X-Ray Binaries
Binary starsystems that contain a compact object, such as a white dwarf, neutron star or a black hole and emit x-rays are referred to as x-ray binaries. This class of stars is furthermore divided into two subgroups. If the companion to the compact object has a mass of less than or equal to the sun (1 solar mass = 2 x 10^30 kg.), then it is called a Low-Mass X-Ray Binary (LMXB for short. Note that the compact object may still be a neutron star or a black hole) and if the mass of the companion exceeds 10 solar masses, then it is referred to as a High-Mass X-Ray Binary (HMXB for short).
Binary starsystems containing a star with an intermediate mass (1-10 MSun) arent classified as x-ray binaries, even though they do emit x-rays. Instead these systems are called cataclysmic binaries, because they vary greatly in brightness (example: during a nova). They are also weaker x-ray sources than binaries containing a neutron star or a black hole.
High Mass X-Ray Binaries - HMXB's
High mass x-ray binaries are binary starsystems containing a massive blue O or B class giant star (M > 10 MSun) revolving around a compact object, which could be a, a neutron star or a black hole. In HMXB, the massive star dominates the emission of optical light while the accretion disc and the compact object's surface stands for the x-ray emission (if the compact object is a neutron star. Light cannot escape a black hole).
The compact object must have been a more massive star earlier in the history of the binary star system, since it evolved faster than the other star. During its final stages it shed the outer layers of hydrogen, helium and carbon which could partly have been captured by the other star. At the end of the massive star's life it blew off as a supernova and left behind this compact object, which is most likely a neutron star or a black hole. The most famous example of a HMXB ought to be Cygnus X-1.
A mass-transfer significant enough to provide material for the generation of x-rays by the compact object could occur through an overflow of the relatively less compact object's Roche lobe, like the case with LMXB's, but a significant amount of mass could also be transferred by stellar wind, which is caught by the compact object. It should be noted that if the massive star hasnt entered a red-giant phase, where it has swollen significantly, the compact object has to be really close to be able to rip off gas and create an accretion disc, especially if the compact object is a white dwarf or a neutron star.
Cygnus X-1
The first strong candidate for a black hole was found orbiting the O9.7 Iab type supergiant HDE 226868 in 1972. The system is located in the constellation of Cygnus, The Swan, at a distance of 2500 parsecs (about 8 150 light years). The binary starsystem is now designated Cygnus X-1.
The main star, (which is the visible blue star with a surface temperature of
31 000 degrees Kelvin) is calculated to have a mass of 20-25 solarmasses and the unseen companion is calculated to have a mass of 7-13 solarmasses. Cygnus X-1 is classified as an HMXB. The binary starsystem revolves around the gravitational center in 5.6 days.
Studies showed that gas was ripped off from the massive blue giant by an unseen companion, a black hole. This gas was highly accelerated into a spiraling orbit towards the black hole. The gas then got so hot - because of friction - that it emitted x-rays. Also, studies of the spectra from the system showed that the companion, which was causing the primary star to wobble had a mass of atleast 7 solarmasses. Since this mass surpassed the Oppenheimer - Volkoff limit, the masslimit a neutron star must pass to become a black hole (3 solarmasses), in time many astronomers became convinced that the companion was a black hole. Another discovery that pointed toward the nature of the unseen companion being a black hole was that light from the system appears to flicker. Had the unseen companion been a neutron star the light from the compact object would have been appear in periodic pulses, which it did not.
Cygnus X-1 is presently among the strongest sources of x-rays in the night sky.
The illustration to the right shows a schematic of how the system is thought to have evolved, and will evolve. (1) Since there is a black hole in the system, which is a stellar corpse, it is reasonable to assume that there was another star in the system's past that had a mass larger than what the present blue giant HDE 226868 (20-25 MSun). (2) The more mass a star has, the faster it evolves, and so the more massive star entered the red giant phase faster than its companion until it went supernova (3) and spread its mass many light years away. Some of that mass may have been captured by HDE 226868. (4) The supernova left behind a stellar corpse that had a mass greater than 3 solarmasses (7-13 MSun), which is the upper limit for a neutron star. Astronomers believe this corpse is a black hole. 5 is the current status of the system where the black hole has moved closer to the star HDE 226868. As a result it is able to increase in mass by absorbing the stellar wind from HDE 226868, and it is also able to rip off gas from the surface.
At stage 6 HDE 226868 has entered its red giant phase and the black hole is able to rip off gas in an increasing rate. (7) HDE 226868 goes supernova and it is quite possible that the black hole absorbs much of the gas ejected from the explosion. It may grow in mass. (8) When the gas has been ejected, it is possible that yet another black hole will form, unless the first black hole has stolen too much mass from HDE 226868 in its previous stages.
Low Mass X-Ray Binaries - LMXB's
Low mass x-ray binaries are binary starsystems where a neutron star or a black hole is present, along with a normal star (mass: less than 1 MSun), which may be a sunlike main-sequence star, a star that has swollen or a white dwarf. The compact object rips off gas from the surface of the less dense object, forcing the Roche lobe to overflow. This gas is accelerated towards the compact object and heated to more than 1 million degrees K, which makes it emit x-rays. The brightest part of the LMXB's is the accretion disc of matter falling down onto the compact object. Many LMXBs show display bursts of light, which are very rapid increases in the X-ray output, followed by exponential declines (typically lasting seconds to minutes). The bursts are interpreted as thermonuclear explosions on the stellar surface, after material has accumulated due to accretion. During these bursts, x-ray emission from the vicinity of the neutron star dominates that coming from the disk.
Most of the radiation output of LMXB's is through x-rays, and a small fraction of the light is visible light, which makes LMXB's faint in visible wavelengths.
So far astronomers have discovered about 100 LMXB's and about 13 of them are located in globular clusters orbiting the milky way galaxy.
The orbital periods of LMXBs range from ten minutes to hundreds of days.
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