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 become a black hole.

Step-by-step solution

Understand the System

An X-ray binary system consists of a neutron star and a companion star. The neutron star accretes material from its companion.

Introduce Mass Accretion

If the companion star transfers over 3 solar masses of matter to the neutron star, this additional mass will play a crucial role.

Evaluate Neutron Star Limit

A neutron star has a maximum mass limit known as the Tolman–Oppenheimer–Volkoff limit, which is approximately 2 to 3 solar masses.

Determine Consequence of Mass Breach

Since the neutron star will exceed its maximum mass limit when accreting an additional 3 solar masses, it will collapse into a black hole.

Key concepts

Neutron Star

A neutron star is the dense remnant of a massive star that has gone through a supernova explosion. These stars are incredibly compact, with masses about 1.4 times that of our Sun compressed into a sphere with just a radius of about 10 kilometers. Neutron stars are primarily composed of neutrons, the particles found in the nucleus of an atom. The immense gravitational force keeps the neutrons closely packed together.

• These stars have an extraordinarily high density, with one teaspoon weighing billions of tons.
• Neutron stars often have strong magnetic fields and can emit beams of radiation from their poles, making them observable as pulsars if these beams sweep past Earth.

The intense gravitational and magnetic forces of neutron stars make them fascinating subjects of study, especially when they are components of X-ray binary systems.

Accretion Disk

An accretion disk forms when matter from a companion star is pulled toward a more compact object, such as a neutron star or black hole, within a binary star system. The gravitational forces of the neutron star draw material from the companion star, which spirals inward due to angular momentum. This matter forms a disk around the neutron star.

As material in the accretion disk falls closer to the neutron star, it heats up and emits X-rays, which is why these systems are often detected as X-ray binaries. The process of accretion is vital because:

• It can lead to significant mass gain for the neutron star.
• The energy released by infalling material powers strong X-ray emissions.

Accretion is a crucial mechanism that can eventually lead to significant changes in the structure and fate of a neutron star.

Tolman–Oppenheimer–Volkoff Limit

The Tolman–Oppenheimer–Volkoff (TOV) limit is a theoretical upper mass limit for a neutron star. It is analogous to the Chandrasekhar limit for white dwarfs. Once a neutron star exceeds this limit, it can no longer support its own weight against gravitational collapse. Typically, this limit is estimated to be between 2 to 3 solar masses.

• If a neutron star accumulates matter such that its total mass surpasses the TOV limit, it no longer remains stable.
• Beyond this limit, gravitational forces overcome neutron degeneracy pressure.

The exact value of the TOV limit depends on the star's internal composition and the physics governing nuclear interactions at extreme densities. Understanding this limit is essential in predicting the possible transformation of neutron stars into black holes.

Black Hole Formation

Black hole formation occurs when a massive celestial object's core collapses under its own gravity. When a neutron star in an X-ray binary system accretes enough mass to exceed the TOV limit, it can no longer support itself, resulting in its collapse into a black hole. This transformation involves:

• The star undergoing a gravitational collapse, leading to the formation of a singularity where gravitational forces are infinite.
• The event horizon forming, beyond which no light or information can escape.

Black holes are characterized by having such strong gravitational pull that nothing, not even light, can escape once it passes the event horizon. In the context of X-ray binary systems, when the neutron star becomes a black hole, the system's X-ray emissions may change due to different accretion dynamics.