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Evidence for a black hole in the X-ray transient XTE J1859-226.

Author/s: (2011) Corral-Santana, J. M.; Casares, J.; Shahbaz, T.; Zurita, C.; Martínez-Pais, I. G.; Rodríguez-Gil, P.

Reference: MNRAS, tmpL., 215C | Link

Representation of a system similar to XTE J1859+226. The star is distorted by the gravitational influence of the black hole. The material is pulled out from the star and it orbits around the black hole forming an accretion disc.Source/Jesús Corral Santana (IAC). Made with the software BinSim, developed by Rob Hynes, Louisiana State University (USA).
Representation of a system similar to XTE J1859+226. The star is distorted by the gravitational influence of the black hole. The material is pulled out from the star and it orbits around the black hole forming an accretion disc.Source/Jesús Corral Santana (IAC). Made with the software BinSim, developed by Rob Hynes, Louisiana State University (USA).

X-ray binaries are systems formed by a compact object, either a neutron star or a black hole, and a 'normal'-type star. The compact object pulls out material from the star. This material orbits around the compact object making an accretion disc.

Transients are a class of X-ray binaries which stay most of their life in a quiescence state. Accidentally, the system shows an outburst, which is related with an abrupt rising of the accretion rate inside the black hole. This brightening occurs at all wavelengths (up to 6 magnitudes in optical!) and it is then detected by X-ray satellites. After a while in this active state, the system decays to its quiescence level.

In this study we have found that in the X-ray transient XTE J1859+226 there is a black hole of 5.42 solar masses. We have combined optical photometry taken with the Isaac Newton, William Herschel and Nordic Optical telescopes since 2000 and low-resolution spectra taken with the Gran Telescopio Canarias in 2010. The light curve gave us a 6.58h orbital period while spectra provided us a secondary star's radial velocity semi-amplitude of 541 km/s.

Both parameters set a mass function of 4.5 solar masses, which is a lower limit to the mass of the compact object. The equation of state gives an upper limit to the mass of neutron stars in 3 solar masses, so in this binary system the compact object must be a black hole. The lack of eclipses and the depth of the minima in the light curve implies an inclination angle between 40º and 70º, and therefore a  lower limit in the mass of the black hole of 5.42 solar masses.

There are only about 20 dynamically confirmed black holes in this binary systems and its thought that there should be several thousands in the Milky Way, so statistics are quite poor. We need to increase the population of black holes because the shape of their mass distribution is pretty important to understand the evolution of massive stars, supernovae and binary systems.

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