Draft:Intermediate-mass X-ray binary

An intermediate-mass X-ray binary is a type of X-ray binary with an intermediate-mass donor star. Mass ranges differ, but a system with a donor star between ~1-8 solar masses (M_☉) is generally considered to be an IMXB.

Formation

IMXBs form in a manner similar to LMXBs. First, a binary star system forms in which the more-massive (primary) star has a mass greater than 8 M_☉. As the primary object evolves, it will fill its Roche lobe and mass transfer between the two stars will begin, which will typically be dynamically unstable due to the high mass ratio between the two stars. The star can also lose mass from stellar wind. Eventually, the primary star will lose its entire envelope of hydrogen, leaving just a hydrogen-depleted core. Ultimately, this core will become unstable due to loss of hydrostatic equilibrium and an inability to fuse elements heavier than iron and will explode in a type Ib or Ic supernova, forming a neutron star or black hole.

Properties

NS IMXBs are candidates for ultraluminous X-ray sources (ULXs), X-ray sources with luminosities above the Eddington luminosity for a typical stellar-mass black hole.

Evolution

Due to accretion, the mass of the donor star decreases over time. If this effect is extreme enough, an IMXB may lose enough mass to cross below the IMXB mass threshold, thus "becoming" an LMXB.

A 2002 simulation of X-ray binary evolution found that most neutron stars "LMXBs" start their lives with a donor star of 2 M_☉ or higher, but spend most of their lifetimes with a low-mass donor star due to mass loss from accretion. A 2003 follow-up study found that the formation probability of NS IMXBs with 1.5–4.0 M_☉ donor stars is 5 or more times higher than that of NS LMXBs with donor masses less than 1.5 M_☉, corroborating this finding. The formation of LMXBs via IMXB evolution could explain the problem of supernova kicks—in which binary systems remain bound despite the newly-formed compact object receiving a large velocity boost from its supernova explosion—because it is easier for systems with a higher-mass donor star to remain bound.

It has also been suggested that IMXBs may be the progenitors of recycled pulsars.

Observation

Observations of the LMXB Cygnus X-2 sparked further interest in LMXBs in the 1990s and 2000s. Cygnus X-2 hosts a 0.6 M_☉ donor star, but its intrinsic luminosity is much higher than what would be expected for a star of that mass. Thus, it is believed to have originally been an IMXB with a mass of 3.5 M_☉, which would fit better with the observed luminosity of the star.

Notable IMXBs

• Cygnus X-2 (at time of formation)

See also

• Low-mass X-ray binary
• High-mass X-ray binary