Question

Fate of an \(X\) -Ray Binary. The X-ray bursts that happen on the surface of an accreting neutron star are not powerful enough to accelerate the exploding material to escape velocity. Predict what will happen in an X-ray binary system in which the companion star eventually feeds over 3 solar masses of matter into the neutron star's accretion disk.

Short answer

The neutron star will collapse into a black hole after accreting over 3 solar masses.

Step-by-step solution

Understanding X-ray Binary Systems

An X-ray binary is a system in which a normal star orbits a compact object like a neutron star or black hole. As matter from the star is drawn towards the compact object, it forms an accretion disk and emits X-rays as it is heated.

Comprehending the Effects of Accretion

In X-ray binaries, the neutron star can accumulate, or 'accrete', matter from its companion star. This increases the total mass of the neutron star. Typically, neutron stars have a mass slightly above 1.4 solar masses, known as the Chandrasekhar limit.

Analyzing Mass Increase

If the neutron star accumulates 3 solar masses of matter, this would increase its total mass significantly. Neutron stars have an upper mass limit, known as the Tolman–Oppenheimer–Volkoff (TOV) limit, which is around 2 to 3 solar masses.

Predicting the Outcome

When the accreted mass causes the neutron star to exceed the TOV limit, it can no longer remain stable. The gravitational force will overwhelm the neutron degeneracy pressure, leading to the collapse of the neutron star into a black hole.

Conclusion

Extra 3 solar masses can lead the neutron star to surpass its stability threshold, transforming it into a black hole. This process follows the fundamental principles of stellar evolution and gravitational physics.

Key concepts

Neutron Stars

Neutron stars are incredibly dense remnants of massive stars that have undergone supernova explosions. These celestial objects are known for their extreme density and are primarily composed of neutrons, subatomic particles found in atomic nuclei. After a supernova explosion, if the core of the star has a mass between approximately 1.4 and 3 solar masses, it will collapse into a neutron star. This threshold is defined by the Chandrasekhar limit, which is the maximum mass a white dwarf star can have before collapsing under its gravity.

Neutron stars have a radius of about 10 kilometers but hold a mass greater than that of our Sun. This leads to extremely high gravitational and magnetic fields. Their extreme density means a sugar-cube-sized amount of neutron-star material would weigh the same as a mountain on Earth.

In X-ray binary systems, neutron stars play a crucial role due to their ability to accrete matter from a companion star. This accreting matter forms an accretion disk, eventually emitting X-rays. Understanding how these systems evolve helps scientists explore not only the properties of neutron stars but also fundamental aspects of general relativity and high-energy astrophysics.

Accretion Disks

Accretion disks form around massive celestial objects like neutron stars and black holes in processes involving the gravitational pull of matter. When a star in a binary system loses material to its massive companion, this material orbits the dense object, forming a flattened disk that spirals inwards.

Accretion disks are fascinating due to their role in the conservation of angular momentum. As matter spirals closer to the neutron star, it speeds up, creating friction and therefore heat, which causes the disk to glow, often emitting X-rays. This makes them prominent in the study of X-ray binary systems, as these emissions are critical for observing and understanding these complex systems.

The exact structure and dynamics of an accretion disk can provide insight into the characteristics and behaviors of both the disk itself and the central neutron star, helping astronomers determine factors like size, mass, and rotation speeds. This ongoing study assists in piecing together the lifecycle of stars and the evolution of galaxies.

Tolman–Oppenheimer–Volkoff Limit

The Tolman–Oppenheimer–Volkoff (TOV) limit defines the maximum mass a neutron star can have before collapsing under its gravity into a black hole. Named after physicists Robert Oppenheimer, and George Volkoff, this limit is a crucial concept in astrophysics and helps explain the formation of black holes.

Neutron stars hold themselves up against gravitational collapse with a quantum force known as neutron degeneracy pressure. The TOV limit is reached when this pressure can no longer counterbalance the gravitational force due to the star's large mass. Current estimates place the TOV limit between 2 and 3 solar masses.

Surpassing this limit in an X-ray binary system, as seen when a neutron star accretes excess matter, leads to catastrophic consequences. The ensuing collapse is not a gradual process; it happens rapidly when a neutron star's mass increases beyond the limit. This results in the transformation of the star into a black hole, a process connected to extreme gravitational physics and fundamental stellar evolution principles.