km / s and orbital times of hours to days are observed as a minimum . A very high mass of the invisible companion of the main sequence star can be derived from this and it is likely to be neutron stars or black holes . In the infrared, most of the radiation comes from the main sequence star or subgiant. The brightness fluctuates with the phase of the orbit in the minimum and is caused by an ellipsoidal change of light . The strong deformation of the non-degenerate star confirms the high mass of the primary star.

outbreaks

The outbreaks repeat themselves with cycle lengths of decades. Since there are no observations of X-rays over long periods of time, these have been obtained from old optical sky surveillance . This means that the eruptions will not change the binary star system dramatically. During and after the eruptions, superhumps can be detected in some X-ray novae . The periods of these modulations of the light curve deviate by a few percent from the period of rotation and are the result of a precession of the accretion disc.

During the eruptions, radio radiation from the X-ray novae could be detected, with some SXTs at high resolution being able to observe jets like the microquasars . The jets always form parallel to an outbreak and are not active in the minimum phases.

Origin of the optical radiation at the maximum

The maximum of the optical radiation probably arises from two sources. Firstly through matter which has been heated up by the X-rays and which radiates this radiation again in the UV and optical. Second, the optical radiation begins to increase even before the X-ray radiation in the outbreak. The lighting up seems to begin in the optical and spread through the UV to the X-rays. In addition, there is also a short-term variability in the visual and ultraviolet in the order of seconds, which is referred to as " flickering ". Flickering is associated with accretion from the inner rim of an accretion disk onto a degenerate star.

Rest periods

During the rest phases, some X-ray novae show a low variability of the X-ray radiation. The X-ray luminosity can  rise to values ​​of up to 10 27 W for a period of a few hours to days and then decrease again to the normal resting value. These events are referred to as accretion flares, because matter can temporarily fall on the neutron star even during the rest phases.

Quasi-Periodic Oscillations (QPO)

In the case of X-ray novae, quasi - periodic oscillations (QPO) could be detected in the area of ​​soft X-ray radiation . These oscillations are typical of X-ray binary stars and show broad maxima with increased intensity in the spectra. While candidates for black holes show no oscillations above 100  Hz , frequencies of up to a few kHz are observed in identified neutron stars. The QPOs likely arise at or near the inner edge of the accretion disk.

X-ray novae and red dwarfs

No cover light change by a red dwarf could be observed from X-ray novae . Since the X-rays and most of the optical radiation originate in the immediate vicinity of the compact star , it can be deduced from this that the accretion disk is quite thick. Therefore, in systems with a low orbit inclination, the electromagnetic radiation is almost completely absorbed by the pane and does not appear to the observer as an X-ray nova.

Breakout Mechanism

X-ray novae consist of a companion star that fills its Roche volume and transfers matter via the Lagrange point L1 to a neutron star or a black hole. Due to the preservation of the torque, an accretion disk forms around the compact star, in which the plasma loses energy due to internal friction and falls onto the compact star. During the rest phases, around 10 −12 to 10 −10 solar masses per year are transferred from the companion star to the accretion disk, while during the eruption up to 10 −8 solar masses per year flow to the compact star. The variability of the accretion rate of the compact star is caused by a change in viscosity in the accretion disk due to bistability of magnetorotational instability . The outbreaks of the X-ray novae are therefore analogous to the dwarf nova eruptions , which mainly emit electromagnetic radiation in the optical spectral range. Because the compact star in X-ray novae is a neutron star or a black hole with a lower gravitational potential than in the dwarf novae , where the compact star is a white dwarf , the electromagnetic radiation is emitted at shorter wavelengths in the range of the X-ray radiation. There is also an alternative hypothesis, according to which the instability in the mass transfer rate is controlled by the companion star.

Formation of X-ray novae

The mean distance between the compact star and its mass-donating companion is around ten solar radii . This is considerably less than the radius of the previous star, which formed a neutron star or a black hole after a supernova explosion . A common envelope scenario is usually assumed in the literature . After that, the precursor star of the compact star expands so far that the companion star is immersed in its atmosphere and the distance between the two stars is reduced due to friction. However, the mass distribution of the companion stars of X-ray novae does not match the simulated results. While the calculations suggest that low-mass companions mostly merge with the red giant and more massive companion stars survive a common shell phase, most companions of the compact star are K or M dwarfs .

Examples

  • A0620−00 = Nova Monocerotis 1975 = V 616 Mon
  • H 1705-250 = Nova Ophiuchi 1977 = V2107 Oph
  • Nova Muscae 1991 = GU Mus

Individual evidence

  1. ↑ Laura Kreidberg et al .: MASS MEASUREMENTS OF BLACK HOLES IN X-RAY TRANSIENTS: IS THERE A MASS GAP? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.1805 .
  2. ^ YJ Yang, AKH Kong, DM Russell, F. Lewis, R. Wijnands: Quiescent X-Ray / Optical Counterparts of the Black Hole Transient H 1705-250 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.2417 .
  3. ^ Daniel R. van Rossum: Massive NLTE models for X-ray novae with PHOENIX . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.0846 .
  4. ↑ Juthika Khargharia et al: The Mass of the Black Hole in XTE J1118 + 480 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.2786 .
  5. ↑ DE Calvelo et al: Doppler and modulation tomography of XTE J1118 + 480 in quiescence . In: Astrophysics. Solar and Stellar Astrophysics . 2009, arxiv : 0905.1491 .
  6. ↑ Ling Zhu, Rosanne Di Stefano, Lukasz Wyrzykowski: Results from Long-Term Optical Monitoring of the Soft X-Ray Transient SAX J1810.8-2609 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.7570 .
  7. ↑ R. Farinelli et al .: Spectral evolution of the X-ray nova XTE J1859 + 226 during its outburst observed by BeppoSAX and RXTE . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.1270 .
  8. ↑ Rudy Wijnands, Nathalie Degenaar: A low-level accretion flare during the quiescent state of the neutron-star X-ray transient SAX J1750.8-2900 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.3091v1 .
  9. ↑ M. Cadolle Bel et al .: Detailed Radio to Soft gamma-ray Studies of the 2005 Outburst of the New X-ray Transient XTE J1818 −245 . In: Astrophysics. Solar and Stellar Astrophysics . 2009, arxiv : 0903.4714 .
  10. ↑ JM Corral-Santana et al: A Black Hole Nova Obscured by an Inner Disk Torus . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.0034v1 .
  11. ↑ VF Suleimanov, GV Lipunova, NI Shakura: Modeling of non-stationary accretion disks in X-ray novae A0620 -00 and GRS 1124 -68 during outburst . In: Astrophysics. Solar and Stellar Astrophysics . 2008, arxiv : 0805.1001 .
  12. ↑ GV Lipunova, NIShakura: Non-steady state accretion disks in X-ray novae: Outburst Models for Nova Monocerotis 1975 and Nova Muscae 1991 . In: Astrophysics. Solar and Stellar Astrophysics . 2009, arxiv : 0905.2515 .
  13. ↑ Grzegorz Wiktorowicz, Krzysztof Belczynski, Thomas J. Maccarone: Black Hole X-ray Transients: The Formation Puzzle . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1312.5924v1 .