Soft X-ray transient

An X-ray Nova is a short-period low-mass X-ray binary star with circulation periods of several hours. The intensity of the X-ray radiation increases during the few bursts by a factor of 100 to 1.000.0000 over a period of days to weeks. Parallel with the increase of the photon beam, the optical brightness increases by 6 to 10 mag. The X-ray novae are also referred to as soft X -ray transients ( SXT ).

Properties

The X-ray luminosity of X-ray novae is during rest periods 1030-1032 erg / s and the double stars spend about 95 percent of the time in min. Within days to weeks, the luminosity increases to values ​​of 1036 to 1039 erg / s and the eruption lasts for months to years. At the beginning of the outbreak shows predominantly hard X-rays, which will soften over the course of the eruption. Parallel to the rise and fall of the X-ray luminosity to the optical brightness changes by 6 to 10 mag. Because the shape of the light curve resembles those of novae and about a thousand times more X-ray is emitted as optical radiation X-ray Nova these events were called.

In outburst, the optical spectrum is dominated by emission lines, while the minimum shows the spectrum of a late main -sequence star or sub Riesens with the spectral types K or M. In the binary systems very high radial velocities of up to 800 km / s and current lives are observed from hours to days at a minimum. This can be a very high mass of the invisible companion of the main sequence star are derived and it is likely to be either neutron stars or black holes. In the infrared the radiation mainly comes from the main sequence star or subgiant. The brightness varies with the phase of circulation at the minimum and is caused by an ellipsoidal light variation. The strong deformation of the non-degenerate star confirmed the high mass primary star. Some X-ray novae are considered the most secure cases for black holes with stellar mass, since the mass of the primary star are significantly above three solar masses.

The optical radiation in the maximum likely arises from two sources. One of matter, which has been heated by the X radiation and emits radiation in the UV and optical again. Second, the optical radiation begins before the X-ray radiation in the outbreak to rise. In this case, the illumination in the optical seems to start and to spread over the UV to X-rays. In addition, also shows a short-term variability in the visible and ultraviolet in the order of seconds, which is called Flickering. Flickering is associated with an accretion from the inner edge of an accretion disk onto a degenerate star.

The outbreaks recur with cycle lengths of decades. Since there are no observations over long periods of X-rays are those obtained from old sky optical monitoring. This means that the eruptions do 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 differ by several per cent of the orbital period and are the result of the precession of the accretion disk.

During outbreaks radio emission was detected from the X-ray novae, which at high resolution Jets could be observed as in the microquasars in some SXTs. The jets are formed always parallel to an outbreak and are not active in the minimum phase.

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

From X-ray novae not cover light change could be observed by the red dwarf. Since the X-rays and most of the optical radiation in the immediate vicinity of the compact star is formed it can be deduced that the accretion disk is quite thick. Therefore, the electromagnetic radiation is absorbed in almost all systems with a low orbit inclination of the disk and the observer does not appear as X-ray Nova.

During the resting phase, some X-ray novae show a low variability of the X-rays. Here, the X-ray luminosity for the period of several hours to days to values ​​of up to 1034 erg / s rise and then fall back to the normal resting value. These events are referred to as Akkretionsflares, because even during the sleep phase, it may temporarily come to an accretion of matter onto the neutron star.

Outbreak mechanism

X-ray novae consist of a volume filling its Roche companion star, the matter about the L1 Lagrange point to a neutron star or a black hole transferred. Due to the conservation of the torque forms around the compact star an accretion disk, in which the plasma due to internal friction loses energy and falls onto the compact star. During the rest periods are around the 10-12 to 10-10 solar masses per year transferred from the companion star to the accretion disk, while the outbreak flow up to 10-8 solar masses per year on 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 a bistability of the magneto rotational instability. The outbreaks of X-ray novae are therefore analogous to the dwarf nova eruptions which emit predominantly in the optical spectral electromagnetic radiation. Because in X-ray novae, the compact star is a neutron star or a black hole with a deeper gravitational potential than the dwarf novae, where the compact star is a white dwarf, the electromagnetic radiation is emitted at shorter wavelengths in the range of X-rays. There is also an alternative hypothesis that the instability is controlled in the mass transfer rate from the companion star.

Formation

The mean distance between the compact star and the mass -giving companion is about ten solar radii. This is considerably less than the radius of the progenitor star, which has formed after a supernova explosion a neutron star or a black hole. Usually a common envelope scenario is assumed in the literature. After that the progenitor star of the compact star expands so far that the companion star is immersed in its atmosphere and reduces friction by the distance between the two stars. However, the mass distribution of the companion stars of X-ray novae does not fit the simulated results. While the calculations can be that low-mass companion usually merge with the Red Giant and more massive companion stars survive a common envelope phase suggests that most of the compact companion star are K or M dwarfs.

Distinction between

Since the concept of soft X -ray transient has also emit normal novae in their late-stage X-ray radiation is established. In novae X-ray radiation is produced as a direct result of hydrogen burning on the surface of a white dwarf and not in an accretion disk or in a shock front near a neutron star as the X-ray novae.

Examples

• H 1705-250 = Nova Ophiuchi 1977 = V2107 Oph
• Nova Muscae 1991 = GU Mus
• Aql X-1
• A0620 -00 = Nova Monocerotis 1975 = V 616 Mon