Very faint X-ray binary star

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Very Faint X-ray Transients (dt. Translation very faint temporary X-ray sources) are X-ray binaries , whose luminosity in the outbreak values between 10 34 and 10 36 / s erg are obtained and their X-ray brightness is lower in the idle phases at least a factor 10. The intensity of their X-ray radiation in the outbreak is on average 100 times lower than with the classic soft X-ray transients , which show a similar X-ray spectrum as the very faint X-ray transients.

Breakout Mechanism

From the X-ray spectrum and the light curve was derived that it is neutron stars or black holes in binary systems is that a very low mass transfer rate of 10 -13 solar masses per year accrete . The X-ray radiation is emitted in the inner area of ​​the accretion disc , which heats up to a few million K due to its viscosity . In the case of neutron stars, the X-rays are also produced as bremsstrahlung when the incident matter hits backed up matter above the surface of the neutron star and is abruptly slowed down. The light curve of the Very Faint X-ray Transients shows that the X-ray binary star systems remain in the minimum for most of the time and less than 5 percent in the status of the eruption. The outbreak mechanism of the Very Faint X-ray Transients corresponds to that of the X-ray novae , in which the viscosity in the accretion disc increases temporarily due to the magneto-rotation instability and this leads to an increased accretion rate on the compact star .

Bursts

In addition to the classic eruptions, many Very Faint X-ray Transients also show thermonuclear bursts . The X-ray brightness increases over a period of a few seconds and falls back into the idle state after a few minutes. This behavior is interpreted as the ignition of thermonuclear reactions on the surface of neutron stars, with the accreted matter being converted by burning hydrogen or helium . Since this cannot occur with black holes because they have no surface, the compact stars in most Very Faint X-ray Transients are neutron stars.

construction

Because of the low accretion rate, it is assumed that the companions of the compact stars could be red giants in symbiotic X-ray binary stars , where the compact star picks up matter from the stellar wind of its companion. Alternatively, it could be the stellar wind of massive early stars, but this is considered unlikely due to the lack of optical counterparts at the location of the Very Faint X-ray Transients. In addition, the companions of the compact stars in Very Faint X-ray Transients could also be red dwarfs . If these binary star systems are within the period gap, then the red dwarf does not fill its Roche boundary volume and the compact star can only accrete matter from the stellar wind of its companion.

Very-faint Persistent Neutron Star X-ray Binaries

A very small group of X-ray binary stars shows an X-ray spectrum similar to that of the Very Faint X-ray Transients. However, these systems always show an X-ray brightness of a few 10 34 erg / s with maximum fluctuations of a factor of 10. Since these are not temporary sources, they are called Very-faint Persistent Neutron Star X-ray binaries. Like the Very Faint X-ray Transients, they also show brightnesses in the range from 0.005 to 0.05 percent of the Eddington limit . It is not known whether there is a relationship in the structure of these groups of X-ray binary stars.

Individual evidence

  1. ^ AR King, R. Wijnands: The faintest accretors . In: Astrophysics. Solar and Stellar Astrophysics . 2005, arxiv : astro-ph / 0511486 .
  2. ↑ M. Del Santo, L. Sidoli, S. Mereghetti, A. Bazzano, A. Tarana and P. Ubertini: XMMU J174716.1–281048: a “quasi-persistent” very faint X-ray transient? In: Astrophysics. Solar and Stellar Astrophysics . 2007, arxiv : 0704.2134 .
  3. ↑ M. Armas Padilla, N. Degenaar, DM Russell and R. Wijnands: Multiwavelength spectral evolution during the 2011 outburst of the very faint X-ray transient Swift J1357.2-0933 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.5805 .
  4. ↑ M. Armas Padilla et al .: X-ray softening in the new X-ray transient XTE J1719-291 during its 2008 outburst decay . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1104.3423 .
  5. ↑ L. Sidoli, A. Paizis, S. Mereghetti, D. Götz M. Del Santo: XMM-Newton and INTEGRAL observations of the very faint X-ray transient IGRJ17285-2922 / XTEJ1728-295 during the 2010 outburst . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1104.1055 .
  6. ↑ Thomas J. Maccarone, Alessandro Patruno: Are the very faint X-ray transients period gap systems? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.0434 .
  7. ↑ Armas Padilla M., Degenaar N., Wijnands R .: The X-ray spectral properties of very-faint persistent neutron star X-ray binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.6640 .