BP Crucis (x-ray source GX 301-2) is an

X-ray binary 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'', whi ...

system containing a blue hypergiant and a pulsar.

System

BP Crucis is considered as the optical counterpart to the X-ray source GX 301-2. The system consists of a massive

hypergiant A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term ''hypergiant'' is defined as luminosity class 0 (zero) in the MKK ...

star and a

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

in an eccentric 41.5 day orbit. The distance is likely to be between three and four thousand parsecs. It is heavily reddened and has a K-band

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

magnitude of 5.72. There is a mass transfer from the hypergiant to the pulsar which occurs via a dense accretion disc. This produces a cyclotron effect with electron energies of 37 and 48 keV.

Variability

The system shows both optical and x-ray variability. Although no eclipses are observed, the x-ray luminosity varies during the orbit with large x-ray flares being observed during

periastron An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...

passages. The system is an optical variable showing brightness changes of up to 0.08 magnitudes at visible wavelengths. These have been attributed to ellipsoidal variations as the hypergiant rotates and to α Cygni variability. There is an intrinsic pseudo-period of 11.9 days as well as small variations corresponding to the orbital period. That X-ray emission comes not from the neutron star itself, but rather represent a radiation re-emitted by optically thick accretion shell.

Properties

BP Crucis is around 43 times as massive as the Sun, it is also one of the most luminous stars known in the Galaxy, with an estimated

bolometric luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a ...

of around 470,000 times that of the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

and a radius 70 times that of the Sun. The neutron star appears to belong to the "high mass" variety being at least . It is very likely to have a mass less than as the theoretical maximum mass based on the equation of state for a neutron star. The pulsar has a spin period of 685 seconds, but shows relatively large spindown rates thought to be due to its strong magnetic field, and also occasional spinups due to interaction with the accretion disk. It is calculated that a slowly spinning neutron star could be spun up to the current rotation rate by accretion in only ten years.

References

External links

Swift/BAT transient monitor results
{{DEFAULTSORT:BP Crucis Crux B-type hypergiants Emission-line stars Crucis, BP J12263756-6246132 Rotating ellipsoidal variables X-ray binaries Pulsars