X-ray binaries are a class of binary stars
that are luminous in X-rays.
The X-rays are produced by matter falling
from one component, called the donor (usually
a relatively normal star), to the other component,
called the accretor, which is very compact:
a neutron star or black hole.
The infalling matter releases gravitational
potential energy, up to several tenths of
its rest mass, as X-rays.
(Hydrogen fusion releases only about 0.7 percent
of rest mass.)
The lifetime and the mass-transfer rate in
an X-ray binary depends on the evolutionary
status of the donor star, the mass ratio between
the stellar components, and their orbital
separation.An estimated 1041 positrons escape
per second from a typical low-mass X-ray binary.
== Classification ==
X-ray binaries are further subdivided into
several (sometimes overlapping) subclasses,
that perhaps reflect the underlying physics
better.
Note that the classification by mass (high,
intermediate, low) refers to the optically
visible donor, not to the compact X-ray emitting
accretor.
Low-mass X-ray binaries (LMXBs)
Soft X-ray transients (SXTs)
Symbiotic X-ray binaries
Super soft X-ray sources or Super soft sources
(SSXs), (SSXB)
Intermediate-mass X-ray binaries (IMXBs)
Ultracompact X-ray binaries (UCXBs)
High-mass X-ray binaries (HMXBs)
Be/X-ray binaries (BeXRBs)
Supergiant X-ray binaries (SGXBs)
Supergiant Fast X-ray Transients (SFXTs)
Others
X-ray bursters
X-ray pulsars
Microquasars (radio-jet X-ray binaries that
can house either a neutron star or a black
hole)
== Low-mass X-ray binary ==
A low-mass X-ray binary (LMXB) is a binary
star system where one of the components is
either a black hole or neutron star.
The other component, a donor, usually fills
its Roche lobe and therefore transfers mass
to the compact star.
In LMXB systems the donor is less massive
than the compact object, and can be on the
main sequence, a degenerate dwarf (white dwarf),
or an evolved star (red giant).
Approximately two hundred LMXBs have been
detected in the Milky Way, and of these, thirteen
LMXBs have been discovered in globular clusters.
The Chandra X-ray Observatory has revealed
LMXBs in many distant galaxies.
A typical low-mass X-ray binary emits almost
all of its radiation in X-rays, and typically
less than one percent in visible light, so
they are among the brightest objects in the
X-ray sky, but relatively faint in visible
light.
The apparent magnitude is typically around
15 to 20.
The brightest part of the system is the accretion
disk around the compact object.
The orbital periods of LMXBs range from ten
minutes to hundreds of days.
The variability of LXMBs are most commonly
observed as X-ray bursters, but can sometimes
be seen in the form of X-ray pulsars.
The X-ray bursters are created by thermonuclear
explosions created by the accretion of Hydrogen
and Helium.
== Intermediate-mass X-ray binary ==
An intermediate-mass X-ray binary (IMXB) is
a binary star system where one of the components
is a neutron star or a black hole.
The other component is an intermediate-mass
star.
An intermediate-mass X-ray binary is the origin
for Low-mass X-ray binary systems.
== High-mass X-ray binary ==
A high-mass X-ray binary (HMXB) is a binary
star system that is strong in X rays, and
in which the normal stellar component is a
massive star: usually an O or B star, or a
blue supergiant.
The compact, X-ray emitting, component is
a neutron star or black hole.
A fraction of the stellar wind of the massive
normal star is captured by the compact object,
and produces X-rays as it falls onto the compact
object.
In a high-mass X-ray binary, the massive star
dominates the emission of optical light, while
the compact object is the dominant source
of X-rays.
The massive stars are very luminous and therefore
easily detected.
One of the most famous high-mass X-ray binaries
is Cygnus X-1, which was the first identified
black hole candidate.
Other HMXBs include Vela X-1 (not to be confused
with Vela X), and 4U 1700-37.
The variability of HMXBs are observed in the
form of X-ray pulsars and not X-ray bursters.
These X-ray pulsars are due to the accretion
of matter magnetically funneled into the poles
of the compact companion.
The stellar wind and Roche lobe overflow of
the massive normal star accretes in such large
quantities, the transfer is very unstable
and creates a short lived mass transfer.
Once a HMXB has reached its end, if the periodicity
of the binary was less than a year, it can
become a single red giant with a neutron core
or a single neutron star.
With a longer periodicity, a year and beyond,
the HMXB can become a double neutron star
binary if uninterrupted by a supernova.
== Microquasar ==
A microquasar (or radio emitting X-ray binary)
is the smaller cousin of a quasar.
Microquasars are named after quasars, as they
have some common characteristics: strong and
variable radio emission, often resolvable
as a pair of radio jets, and an accretion
disk surrounding a compact object which is
either a black hole or a neutron star.
In quasars, the black hole is supermassive
(millions of solar masses); in microquasars,
the mass of the compact object is only a few
solar masses.
In microquasars, the accreted mass comes from
a normal star, and the accretion disk is very
luminous in the optical and X-ray regions.
Microquasars are sometimes called radio-jet
X-ray binaries to distinguish them from other
X-ray binaries.
A part of the radio emission comes from relativistic
jets, often showing apparent superluminal
motion.Microquasars are very important for
the study of relativistic jets.
The jets are formed close to the compact object,
and timescales near the compact object are
proportional to the mass of the compact object.
Therefore, ordinary quasars take centuries
to go through variations a microquasar experiences
in one day.
Noteworthy microquasars include SS 433, in
which atomic emission lines are visible from
both jets; GRS 1915+105, with an especially
high jet velocity and the very bright Cygnus
X-1, detected up to the High Energy gamma
rays (E > 60 MeV).
Extremely high energies of particles emitting
in the VHE band might be explained by several
mechanisms of particle acceleration (see Fermi
acceleration and Centrifugal mechanism of
acceleration).
== See also ==
4U 0614+091
LS I +61 303
SS 433
Quasar
